Cold-Roof Design Practices And Performance Case Studies

COLD-ROOF DESIGN PRACTICES
AND PERFORMANCE CASE STUDIES
LINDA M. MCGOWAN, PE,AIA; AND
LOREN D. FLICK, PE
BUILDING CONSULTANTS & ENGINEERS, INC.
1520 West Canal Court, Ste. 240, Littleton, CO 80120
Phone: 303-350-1000 • Fax: 303-350-1004
E-mail: loren@building-c-e.com and linda@building-c-e.com
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ABSTRACT
Cold-roof systems have been used for many years in alpine environments to manage
snow and ice accumulations on roofs. The design of these systems includes some unique
criteria such as the following:
• Ventilation within the cold-roof space, and inlet and outlet venting
• The effects of the interior environment on the cold-roof system, including attic and
rafter ventilation
• The geometric complexity of the roof plan
• Detailing of the roofing system and cold-roof ventilation space to reduce the potential
for leakage
This paper explores some of the published information on cold-roof design and the
authors’ opinions regarding reasonable design practice based on the authors’ experiences in
evaluating the performance of cold roofs in the western U.S. over the last 20 years. Several
case studies are presented of the performance and repair of large cold-roof systems in the
alpine regions of the western U.S..
SPEAKER
LOREN D. FLICK, PE — BUILDING CONSULTANTS & ENGINEERS, INC.
For more than 30 years, LOREN D. FLICK, senior principal of his firm, has consulted on
thousands of projects nationwide involving the evaluation of and design of repairs for building
envelope systems, structures, foundations, and construction materials. Included in
these projects have been cold-roof systems on large buildings in resorts in the alpine climates
of the western United States. Based on this background, Mr. Flick brings a practical
perspective on the design practices and details that have resulted in good, long-term coldroof
performance in the western U.S. and those that have not.
NONPRESENTING COAUTHOR
LINDA M. MCGOWAN, PE, AIA — BUILDING CONSULTANTS & ENGINEERS, INC.
LINDA MCGOWAN, PE, AIA, now president and principal at her firm, began her career
designing various structures for paper and chemical manufacturing facilities in Georgia.
Later, while employed with an environmental engineering firm in Virginia, Ms. McGowan’s
projects included the architectural and structural design of several solid-waste-handling facilities,
large-equipment-maintenance facilities, and water and wastewater treatment plants.
Ms. McGowan has investigated numerous water leakage and deterioration problems in plaza
and below-grade waterproofing systems, including related structural damage, and has
designed and observed repairs. She has extensive experience in the evaluation and design of
snow-country roofing systems, including cold roofs, cold attics, and superinsulated roofs; she
also has extensive experience in the design of snow and ice management systems.
INTRODUCTION
Cold-roof systems have been utilized for
years in alpine environments as a means to
manage snow and ice accumulations on
sloping roofs and to reduce the formation of
ice and ice dams on the roofs. The cold roof
system discussed in this paper will be the
“double roof” system, where there is an upper
roof surface (the surface that has roofing
materials on it) and a lower roof surface
positioned below the upper roof surface.
The cavity between the two roof surfaces is
intentionally left open to create a cold-roof
ventilation space between the upper and
lower roof surfaces. The lower roof surface
may be positioned above an attic or rafter
spaces (either ventilated or nonventilated).
Figure 1 illustrates a cross-section
through a cold roof on a building.
Cold-roof systems are also only utilized
for sloping roofs, typically with slopes of
4:12 or greater. Roofs with slopes less than
4:12 typically do not function properly as
cold roofs, and their snow and ice management
is typically addressed in a different
manner.
In general, when ambient air temperatures
are below freezing, there are primarily
two mechanisms that can cause snow accumulations
on roofs to melt and create meltwater
on the roofing surface: a warm roofing
surface caused by heat loss from the
interior of the building, and solar heating.
The meltwater flows down the sloped roofing
and will encounter a roofing surface
whose temperature is below freezing, typically
near the roof eaves, which extend out
beyond the exterior walls of the building
and are therefore usually near ambient
temperature. When the meltwater encounters
these colder roofing temperatures, it
can freeze before it runs off the roof eave
edge. Over repeated cycles of this process,
an ice dam and icicles can begin to form.
When ice dams build up to a certain depth,
additional meltwater seeping down the roof
can pool behind the ice dam. Sloped roofing
systems are not intended to be watertight
under ponded-water conditions, and as a
result, the ponded water can leak through
the roofing system and into the occupied
space below. Uncontrolled ice dam formation
can also lead to excessive loads on the
eave edges of the roof.
In a properly functioning cold-roof system
during cold weather, a chimney effect is
created in the sloped ventilation space by
heat loss from the interior of the building.
This effect draws exterior air into the coldroof
ventilation space at the inlet vents,
which are primarily located along the eaves.
The air in the ventilation space is exhausted
at the outlet vents, usually located along
the ridges, headwalls, and, in some cases,
along hips. Cold-roof systems function by
allowing the heat that is lost from the interior
of the building to be continuously
flushed out via the cold-roof ventilation
space so that the temperature of the upper
roof surface (where snow and ice accumulate)
remains near ambient exterior temperatures,
thus reducing the formation of ice
dams on the upper roof surface. However, it
should be recognized that cold roofs can
only perform in this manner when the flow
of cold air in the ventilation space is sufficient
to offset the effects of heat loss from
the interior of the building. Further, cold
roofs cannot prevent ice formations on roofs
due to the solar melting of snow on the roof
surface and the refreezing of the meltwater
at eaves and at shaded portions of the roof.
Therefore, it should be understood that
while cold-roof systems are intended to
reduce the amount of meltwater and resulting
formation of ice dams and icicles that
are caused by heat lost from the interior of
the building (and thus decrease the risk of
leakage inside the building), ice dams can
form in some circumstances even with a
properly designed and functioning cold-roof
system. Thus, the design should include
provisions to reduce the risk of leakage
caused by water ponding upslope of ice
dams, even with a cold-roof system.
Another type of cold-roof system is a
cold attic, which functions similarly to a
cold roof by using the attic space as the
cold-roof ventilation space. However, there
are some significant differences in design
approaches, and for this reason, this paper
does not include a discussion of cold attics.
DESIGN CRITERIA
There are no building code requirements
or other widely recognized industry
standards for the design of a cold-roof system.
A number of papers and articles have
been presented that discuss cold-roof systems
and mitigating ice dam development
on roofs in cold environments. Relevant
papers and articles are listed in the
attached bibliography. This paper reflects
the authors’ review of the information in
these papers and articles, tempered by the
COLD-ROOF DESIGN PRACTICES
AND PERFORMANCE CASE STUDIES
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authors’ evaluations of the performance of
existing cold-roof systems since the 1980s
and the performance of repairs and improvements
that the authors have designed
for existing cold-roof systems.
For sloped roofs in alpine climates,
there are a number of options for roof
designs and assemblies for managing snow
and ice accumulations. The selection of a
cold-roof system should be carefully evaluated
by the designer and the owner. In addition
to the multitude of factors related to
the selection of roofing forms, materials,
and assemblies, the following items should
be considered in conjunction with a coldroof
system:
• Wintertime environment, including
amount of snowfall, average and
extreme temperatures, and the
amount of solar radiation
• Overall shape of the roof—particularly
the complexity of the roof and
suitability of a cold-roof system for
the roof design
• The need to manage snow and ice
accumulations on the roof surface,
and the risks associated with snow
and ice falling from the roof surface
• The feasibility of various cold-roof
assemblies, including the additional
cost to construct a cold-roof system
Environment During Snow Season
The additional cost of a cold-roof system
is a reasonable option for buildings constructed
in locations subject to heavy snowfall
and cold temperatures during snow season,
which would include alpine ski resorts
in the western U.S. Average daily ambient
temperatures during the snow season
should be below freezing to allow most of
the snow that accumulates on roofs to
remain in place except for the upper portions
of the snow that will gradually sublimate
if exposed to solar heating.
