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Cold-Weather Considerations for Glass-Reinforced Asphalt Shingles

May 15, 2002

By Raymond L. Corbin and L. D. Hogan
Glass-reinforced, asphalt shingles are designed for application
in a wide range of weather conditions; however, care
should always be exercised whenever approaching climate
extremes. While there are also hot weather precautions, this
article focuses on matters of cold weather application.
Snow, ice, and water accumulation can exert considerable
force, exceeding the allowable rooftop loads (as calculated
for the design snow load). Additionally, ice dams can
hold a significant amount of water. Through hydrostatic
pressure, they can cause water to enter the structure that
otherwise would not occur during ordinary rainfall conditions.
Aspects of attic ventilation are explored at length.
These and related issues are examined in this article.
Cold Temperature
ARMA (the Asphalt Roofing Manufacturing Association) and
most glass fiber shingle manufacturers caution against the installation
of glass-reinforced shingles in temperatures at or below 40 degrees F.
At these temperatures, the shingles may be stressed, fractured, or
broken as they are handled and installed. The damage inflicted
may not be evident until after a year or two of weathering.
During cold-weather application, the fiber glass shingle’s mat
weight can make a difference. The heavier mat weights generally
improve installation attributes in cold weather. Once the shingle
is installed, however, properly-designed and manufactured shingles
on a lighter glass mat can perform just as well.
Behavior and Influence of Ice on
Shingle Roofs
In colder areas, ice can form on the shingle and lock to the
granular surface. Through repeated cycles of melting and refreezing,
the shingle can split and eventually be pulled apart.
With cyclical expansion and contraction, the tensile strength of
the asphalt shingle may be exceeded by the volume change of
the ice bonded to it. This exceptional behavior of ice is its
“phase transition” during which it shrinks (or contracts) by
approximately 10% upon melting and expands in similar fashion
during re-freezing.1
While a remote occurrence, glacier-type movement of ice on
roofs (especially in alpine areas) can cause abrasive damage to
the shingles. The loss of protective granular surfacing will
expose the asphalt to harmful ultraviolet radiation, eventually
leading to premature failure. Shingles that exhibit abrasion damage
should be replaced.
Interior heat, escaping through an inadequately insulated and
poorly ventilated roof system, can cause snow to melt. When the
melted water refreezes at a colder, lower portion of he roof, an
ice dam is created. It usually starts at the colder eaves, and without
a proper waterproofing underlayment installed, capillary
action will cause the melted water to invade beneath the shingles.
Leakage and the potential for structural damage are the
likely result.
Ice dams can form elsewhere on the roof, and waterproofing
measures may be required on areas other than the eaves. Features
giving off shadows (such as large roof projections, dormers, and
higher roof levels) are prone to foster ice damming. These areas
are blocked from the sun and can re-freeze, causing an ice dam
and resultant leakage. Figure 1 depicts ice damming on a roof
where an ice and water membrane was not provided.
It should be noted that, while ice and water membranes are
crucial, installing the product beyond the recommended boundaries
can be unwise. For structures in cold climates having high
interior humidity, water vapor can condense beneath the “barrier,”
resulting in unexplained leaks.2
24 • Interface January 2002
Wind Influence on
Steep Roof Shapes
Wind has an impact
upon the performance of the
roofing system, particularly
in cold settings. Poorlyattached
or inadequatelysealed
shingles are
vulnerable to blow-off. It
has been shown that fastening
as little as one inch
above the intended line
increases blow-off potential
by 20 percent.3
In cold weather, it is critical
that a bond be formed
among the respective courses
of shingles at the earliest
opportunity. The heat-activated,
sealing adhesive may
be some months in developing
a functional bond in
cold climates. A late fall
installation in a northern tier
state may not be fully bonded
until mid spring of the
following year.4 The installation
is vulnerable to wind
loss in the meantime. Hand
tabbing of shingles (applying adhesive or
flashing cement beneath tabs) may be
appropriate in these settings.