Roof Design and Complexity
To the extent practical, the roof forms
should be as simple as possible to allow
ambient air to flow from the inlet vents
through the ventilation cavity to the
exhaust vents in a relatively straight line up
the slope of roof. Simple gable roof forms or
single-slope roof forms are ideal for the use
of cold roofs. Complex roof forms increase
the difficulty in achieving an even, continuous
flow of air in the ventilation cavity from
the inlet vents to the outlet vents, which can
often result in a cold roof that does not perform
well. In addition, complex roof forms
can concentrate areas of snow, ice, and
meltwater, exacerbating problems with ice
dam formation.
Other design-related considerations for
the roof form should include these:
1. Design the roof to reduce the number
of valleys and intersecting roof
planes to the extent reasonable and
practical. Valleys and intersecting
roof planes all interrupt the flow of
air within the cold-roof ventilation
space. Hips can be made to function
properly if the hip is properly
designed as an exhaust (or transfers
air flow to the outlets at the ridge),
but hips can create cold-roof performance
problems if not properly
designed.
2. Make gabled dormer roofs relatively
steep, and do not locate dormers
close together. Closely spaced gable
dormers restrict the amount of air
that can be drawn into the inlet
vents along the eaves and concentrate
snow and ice into narrow
“chutes” between adjacent dormers.
The accumulation of snow, ice, and
meltwater in these areas, coupled
with heat loss from the dormer sidewalls
and the likely restriction of air
within the cold-roof ventilation
space, increase the risk for ice dam
formation and potential leakage. The
risk of leakage may also be
increased due to the difficulty of
adequately installing roofing, flashing,
and waterproofing underlayment
materials.
3. If possible, use shed dormers instead
of gabled dormers. Shed
dormers have the same issue with
ice dam formation along the eave
edge but do not contribute to the
concentration of snow and ice created
by valleys of gabled dormers. The
cold-roof air inlets along the eaves of
the shed dormers can also be integrated
into the cold-roof systems to
provide proper airflow within the
ventilation cavity.
4. Avoid placing chimneys in the path
of moving snow, especially near the
bottom edge of sloping roofs and
near valleys. If possible, locate chimneys
near a ridge or rake edge.
Chimneys not only block the coldroof
ventilation space, but also promote
snow and ice accumulations on
the roof surface. Provide properly
designed crickets on the upslope
side of chimney walls, with provisions
to drain meltwater on the
cricket around the chimney. If chimneys
are “oversized” for aesthetic
reasons, make the penetration
through the cold-roof space as small
as practical to decrease the amount
of air that is blocked by the chimney
in the cold-roof space. Further, other
penetrations through the roof (such
as plumbing vents, furnace flues,
and lightning terminals, etc.) should
not be placed near the bottom edge
of sloping roofs or valleys and should
be protected from sliding snow where
installed elsewhere on the roof.
5. All details necessary for a contractor
to build a cold roof should be shown
in the design documents. This is
particularly true for complex roof
forms, which require a comprehensive
plan for ensuring adequate
cold-roof ventilation over the entire
roof surface. In the authors’ opinion,
it is not reasonable for a designer to
expect a qualified contractor to “figure
out” how to build all the unusual
details associated with a cold roof
in such a way that it will perform as
intended by the designer.
Cold-Roof Assembly
The following describes a typical “double-
roof” cold-roof assembly (from top to
bottom):
• Roofing material (i.e., asphalt shingles,
wood shakes, standing-seam
metal, etc.)
• Waterproofing underlayment
• Upper roof deck (typically a rated
sheathing like OSB or plywood)
• Cold-roof ventilation space (often
created by 2×4 wood framing members
turned on edge running up the
roof slope, and thus creating a 3½-
in.-high cold-roof ventilation space)
• Roofing underlayment (may be a
waterproofing underlayment at eave
edges, sidewalls, and at penetrations)
• Lower roof deck, which is typically
the structural roof deck (typically
rated sheathing or gypsum sheathing
over a structural steel deck)
• Roof structural framing (trusses or
joists)
• Roof insulation
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• Interior air barrier (which could also
function as a vapor retarder) and
ceiling
There are numerous other assemblies
and configurations for cold-roof systems,
depending on the type of construction, the
complexity of the roof forms, the materials
of construction, and other factors. Each
layer within the assembly should be
designed and selected to provide the intended
performance and compatibility with each
other to meet the design objectives.
Other common cold-roof assemblies
include concrete tiles on battens and crossbattens,
which results in a cold roof below
the concrete tiles and above the roof deck.
This provides a type of double roof system
and, while the concrete tiles provide the primary
weathering surface, the lower roof
surface is the primary weathertight surface.
It should be recognized that while the crossbattens
(which extend perpendicular to the
slope of the roof) provide cross ventilation,
they are not part of the cold-roof ventilation
space; the cold-roof ventilation space is
determined by the battens that run up the
roof slope.
In some applications, ventilated “nailbase”
panels have been utilized in cold-roof
applications. These typically consist of an
upper roof deck surface (typically oriented
strand board) supported on wood or rigid
foam blocks that are attached to a rigid
foam insulation layer (commonly polyisocyanurate).
These nailbase panels are
attached to the structural roof deck with
fasteners in a specific pattern, and the roofing
underlayment and roofing materials are
applied directly to the top of the nailbase
panels. The clear space between the blocks
and the height of the blocks determines the
cold-roof ventilation space. In some cases,
the pattern of the blocks provides for cross
ventilation within the ventilation space. In
the authors’ experience, ventilated nailbase
panels with cross ventilation can provide
some attributes of a cold-roof system, and
the added insulation provides additional
benefits; but generally, the ventilation space
is insufficient to be considered a true coldroof
system.
Issues related to types of construction
and fire ratings should be carefully evaluated
by the designer for the particular building
and application. The very nature of coldroof
systems utilizes a “chimney effect”
within the cold-roof ventilation space.
Therefore, fire from outside the building
could enter into the cold-roof air space, creating
a significant hazard. Although they
are beyond the scope of this paper, these
issues need to be considered by the designer
in the overall design of the building and
cold-roof system.
Cold-Roof Ventilation Space
The cold-roof ventilation space must be
sufficiently large to allow an ample amount
of air to flow within the space in a relatively
unobstructed manner from the inlets to the
outlets. This is typically a function of the
roof slope and the distance from the inlet
vents (usually at the eaves) and exhaust
vents (usually at the ridges, headwalls, or
hips).
The building-code-mandated ventilation
for attics and rafter spaces is intended to
reduce the risk of condensation within
attics and rafter spaces; it is not intended to
serve as the basis of design for a cold-roof
system. Nevertheless, the code-mandated
ventilation of attics and rafter spaces can
affect cold-roof systems and is discussed in
more detail later in this paper.
It is the authors’ opinion that a 3½-in.
height in a cold-roof ventilation cavity
should typically be sufficient for roof slopes
greater than 5:12 when the distance from
the eave to the ridge is less than about 50
ft. The height of the cavity should be evaluated
for shallower roof slopes or longer
lengths of roof. For roof lengths less than
about 50 ft., it is not usually practical to
reduce the height of the cold-roof ventilation
space, since ensuring an ample
amount of air in the cold-roof ventilation
space can be difficult to achieve with
heights less than about 3½ in.
The authors have found guidelines for
“double-roof” venting as presented in the
Concrete and Clay Tile Roof Design Criteria
Manual for Snow and Cold Regions to be a
useful starting point for evaluating the
height of the cold-roof ventilation cavity.
Findings by Tobiasson, Buska, and
Greatorex in Guidelines for Ventilating Attics
and Cathedral Ceilings to Avoid Icing at
Their Eaves, which is referenced in the
National Roofing Contractors Association
Roofing and Waterproofing Manual, indicate
that ventilation heights greater than 1¾ in.
do not improve ventilation appreciably. In
the authors’ experience in the western U.S.,
poorly performing cold roofs often have a
ventilation cavity height of less than 2 in.,
among other characteristics that affect the
performance of the cold roof. In the authors’
opinion, a practical way to offset some
inherent design and construction limitations
is to intentionally provide more ventilation
space by reasonably increasing the
height of the ventilation space. For this reason,
since most cold-roof cavities are created
by using standard-size wood-framing
members, it is reasonable to design the cavities
to be 3½ in. high for most of the cold
roofs that we have observed.
The amount of insulation in the roof
assembly, as well as the amount of air leakage
from the building into the cold-roof ventilation
space, can affect the performance of
the cold roof. Lesser amounts of insulation,
or poorly installed insulation with numerous
gaps and thermal bridges, will increase
heat loss into the cold-roof ventilation space
and, thus, increase the need for more air in
the cold-roof ventilation space. Likewise, if
air leakage from inside the building is
allowed to easily pass into the cold-roof ventilation
space, more air is needed in the
cold-roof ventilation space to flush the
warmer air from inside the building to prevent
warming of the upper roof surface.
However, both of these heat- and air-loss
considerations increase the chimney effect
within the ventilation cavity, probably
increase the flow of cold air through the
cavity, and probably somewhat offset the
negative effect of increased heat and warm
air being lost into the cavity from the building
interior.
Once the height of the cold-roof ventilation
cavity is determined, the net free ventilation
area (NFVA) of the cold-roof system
can be determined.
Continuity of Cold-Roof Ventilation
Space
Ideally, the NFVA of the cold-roof ventilation
space should be maintained continuously
from the inlet to the outlet, but this
can be challenging to accomplish due to the
inherent airflow interruptions within the
cavity and penetrations through the coldroof
system. These are difficult to account
for in the design; therefore, interruptions,
penetrations, and other obstructions to the
flow of air should be kept to a minimum,
resulting in a simpler roof form.
Where penetrations through the roof
interrupt this ventilation (such as at a
chimney shaft), the design should be such
that air within the cold-roof ventilation cavity
is allowed to flow around the penetration,
and the penetrations kept to a minimum.
Where such penetrations exist, the
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design details should show how to mitigate
the airflow obstruction and how the framing
should be detailed to encourage the flow of
air around the penetration to the extent
possible. The designer should also consider
how snow and ice accumulations around
such penetrations might affect the performance
of the roof.
Similarly, other interruptions to air flow
within the cavity, such as at valleys and at
unvented hip roofs, create a disruption in
the flow of air from the eave to the ridge
within the cold-roof ventilation space.
Valleys will allow for a significant amount of
outlet ventilation but minimal inlet ventilation,
and hip roofs will allow for a significant
amount of inlet ventilation but minimal
outlet ventilation (unless ridge vents
are incorporated into the design along the
hip). The design should show how the inlet
or outlet ventilation is to be increased to
account for the interruption without
decreasing the NFVA. It should detail the
cavity framing to encourage lateral air
movement within those portions of the roof
not otherwise directly in the path of continuous
ventilation from the inlet to the outlet.
The designer should consider how the valleys
affect the performance of the roof, such
as by concentrating snow and ice accumulations
in the valleys and at roof areas below
the ends of valleys.
Where headwalls are positioned along
the upper portions of the roof, outlet ventilation
should be accomplished by the use of
headwall vents. In the authors’ experience,
windows are often positioned on these
headwalls and, in some cases, can prohibit
the installation of headwall vents below the
windows. In these cases, the design should
incorporate provisions to allow for adequate
cross ventilation so that the headwall vents
can adequately provide the necessary outlet
ventilation without reducing the NFVA.
Inlet and Exhaust Vents
Inlet and exhaust vents must be of sufficient
size so that the NFVA of the cold-roof
ventilation space is not restricted; in other
words, the NFVA at the inlet vents and at
the exhaust vents should be approximately
equal to the NFVA within the cold-roof ventilation
space. This must take into account
the reduction in NFVA caused by screens on
the inlet and exhaust vents, which can significantly
reduce the NFVA of the vent,
depending on the type of screening (some of
which is more susceptible to debris accumulation
during the life of the building,
which lowers the effective NFVA). The
authors have found metal mesh with ¼-in.
openings to be a reasonable compromise to
keep birds and insects out of the cold-roof
ventilation space while retaining as much
NFVA as practical.
To the extent possible, the design intent
should be for the NFVA of the inlet to be
equal to the NFVA of the exhaust. If necessary,
the NFVA of the inlet may be slightly
greater than the NFVA of the exhaust, since
the “chimney effect” will tend to push air
through the exhaust vent; however, the
NFVA of the exhaust vent should generally
not be greater than that of the inlet vent.
Also, the intended flow of air from the inlet
to the exhaust should be in a straight line
up the slope of the roof in an unobstructed
and continuous manner. However, if the
inlet or exhaust ventilation must supply or
exhaust additional areas of the roof (beyond
that simply above and in direct line with the
vents), then inlet or exhaust NFVA should
be increased to accommodate the additional
ventilation demand.
In some jurisdictions and for some building
types, soffits are required to be fire-resistance-
rated and are not permitted to have
openings. In these cases, inlet vents cannot
be located in the soffits. In some instances,
inlet vents may be positioned in the fascia
(between fascia and subfascia boards).
Also, in some locales, inlet vents cannot
be positioned in soffits or in the fascia due
to wildfire mitigation regulations. In these
cases, this likely makes cold-roof assembly
impractical.
Effects of Ventilation of Attic and
Rafter Spaces
The ventilation in a “double-roof” system
is independent from the building-coderequired
ventilation of the attic and rafter
spaces. The ventilation of attic and rafter
spaces can affect the performance of the cold
roof—in particular, how warm the lower roof
surface might be (due to limited ventilation of
the attic and/or rafter ventilation) and how
well the cold-roof ventilation can exhaust air
warmed as a result. This issue can also be
mitigated by providing sufficient roof and/or
ceiling insulation and reducing air infiltration
into the attic and rafter spaces.
When ventilation of the attic and rafter
spaces is provided, the inlet and exhaust
vents for the attics or rafter spaces and the
cold-roof ventilation space may be combined,
with the understanding that such
vents service two areas to be ventilated and
thus need to be sized to accommodate air
flow to and from both areas. There is also
the potential for more air to be drawn into
the cold-roof ventilation space than into the
attic or rafter spaces, which could result in
less-than-anticipated airflow into the attic
or rafter spaces; therefore, this potential
should be evaluated by the designer.
As an alternative to ventilation of attic
and rafter spaces, consideration may be
given to designing nonventilated attics or
rafter spaces. Provided the insulation is sufficient
and thermal bridging is minimized,
cold-roof systems over these assemblies can
perform successfully.
As illustrated in the following case studies,
the lack of adequate attic or rafter ventilation
does not necessarily mean that condensation
problems will occur on the
underside of the lower roof deck. In the
authors’ experience, such condensation
problems are relatively infrequent and are
usually caused by unusually high indoor
humidity levels (such as by mechanical
humidification, indoor pool or hot tubs, or
soil-moisture sources).