Consideration must be given to the
complex behavior of the wind as it
impinges on roofs. Variables to
consider are:
• Wind velocity (both instantaneous
and average)
• Real wind speed and duration
of gusts
• Pattern of wind acceleration
• Air density
• Roof shape and slope
• Height of ridge and eaves
• Shape of shingle unit and its adhesive type
• Shingle position relative to roof penetrations
• Air temperature
• Building orientation (leeward vs. windward)
Rooftop Snow Removal
Unless the framing system is designed for snow loads, the
rate at which a roof can shed its snow load is important to the
integrity of the structure. A “rain-on-snow” scenario can have
calamitous influence on certain types of framing systems.
Not only is trafficking the snow pack an unsafe practice, substantial
damage to the shingle product is more than probable.
However, to maintain the structural integrity, it may become
necessary to remove snow accumulation from the roof. The work
should not be attempted by hand, as it is
unsafe and can damage the roof covering.
Snow rakes are available to assist in this
effort, which is performed from the
ground. It may also be prudent to engage a
reputable contractor to carefully and safely
remove the snow load.
This snow removal measure should not
be confused with an attempt to remove an
ice dam, which should not be removed
from a roof. Chiseling away the ice accumulation
is virtually certain to damage the
shingle product. A preemptive strategy
would be the use of heated cables along
the eaves to melt the snow and prevent
Moisture Gain and the
Ventilation Aspect
Ventilation of the attic region is
extremely important to both the roof and
the remainder of the building. Many investigations have identified
substandard ventilation as the culprit in poor roof performance.
Minimum ventilation area for steep roofing has been
established at one sq. ft. of net free ventilation area for every 150
sq. ft. of attic space (1:150). Half of this total vent opening
should be provided at the eaves and half at the ridge.
In cold conditions, lack of ventilation can lead to excessive
moisture build up, which can adversely affect performance of the
structure as well as the shingle product. As moisture reaches the
cooler deck surface, it may condense and create the appearance
of a roof leak. Moisture absorption into wood components can
occur long before droplets of free water are formed on the
underside of the deck. Moisture gain can lead to:
January 2002 Interface • 25
Figure 1A and 1B (below)—Ice damming on a roof
where ice and water membrane was not provided.
26 • Interface January 2002
• decayed wood;
• expansion, warping, and buckling of
the sheathing (Figure 2); and
• dripping into the building interior5
At the very least, movement of the
deck from moisture gain will place additional
stress upon the shingle product.
When ventilating a steep roof system,
it is important to have a liberal air space
between the insulation and the deck.
With cathedral ceilings, the narrow airway
produces greater resistance to air
flow, and rather large airway heights are needed. Indeed, air
flow virtually stalls when batt insulation expands in a cathedral
assembly (Figure 3). Consequently, the ventilation aspect should
be adjusted upward. Note also that 1“ rigid board insulation
can restrain the batt insulation while providing its own
thermal benefit.
Attic insulation must be held back from the soffit region in
order to serve as the intake port (Figure 4), and blown-in insulation
is routinely over-sprayed into the eaves. A tightly-installed
air barrier can reduce the tendency of the insulation to fill the
cathedral passageway. It also increases the net insulating ability
of the fiberglass insulation by keeping airflow away from it. A
pre-manufactured baffle can also be used to keep blown-in insulation
from becoming a ventilation impediment.
It should be noted that screens and louvers placed over vents
reduce the effective airflow. Similarly, multiple coats of paint can
render soffit openings virtually useless (Figures 5a and 5b). Other
conditions have impact on venting
effectiveness. Figure 6 depicts a
mold/mildew occurrence in a
humid, southern climate (hot and
dry at the time of the study). Attic
insulation was properly held back
at the edge, and the openings were
present in the vinyl soffit cladding.
However, carpentry was continuous
along the eaves, so the soffit feature
was completely useless. The
mold/mildew colonies were fostered
from a wintertime condensation
scenario, drying to the powder
shown during the hot season.
Wood responds to changes in
temperature and moisture.