Roof and Ceiling Insulation
Sufficient insulation should be provided
to comply with roof and ceiling insulation
requirements of the building code (and the
energy code, if applicable) and to reduce the
rate of heat loss from the building. This
includes adequate insulation around and
on top of heat sources such as recessed
light fixtures and bathroom exhaust fans.
The type, amount, and installation methods
of the insulation should be evaluated to prevent
blocking of ventilation of the attic and
rafter spaces. Also, designs should avoid
placing mechanical equipment and ductwork
above the insulation and below the
lower roof deck, since heat generated by
this equipment and heat loss from ductwork
can increase the heat in the space
above the insulation and below the lower
roof deck, thus warming air in the cold-roof
ventilation space and increasing the risk of
melting snow on the upper roof deck.
Air Barrier Above Interior Spaces
The interface between the building interior
and the cold roof should be as airtight
as possible. If air sealing on the underside
of the lower roof deck is not provided, this
would include air-sealing penetrations in
the ceiling assembly (such as recessed light
fixtures, speaker boxes, and bathroom
exhaust fans mounted above the ceiling)
and is usually accomplished by adequately
sealing the air barrier at these penetrations.
Therefore, an adequate air barrier should be
designed and detailed to limit the amount of
airflow from the building interior. This
should include sealing all penetrations
through the air barrier.
The air barrier may also function as a
vapor retarder for the attic or rafter space.
The designer should evaluate the need for a
vapor retarder within the roof and ceiling
assembly and should evaluate air barrier
materials relative to their performance as
vapor retarders.
Weathertightness of Upper Roof Deck
Adequate provisions should be implemented
to minimize leakage in areas where
meltwater could pond on the upper roof
deck upslope of ice dams and packed snow
accumulations. These provisions, usually in
the form of a full and continuous coverage
of waterproofing underlayment, should
exist on the upper roof level and be continuous
with adjacent intersecting roofs, sidewalls,
chimney walls, and headwalls. In the
authors’ experience, self-adhering sheet
waterproofing underlayments perform better
than loose-laid underlayments in reducing
the risk of leakage. Because of the airflow
within the cold roof, a full coverage of
waterproofing underlayment on the upper
roof deck is not a concern from a watervapor-
permeance and condensation perspective.
Waterproofing underlayments
should typically extend up headwalls, sidewalls,
and chimney walls to a height of at
least 24 in., or higher where greater accumulations
of snow and ice are anticipated.
The authors have found it beneficial to
bed the waterproofing underlayment in a
cove bead of 100% solids, internally cured
liquid membrane (before the liquid membrane
has set up) where the roof deck meets
the wall sheathing. Mastics or sealants that
require exposure to air to cure should not
be used. The use of a liquid membrane
under the waterproofing underlayment
reduces the risk of bridging of the underlayment
at these transition areas and reduces
the risk of leakage. The waterproofing
underlayment, weather-resistive barriers,
and sheet metal flashings should be properly
and sufficiently lapped to minimize leakage
at roof-to-wall transitions.
The authors have also found it beneficial
to require two layers of waterproofing underlayment
along valleys and eave edges. In
order to avoid bridging along the valley centerline
(which can cause the waterproofing
underlayment to shrink and split), the
sheathing along the valley centerline should
be blocked from below to prevent differential
movement of the sheathing along this line,
or sheet metal can be utilized under the
waterproofing underlayment in the valleys to
provide a solid substrate for the waterproofing
underlayment across the valley centerline.
The waterproofing underlayment may
also be bedded in liquid membrane (before it
has set up) along the valley centerline.
At cricket locations, we recommend that
small crickets generally be covered with
sheet-metal flashings and waterproofing
underlayment rather than roofing materials
that require multiple exposed fastener penetrations.
At large crickets and at roof areas
that are not steep enough for the roofing
materials, the authors recommend that
consideration be given to the use of exposed
roofing membranes appropriate for this
exposure and abuse. Details for the interface
of the roofing membranes with the roofing
materials should be included in the
design. Other details for weathertightness
of the upper roof deck appropriate for the
exposure to snow and ice accumulations,
potential ice damming, and pooled meltwater,
should be developed and incorporated
into the design.
Weathertightness of Lower Roof Deck
The surface of the lower roof deck
(which is the bottom surface of the cold-roof
ventilation cavity) should be weathertight to
reduce the risk of leakage into the building
caused by wind-blown snow and rain that
will enter into the cold-roof ventilation
space along the exhaust vents. In many
cases, this is accomplished with a vaporpermeable
roofing underlayment, along
with waterproofing underlayment at locations
vulnerable to snow and ice accumulation
and leakage (such as at sidewalls,
chimney walls, valleys, and eaves). The
vapor permeance of the roofing underlayment
may be a concern from the perspective
of the risk for the potential formation of
condensation on the underside of the lower
roof deck, and typically the underlayment
should be highly vapor-permeable. It
should be recognized that the temperature
of the lower roof deck in a double roof coldroof
system is generally colder than the
temperature of a roof deck without a cold
roof on which snow can accumulate. This is
because the cold-roof ventilation space
negates the insulative value provided by the
snow on the roof. This issue should be evaluated
by the designer and may coincide
with the design of the ventilation of the attic
and rafter spaces discussed elsewhere in
this report.
Additionally, provisions should be made
to manage such snow and rain inside the
cold-roof ventilation space so that when it
drains out, it does not leak behind the
weathering surfaces. In the authors’ experience,
flashing the cold-roof ventilation
space so that moisture in the cold roof is
discharged outboard of the exterior wall
cladding is critical, particularly if the upper
roof deck is not watertight (such as with
concrete tiles on battens) or if snow or rain
may enter the inlet or exhaust vents.
Failure to direct water from the cold-roof
ventilation space could allow water to drain
into the exterior wall framing, creating leaks
and significant damage to the building.
Other Factors Affecting Snow and Ice
Management on Roofs
While cold roofs are intended to manage
snow and ice on sloping roofs and to reduce
the risk for the formation of ice dams, there
are a number of other factors that affect the
performance of roof systems in alpine environments.
These include, but are not limited
to, the following:
1. Roofing Materials
Some roofing materials, such as asphalt
shingles, wood shakes, and concrete or clay
tile, have a relatively rough surface texture
and tend to hold snow and ice in place on
the roof surface. Other roofing materials,
such as metal roofs and some synthetic
shingles, are relatively smooth and are
more prone to allowing the instant release
of snow and ice accumulations. The risk of
snow and ice avalanches off of sloped roofs
is dependent on many factors, including
roof slope, solar orientation, roof geometry/
complexity, the presence of snow retention
devices on the roof, the “slipperiness” of
the roofing materials, etc.
The ability of roofing materials to
restrain the sliding of snow and ice on the
roof should be evaluated by the designer. In
the authors’ experience, cold roofs with relatively
rough roofing surfaces tend to
restrain snow accumulations better than
those with smoother roofing surfaces, but
we have also observed copper roofing at a
9:12 slope with several feet of snow on it
that did not slide for months. In general,
though, the authors have made use of metal
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roofing along eave edges downslope of snow
fences to allow these portions of the roofs to
shed their snow accumulations before they
became too large.
2. Snow Drop Zones
The areas below the eaves of sloping
roofs should be evaluated for the potential
hazard of snow and ice falling from the
roofs. In the design of the overall roof form,
the designer should consider pedestrian and
vehicular circulation around the building
and design the roof forms to provide protection
for these areas and avoid placing them
under the low end of roofs whenever possible.
Provisions should be implemented so
that areas of pedestrian and vehicular traffic
are protected from falling snow and ice, as
well as areas below roofs that could form icicles,
and where roof run-off occurs (as meltwater
can refreeze on the ground). In the
authors’ experience, areas easily accessible
to the public, such as sidewalks and roads,
should be provided a high level of protection
(which is sometimes also required by local
jurisdictions). Other semiprivate areas, such
as courtyards, balconies, and private driveways,
often require a high level of protection,
depending on the performance expectations
of the owner for these spaces.
In areas that are not readily accessible
to pedestrians and vehicles, snow may be
permitted to fall from the roof in some circumstances.
These areas should be designated
as snow drop zones, and signage may
need to be employed to prevent or inhibit
access into these areas. The potential hazards
and effects of falling snow and ice from
the roof surfaces above the snow drop zones
should be evaluated. For instance, large volumes
of snow and ice cascading from a roof
surface a distance above the ground can be
problematic, and roof run-off and meltwater
can cause splash problems on exterior
building walls and refreezing problems on
the ground. The effects of falling snow and
ice on ground-level landscaping can also be
problematic.
3. Snow Retention Systems
Snow retention systems usually take
the form of snow fences and snow clips.
Snow fences usually consist of one or more
rows of bars or materials spaced at various
intervals up the slope of the roof. Depending
on their configuration, snow fences can be
connected to the structural roof framing, to
the upper roof deck, or to the roofing materials
(as in the case of clamp-on style snow
fences on a standing-seam metal roof).
Snow clips consist of individual brackets
installed in various configurations, usually
cover the entire roof surface, and are normally
attached to the upper roof deck or to
the roofing materials (as in the case of
clamp-on style brackets on a standingseam
metal roof or adhesively attached to
the roofing material).
In the authors’ experience, in most
alpine environments, snow fences attached
to the structural roof framing provide the
greatest protection from snow and ice slides
from the roof surface. These often take the
form of custom-designed snow fences with
vertical steel supports at 4- to 6-ft. intervals
attached to the structural roof framing, and
a number of steel pipes spanning between
supports. The height of the snow fence is
typically about 12-18 in. but is dependent
on the slope of the roof and the length of the
roof area above the snow fence. In some
cases, two or more rows of snow fences may
be necessary. In the authors’ experience,
multiple rows of snow fences are typically
spaced 12-16 ft. apart.
In many instances, snow fences are
designed to retain snow and ice on the roof
for the duration of the winter. However, for
locations with extreme snowfalls, it may be
necessary to periodically remove snow and
ice from the roof surface to prevent overloading
the roof structure. Therefore, the
structural design of the roof system should
take into account the design and layout of
the snow fences in this regard and determine
at what depth of snow removal from
the roof surface may be necessary.
Additionally, in the event that snow removal
is determined to be necessary at certain
times, snow removal plans should be developed
and implemented by the building
owner or user, and provisions for safely
accessing the roof to perform this activity
and the protection of areas below the roof
should be incorporated into the design.
The spacing of the snow fence bars
should be evaluated to reduce the risk of
snow and ice extruding between the bars
and between the lowest bar and the roofing
surface. Special attention to watertight
detailing of the penetration of the vertical
support and to protecting the vertical support
from condensation due to thermal
bridging should be provided in the design
drawings. The authors have found that
snow fences that extend continuously
across valleys perform better in retaining
snow (since snow often accumulates in and
slides down valleys). The snow fence structure
is made stronger by forming a corner at
the valley, but this can increase the difficulty
of the installation of the snow fence.
In general, snow fences should be positioned
as close to the eave edge as possible;
however, structural considerations should
be evaluated. Where snow fences must be
held back from the eave edge of the roof,
other methods of snow retention (such as
snow clips) can be utilized to retain the
snow on these portions of the roof. If snow
is permitted to slide below the snow fences
onto areas below, the designer should evaluate
the potential hazards and effects this
may cause. The design may include the use
of a more slippery roofing material to promote
sliding of snow at the roof edge.
For snow fences that are to be attached
to the upper roof deck surface, special
attention should be given to the attachment
mechanism and loading on the deck and
structure. This may limit the height of the
snow fence and may require additional rows
of snow fences to provide proper restraint
for snow and ice accumulations. The potential
hazards and effects of snow slides above
the snow fence should be evaluated. Special
watertight detailing of the snow fence base
plate should be included in the design,
including potential deflection of the base
plate under full loading.
For snow fences attached to the roofing
material, the designer should evaluate how
the loading of the snow fence affects the
attachment of the roofing material to the
structure and if special detailing of the
attachment is necessary. In the authors’
experience, the height of these types of
snow fences is usually relatively short
(approximately 3 to 4 in.). They often have
limited load-carrying capacity and may
require additional rows of snow fences at
more frequent spacing. The potential hazards
and effects of snow slides above the
snow fence should be evaluated. Should the
snow fences become overloaded, the potential
risks of damage to the snow fence and
roofing materials should also be evaluated.
In the authors’ opinion, snow clips can
be useful in keeping shallow depths of snow
on the roof surface, but they are not sufficiently
effective in protecting against the
avalanching of snow, particularly with
greater snow depths and steeper roof
slopes. Snow clips have the advantage, in
some instances, of limiting snow and ice
migration down the slope of the roof, thereby
limiting damage to the roof surface.
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4. Gutters, Downspouts, and Heat
Tracing
Meltwater should be handled as any
roof drainage from rain. Heavy flow of water
should be expected at valleys and other roof
areas where snow is allowed to accumulate.
Unless the meltwater is properly controlled
and is directed away at ground level, areas
of ponding are likely. Ponded water, in turn,
may freeze and become a hazard for pedestrians
and vehicular traffic. Gutters should
be protected from damage from avalanches
from snow and ice sliding off of the roof. In
the authors’ experience, snow fences should
be installed on roofs adjacent to gutters to
reduce such damage. Any configuration of
gutter is usually acceptable; however, the
eave edge flashing should extend into and
lap over the back of the gutter, with an additional
layer of waterproofing underlayment
protection extending behind the gutter (to
protect the fascia).
Heat tracing should be installed in all
gutters and downspouts to the point of discharge
out of the downspout to keep them
free of ice and allow them to drain. If possible,
two rows of heat tracing should be
installed in each gutter, one located just
under the drip edge of the roof, and one
located in the bottom of the gutter. The
authors recommend that the downspouts
be located outside the building walls, since
downspouts that are routed inside the
building can create leakage problems. If
downspouts are to be routed inside the
building, special consideration should be
given to their design to reduce the risk of
leakage, freezing, and the formation of condensation.
Additionally, consideration may
be given to using three-sided or “openfaced”
downspouts to reduce the risk of ice
blockage and damage to the downspout.
While downspouts from upper portions
of the roof may discharge at the top of the
slope of lower roofs, the roof run-off may
refreeze and increase the risk of ice
damming and icicle development. If possible,
downspouts should be routed so that
they discharge into a place where the meltwater
will be kept in liquid form until it is
removed from the site.
In the authors’ experience, electric heat
tracing draped along the surface of the roof
is not always helpful in controlling snow
and ice accumulation; and in some instances,
it can create additional icing problems
on portions of the roof below the heat
tracing. One primary benefit of properly
designed and installed electric heat tracing
is to provide a melted pathway for pooled
meltwater behind ice dams to drain, thus
reducing the risk of leakage. Additionally,
electric heat tracing is often costly to install,
maintain, and operate. Therefore, the
authors do not generally recommend the
use of heat tracing except in certain specific
situations, such as when placed in gutters
and downspouts to keep them free from
ice blockage.
There are a variety of electric roof snowmelting
systems available on the market in
which the heat tracing elements are protected
by metal covers or the roofing materials.
These systems are proprietary and
have various configurations and associated
advantages and disadvantages. The use of
such products should, in the authors’ opinion,
be carefully considered for each application
and location to determine their suitability
and performance history in similar
applications and environments. In general,
electric roof-snow-melting systems should
be used at locations equipped with a heattraced
gutter to avoid meltwater creating
icing problems on the ground.
Over the last several years, the authors
have been involved with several large roofs
that have metal roofing along their eave
edges. An electric heat cable was installed
under the metal drip edge along the roof
eaves, and this detail has essentially eliminated
the formation of icicles, which on tall
buildings in alpine climates can present a
significant public safety concern.
The authors have found that the installation
of exterior electrical outlets (or electrical
junction boxes for future hookup) at the
roof level during the original construction
can be beneficial to allow for electric heat
tracing installation based on in-place performance
of the roofs. The electrical outlets or
junction boxes should be positioned at regular
intervals near dormers, which may
require additional heat tracing, and near
chimneys and other large valley areas. The
outlets should be located on the wall near
the roof but should not penetrate through
the roof itself. Preferably, the outlets should
be positioned in locations protected from
sliding snow and ice. The designer should
coordinate this with the electrical engineer
to make sure power is provided where it may
be needed in the future.
CASE STUDIES
The case studies below are based on the
authors’ experience with a variety of coldroof
systems in alpine environments in
Colorado. The case studies are intended to
exemplify a variety of cold-roof assemblies
that have performed poorly and required
remediation, as well as one cold-roof assembly
that has performed relatively successfully
but has aged. Also discussed are some
of the unique challenges associated with
cold roofs on differing building types.
The case studies are presented in
ascending order of the age of the buildings.
As a comparative measure, the following
primary factors for the cold-roof systems
described in the case studies below are
shown in Figure 2 and Table 1.
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Figure 2
Case Study Building 1 –
Beaver Creek, CO
A multiunit condominium property was
constructed in the early 1980s and experienced
significant ice damming, icicle development,
and roof leakage during the first
few winters of operation, prompting an
extensive roof evaluation and remediation
project. The roof form was moderately complex,
with a central ridge with numerous
valleys and hips, gabled and shed dormers,
and chimneys. The roof form is shown in
Photo 1. Photo 2 shows the new cold-roof
framing being installed in the mid-1980s.
The original cold-roof design consisted
of 2x4s laid flat, creating a 1½-in.-high
cold-roof ventilation space. The cold-roof
ventilation space was not continuous at valleys
and hips, and the headwalls located
below the front face of the dormer walls did
not provide any exhaust ventilation. Inlet
ventilation along the eaves was relatively
small and covered with small screening,
further reducing the NFVA. The outlet ventilation
was located along the ridges only
and also had small screening, thereby limiting
its NFVA. Typical slopes were about
6:12, with shallow slopes on the shed dormers.
The roofing materials consisted of concrete
tiles.
The repairs to the cold-roof system were
made in about 1985 and included complete
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Table 1
Building Name Building 1, Building 2, Building 3, Building 4, Building 5,
and Location Beaver Creek, CO Beaver Creek, CO Beaver Creek, CO Steamboat Spring, CO Vail, CO
Height of cold-roof Original: 1½ in. 3½ in. (plus 3½ in. 2½ in. (plus 3½ in.
ventilation space New: 5 in. 1½-in. battens for ventilation from upper
cross-ventilation) corrugated roof deck)
Length of cold roof Approx. 40 ft. Approx. 40 ft. Approx. 25 ft. Approx. 50 ft. Approx. 35 ft.
Roof slope Approx. 6:12 Approx. 6:12 Approx. 9:12 6:12 5:12
Size of inlet Original: Small 4 in. wide, with Original: approx. Entire soffit consists About 5 in.
ventilation and covered with tight screening 4 in. with of 6-in.-wide wide with
tight screening tight screening perforated ¼-in. mesh
New: framed with New: approx. 4 in. steel panels
2x8s and covered with ¼-in. mesh
with ¼-in. mesh
Size of exhaust Original: covered Approx. 5 in. Original: approx. Approximately 4 in. About 5 in.
ventilation with tight screening high, with 4 in. with high with wide with
New: covered with tight screening tight screening tight screening ¼-in. mesh
¼-in. mesh and new New: approx. 4 in.
outlet vents at with ¼-in. mesh
hips and headwalls
Photo 1 – Roof form of Building 1, from Google Maps
satellite image, 2012.
Photo 2 – Cold-roof framing being installed.
From Google Maps satellite image, 2012.
reconstruction of the cold-roof assembly.
The cold-roof ventilation space was greatly
increased to include 2x4s laid on edge
above the 2x4s laid flat, thus creating a
cold-roof ventilation space about 5 in. high.
The cold-roof ventilation was improved
along valleys, allowing air to flow into
spaces previously blocked off due to the
framing layout. The inlet ventilation was
improved, framing the inlet vents with 2x8s
and utilizing steel mesh with ¼-in. openings.
The screening of the outlet vents along
the ridge was also changed to ¼-in. mesh.
Exhaust vents were also added along the
hips and at headwalls.
Additional insulation was also added in
the attic spaces to lower the temperature of
the lower roof deck. This resulted in freezing
some fire sprinkler pipes within the attic
spaces, which was later rectified by modifications
to the fire sprinkler piping. To
address the water leakage problems, full
coverage of waterproofing underlayment
was used on both the upper and lower roof
decks.
Case Study Building 2 –
Beaver Creek, CO
Building 2 in Beaver Creek, CO, was
constructed in 1982 as a condominium
building above a large conference center.
The roof forms are relatively simple with
slopes of about 6:12 to 8:12 and gabled
dormers along the roof perimeters, as
shown in Photo 3. The roof assembly, pictured
in Photo 4, consists of the following
(from top to bottom):
• Clay roofing
tiles
• R o o f i n g
felt
• 2×4 horizontal battens spaced at 11
inches on center (which provides
some cross-ventilation for the coldroof
space)
• 4×4 wood sleepers extending up the
slope (creating a 4-in. cold-roof ventilation
space)
• 2-in. extruded polystyrene insulation
(R-10) supported by 2×6 purlins
running horizontally (through which
the 4x4s are fastened)
• Waterproofing underlayment
• ½-in. gypsum board
• Steel roof deck
• Steel roof joists and beams with R-
19 paper-faced fiberglass batt insulation
(joints not taped, and no ventilation
above insulation)
Despite the relatively small amount of
insulation in the roof assembly, the lack of
ventilation above the batt insulation, the
lack of a continuous air barrier, and thermal
bridging created by the steel framing,
the roof assembly has generally performed
well during its 27 years, from a snow and
ice management perspective as well as from
a condensation perspective. With the exception
of a few isolated problems, the roof has
not leaked.
In some locations, however, broken roof
tiles, deteriorated roofing felt, and debris
have filled the cold-roof ventilation space at
the eave inlets and blocked the flow of air
into the cold roof, resulting in some localized
leakage, ice damming, and icicle development.
Due to the age and condition of the
roofing tiles and roofing felt, it is anticipated
that this problem will become more pronounced
over the next five to ten years.
Combined with an increased awareness of
potential safety problems presented by roofing
tiles or pieces of tiles falling from sloped
roofs, a phased renovation of the roof
assembly is being planned.
Special consideration is being given to
the fire rating of the assembly for the
phased renovation, as the wood framing
used originally as part of the cold-roof system
is not fire-rated and does not satisfy
fire-rating requirements of the local jurisdiction.
Consideration is also being given to
making the cold roof a true double roof
assembly, with a solid, upper-roof deck
above the cold-roof space to keep tiles and
debris from blocking the cold-roof space.
The design is currently under way for the
renovation work, and construction had not
yet commenced at the time of preparation of
this paper.
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Photo 3 – Building 2 roof plan. From Google
Maps satellite image, 2012.
Photo 4 – Photo of cold-roof
framing at an addition.
Case Study Building 3 –
Beaver Creek, CO
Building 3 is a multistory condominium building built in about
1987. The authors were originally retained to evaluate and design
repairs to address problems with
the roof system, which included
problematic snow and ice accumulations
on the roof, avalanches,
seepage out of the soffit vents, and
debris accumulations on the soffit
vent screen. Further, extensive
damage to the exterior wall framing
had occurred as the result of
water infiltration behind the exterior
stucco cladding, mostly due to
poor roofing drainage details from
the lower roof deck level, as shown
in Photo 5.
The roof forms were relatively
complex, since closely spaced
gabled dormers along the eave
edges of the roof limited inlet ventilation
and concentrated snow and ice accumulations on the roof, as
shown in Photo 6. The original roofing assembly consisted of the following
(from top to bottom):
• Concrete roofing tiles
• 2×4 wood horizontal battens (installed flat)
• 2×4 battens turned on edge extending up the roof slope and
spaced 16 inches on center connected to the roof sheathing
• Light-gauge L-shaped steel clips
• Waterproofing underlayment
• ½-in. plywood roof sheathing
• 2×12 wood roof joists spaced at approximately 16 inches on
center, with unfaced fiberglass batt insulation packed in
between the roof joists (no ventilation above insulation) or
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Photo 5 – Ice below roofing and in cold roof.
Photo 6 – Building 3 roof plan. From
Google Maps satellite image, 2012.
Photo 7 – Original eave
condition.
Photo 8 – Repaired
eave condition.
steel roof joists and beams with R-
19 paper-faced fiberglass batt insulation
(joints not taped, and no ventilation
above insulation)
Repair work included removal and
replacement of the cold-roof ventilation system,
with improvements to the layout of the
cold-roof framing at eave edges, valleys, and
headwalls. The new cold-roof framing
remained 2x4s turned on edge, but a new
plywood upper roof deck was installed with
a full coverage of waterproofing underlayment,
new battens, and new and reused
concrete roofing tiles. New flashing details
were implemented at the eave and sidewall
locations to mitigate problems with leakage
into the exterior walls (see Photos 7 and 8).
Structural repairs to the exterior wall framing
and sheathing were also made. No
repairs were made to address the lack of
attic ventilation, as investigation of the attic
spaces did not reveal problems with condensation.
The original snow fences installed on
the building were not adequately designed
for the snow and ice loading, resulting in
damage to the lower roof deck where the
fence attachment failed, as shown in Photo
9. New snow fences attached to the roof
framing were designed and installed.
Since repairs were made in about 2006,
the wintertime performance of the roofs has
been successful, as illustrated in Photo 10.
Case Study Building 4 –
Steamboat Springs, CO
Building 4 is a hotel built in 1998 with
a 6:12 sloping roof above the main portion
of the building, located five to seven stories
above grade. A lower sloping roof, located
above the first-floor level at grade, was positioned
around portions of the exterior of the
building. Photo 11 shows the overall roof
plan of the upper roof. As shown in Photo
12, the roof assembly consists of the following
(from top to bottom):
• Dark-green-colored, prefinished,
interlocking steel shingles secured
to the steel decking below with steel
clips that are screwed into the decking
• Galvanized corrugated steel decking
with corrugations approximately 1½
in. deep extending up the roof slope,
screwed into the underlying steel
hat channels (steel decking was
used in lieu of fire-retardant plywood
based on a fire-rating requirement
set forth by the local jurisdiction)
• 1½-in.-tall light-gauge steel hat
channels spaced at 2 ft. on center
(extending horizontally)
• 4-in.-tall light gauge steel C-shaped
channels spaced at 2 ft. on center,
extending up the roof slope
• 1½-in.-thick, loose-laid glass matfaced
polyisocyanurate insulation
between the channels (the cold-roof
ventilation space consists of the 2½-
in. air space above the insulation
between the channels)
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Photo 10 – Wintertime
performance.
Photo 9 – Failed snow fence
attachment.
• Waterproofing underlayment
• Paper-faced gypsum sheathing
• Steel roof deck spanning approximately
4 ft. between structural steel
roof beams
• 12-in.-thick (R-38) foil-faced fiberglass
batt insulation between beams
and below steel roof deck (no ventilation
above insulation)
One of the biggest problems with the
high-sloping roofs was the snow and ice
avalanching off of the roofs and creating
public safety concerns and damaging the
building features below. The building management
implemented a program to shovel
snow off of the roof when the snow depth
exceeded 6 in., resulting in shoveling the
roof after nearly every snowfall at significant
annual expense. Ice dams and icicle formation
on and near some of the eaves were
also problematic but partially resolved by
the addition of electrical heat cables and by
the building maintenance staff accessing
the upper balconies and using a long pole to
knock the icicles off periodically. Roof leakage
was not a major
problem and was handled
on a case-bycase
basis.
The primary cause
of the snow and ice
management problems
is believed not to
be the result of failure
of the as-constructed
cold-roof assembly
but the nature of the
metal roofing system,
coupled with an inadequate number of
snow fences and snow fences of inadequate
height. Repair recommendations included
adding new, taller snow fences, increasing
the height of the existing snow fences, and
extending the snow fences across the valleys.
Trial repairs to the snow fences on a
portion of the roof were implemented in late
2011. Some small portions of the roof were
also modified, particularly where snow and
ice accumulations on the roof surface were
problematic. These repairs included the
installation of a new solid substrate under
the metal shingles with a complete covering
of waterproofing underlayment (extending
up sidewalls and across valleys). While the
snowfall in 2011-2012 was less than normal,
indications are that the roof remediation
efforts met the performance expectations,
and a phased repair approach is
being considered.
Case Study Building 5 – Vail, CO
Building 5 is a recently constructed
public transit facility. The roof form is relatively
modern and simple, with a single
ridge and a 5:12 slope. Photo 13 shows the
overall form of the roof and building. Due to
a relatively light snow season in 2011-2012,
the building has yet to experience a “normal”
winter in its final constructed state.
The roof assembly generally consists of the
following (from top to bottom):
• Metal roofing shingles
• High-temperature-resistant waterproofing
underlayment
• 5/8-inch plywood roof sheathing
• 3½-in.-tall cold-roof ventilation
space created by 2x4s turned on
edge
• High-permeability roofing underlayment
• 5/8-in. plywood sheathing
• 10-in. polyisocyanurate roofing
insulation (R-60) installed between
2×4 wood framing members at 48 in.
on center installed on edge, running
parallel with the roof slope
• Roofing underlayment (to serve as
an air barrier)
• 5/8-in. plywood sheathing
• Wood-plank roof decking on wood
structural framing (with no insulation
on the interior)
Some of the design features implemented
for this cold roof consist of a superinsulated,
unventilated roof assembly. A substantial
raised gutter (shown in Photo 14)
with electric heat tracing, rather than a
built-in gutter or eave-edge-mounted gutter,
was utilized to avoid potential problems
caused by the weight of snow cornices and
icicle formation common with traditional
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Photo 12 – Cross-section through roofing.
Photo 11 – Building 4 roof plan. From Google Maps
satellite image, 2012.
gutters and to minimize
the potential for public
safety problems caused by
ice and icicle formations
along the eave edges.
Three-pipe steel snow
fences were utilized with
the upper pipe being at
least 18 in. above the surface
of the metal roofing,
the lowest pipe having a
clearance of no more than
4 in. above the surface of
the metal roofing, and a
robust steel mesh fastened
to the lower two
pipes and extending down
to within about ½ in. of
the surface of the metal
roofing to restrain thin
layers of ice and snow
from sliding under the
snow fence.
One of the unique roof
features for this modernlooking
roof resulted in the rake edges not
being perpendicular to the eaves and forming
a “prow.” As a result, snow near the
rake edges could slide down the roofs and
fall from these rake edges unless some form
of restraint was installed along these rake
edges. In this application, a 10-in.-tall
structural raised curb was implemented to
manage snow slides and control water runoff
by discharging into the raised internal
gutter (see Photo 14).
CONCLUDING THOUGHTS
For relatively simple roof forms, properly
designed and constructed cold-roof systems
have been demonstrated to perform
well in managing snow and ice on roofs in
alpine environments. This paper describes
some of the design practices and illustrates,
through case studies, cold roofs that have
performed well. Designers should carefully
weigh the advantages of a cold-roof system
against the desire for architectural expression.
For complex roof forms, cold roofs are
probably not a preferred option, and alternative
roofing assemblies should be
explored.
In the design of a cold-roof system, the
ability for adequate cold-roof ventilation
over the entire roof surface, and the constructability
of the cold-roof system should
be considered by the designer, and details
for the cold-roof ventilation system and
weathertightness details should be incorporated
into the design documents.
Issues associated with the use of fireresistant
materials to construct the coldroof
assembly, as well as wildfire-related
risks, should be evaluated by the designer
in the design process. It is helpful to get
input from the building official in the local
jurisdiction during the initial stages of
design; as such, input may dictate limitations
on cold-roof assemblies and materials.
In the authors’ opinion, these issues will
likely become more significant as trends
toward more fire-resistant buildings in
alpine environments continue to evolve.
The authors believe that more published
case studies of successful and unsuccessful
as-built cold roofs would be useful in developing
basic, proven guidelines for cold-roof
S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • OC T O B E R 2 0 1 2 MCGOWA N A N D F L I C K • 6 5
Photo 13 – Overall view of Building 5.
Photo 14 – Raised gutter and curb at “prow” of Building 5.
designs. Such case studies should indicate
the cold-roof ventilation-space height and
length, materials of construction, the complexity
of the roof forms, and unique features
of the roof (if any), similar to the information
provided in this paper.
BIBLIOGRAPHY
1. M.C. Baker, “Ice on Roofs,”
Canadian Building Digest (CBD-89),
May 1967.
2. Maxwell C. Baker, Roofs: Design,
Application and Maintenance, National
Research Council of Canada,
1980.
3. A.M. Baumgartner, R.L. Sack, and
J.J. Scheldorf, “Approximate Analysis
of a Double Roof,” Cold Regions
Science and Technology Symposium
16, 1989.
4. James Buska and Wayne Tobiasson,
“Minimizing the Adverse Effects of
Snow and Ice on Roofs,” International
Conference on Building Envelope
Systems and Technologies,
Ottawa, Canada, 2001.
5. Concrete and Clay Tile Roof Design
Criteria Manual for Cold and Snow
Regions, National Tile Roofing
Manufacturers Association (NTRMA)
and Western States Roofing Contractors
Association (WSRCA), produced
by Leland E. Gillian and Terry
Anderson, 1998.
6. Paul Fisette, Preventing Ice Dams,
Building Materials and Wood Technology,
University of Massachusetts
– Amherst, 2006.
7. Timothy Larson, Lewis Hendricks,
and Patrick Huelman, Ice Dams,
University of Minnesota Extension,
2002.
8. Joseph Lstiburek, “Damn Ice Dam!”
ASHRAE Journal, February 2011.
9. Joseph Lstiburek, “Understanding
Attic Ventilation,” ASHRAE Journal,
April 2006.
10. Ian Mackinlay, The Neglected Hazards
of Snow and Cold, American
Institute of Architects, 1983.
11. Ian Mackinlay and Richard Flood,
“Snow Distribution on Complex
Roofs,” Proceedings of the Fifth International
Conference on Snow Engineering,
Davos, Switzerland, July
2004.
12. Ian Mackinlay, Richard Flood, and
Anke Heidrich, “Roof Design in
Regions of Snow and Cold,” Proceedings
of the Fourth International
Conference on Snow Engineering,
Trondheim, Norway, June 2000.
13. Michael Noda, “Avoiding the Slippery
Slopes – New Ways of Designing
Ski Resorts for the 21st Century,”
Colorado Construction, February
1999, pp. 10-11.
14. Jonathan Paine and Lee Bruch,
“Avalanches of Snow From Roofs of
Buildings,” International Snow Science
Workshop, Lake Tahoe, CA,
October 1986.
15. Proceedings From the First International
Conference on Snow Engineering,
Santa Barbara, CA, July
1988; National Science Foundation,
Washington, DC; printed February
1989.
a. A.M. Baumgartner, R.L. Sack,
and J.J. Scheldorf, “Thermal
Characteristics of a Double
Roof.”
b. Ian Mackinlay, “Architectural
Design in Regions of Cold and
Snow.”
c. Wayne Tobiasson, “Roof Design
in Cold Regions.”
d. Jonathan C. Paine, “Building
Design for Heavy Snow Areas.”
16.James Buska, Wayne Tobiasson,
Alan Greatorex, and William Fyall,
“Electric Heating Systems for
Combatting Icing Problems on Metal
Roofs,” Proceedings From the Fourth
International Symposium on Roofing
Technology, September 1997; U.S.
National Institute of Standards and
Technology et al.
17. Jonathan C. Paine, “Design Review
for Snow Country,” Proceedings
From the Second International
Conference on Snow Engineering,
Santa Barbara, CA, June 1992;
National Science Foundation,
Washington, D.C. et al.; printed
December 1992; Wayne Tobiasson
and Edmund Wright, editors.
18. Roofing and Waterproofing Manual –
Fifth Edition, National Roofing Contractors
Association (NRCA), 2001,
pp. 832-840.
19. William B. Rose, “Roof Ventilation
Update,” Journal of Light Construction,
October 2007.
20. William B. Rose and Anton
TenWolde, “Venting of Attics and Cathedral
Ceilings,” ASHRAE Journal,
October 2002.
21. Richard Seifert, Attics & Roofs for
Northern Residential Construction,
Alaska Cooperative Extension of the
University of Alaska Fairbanks,
December 2003 (HCM-00559).
22. Wayne Tobiasson, James Buska,
and Alan Greatorex, “Snow Guards
for Metal Roofs,” Eighth International
Conference on Cold Regions
Engineering, American Society of
Civil Engineering, August 1996.
23. Wayne Tobiasson, James Buska,
and Alan Greatorex, “Attic Ventilation
Guidelines to Minimize Icing at
Eaves,” Interface, Roofing Consultants
Institute (RCI), January 1998.
24. Wayne Tobiasson and James Buska,
Standing-Seam Metal Roofing Systems
in Cold Regions, National Roofing
Contractors Association, April
1993.
25. Wayne Tobiasson, Thomas Tantillo,
and James Buska, “Ventilating
Cathedral Ceilings to Prevent
Problematic Icings at Their Eaves,”
Proceedings of the North American
Conference on Roofing Technology,
National Roofing Contractors Association
(NRCA), Toronto, Canada,
September 1999.
26. Wayne N. Tobiasson, James S.
Buska, and Alan R. Greatorex,
“Guidelines for Ventilating Attics
and Cathedral Ceilings to Avoid
Icing at Their Eaves,” Buildings VIII,
American Society for Heating,
Refrigeration, and Air-Conditioning
Engineering, Inc. (ASHRAE), Cold
Regions Research and Engineering
Laboratory (CCREL), and United
States Army Corp of Engineers
(USACE), December 2001 (MP-02-
5778).
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