However, it expands more from
moisture increases than it does
from thermal change. Even in cold conditions, when thermal
expansion is usually not a concern, it is important to prevent
excessive moisture migration into the system. Mold and mildew
can be the signature of substandard ventilation. Development of
mold and mildew is largely a cold-season phenomenon, but it is
certainly not exclusive to ”cold country.” The colonies will dry to
a powdery substance from the heat of the summer months.
However, the interior of the structure may well experience pervasive
indoor air problems during any season from the toxins
released by the molds. Poor attic ventilation plays a role in all
of this.
Moisture gain can cause the edges of deck sheathing to separate
and rise. The respective sheets need to be spaced by 1/8″,
lest moisture cause swelling and buckling (Figure 7). Cyclical
moisture change in wood decking can also cause nails to back
Figure 2—Moisture will accumulate in a
wood deck long before water droplets are
formed. Note the buckled plywood deck,
mold formation, and soffit region blocked
with glass fiber insulation
Figure 3—Air flow can virtually stall when batt insulation expands
in a cathedral ceiling assembly. Photo courtesy of Wayne Tobiasson.
out, compromising attachment
of the sheathing to the framing
supports. Deck movement can
be slow and difficult to observe,
usually becoming manifested
over a long period.
Finally, excessive moisture
weakens the shingles, reducing
physical properties such as tear
strength. Moisture drive will
prematurely age shingles, weakening
the bond of the shingle
matrix to the fibrous glass mat.
When excess moisture is combined
with heat, the aging effect
is accelerated.
Cold Decks and
Super-insulated Attics
There is substantial misunderstanding about ice
dams. A roof surface having a snow pack over the
entire surface may experience no ice damming whatsoever.
Conversely, a roof with very little snow accumulation
may have pervasive leakage stemming from
ice dams. As shown in Figure 8, the actual ice dam is a
discrete portion of the total snow pack, mostly occurring
at outlying walls of the structure.
A “cold roof deck” ordinarily has air space above
the attic insulation and below the nailing deck. This
arrangement fosters a cooler roof surface by allowing
outside air to pass from the soffit, washing the
underside of the nailing deck. This configuration
enables the shingles to remain cooler in the winter
by removing any heat escaping from the structure
before it reaches the shingles. A cold roof helps prevent
uneven melting of the covering snow and the
subsequent re-freezing. Proper ventilation can reduce
the formation of ice dams by maintaining a cold
roof surface.
Figure 4—Attic insulation must be
held back from the soffit region in
order to serve as the intake port. A
pre-manufactured baffle can be used
to keep blown-in insulation from
becoming a ventilation impediment.
Figures 5a and 5b—Multiple coats of paint can render
soffit openings virtually useless. Airways and vent openings
should be slightly larger than would be implied in the
sizing calculation.
January 2002 Interface • 27
For a cold roof to function properly,
its ventilation system should be sized to
keep a snow-covered roof below freezing
whenever the outside temperature is 22
degrees F (-5.6 degrees C) or colder. At
warmer temperatures, the melted snow
rarely re-freezes at the eaves.
Super-insulated attic floors will allow
attic air to become colder as heat loss from
the interior is reduced. The colder that air
becomes, the less it is able to absorb and
hold moisture, which is then free to condense
on various surfaces in the attic.
Summary Comments
The intent of this article is to address some of the concerns
regarding cold weather parameters for glass-reinforced shingles.
When given the choice, the installer should wait for warmer
weather. Otherwise, exercise as much care as possible during
the application of glass-reinforced shingles. When designing
for colder regions, take liberal precautions regarding
ventilation aspect, moisture control, potential snow loads,
and ice management.
Even the best products will not perform as intended if the
roof system is not adequate for the cold conditions or if the
application is improper. When proper application techniques
are ambiguous or debatable, follow the guidelines as set forth by
the shingle manufacturer, the National Roofing Contractor
Association (NRCA), and the Asphalt Roofing Manufacturers
Association (ARMA). ■
Figure 7—Moisture gain can cause the edges of deck sheathing to separate
and rise. The respective sheets need to be spaced by 1/8“, lest moisture
cause swelling and buckling.
Figure 8—There is considerable misunderstanding about ice dams. The “ice
dam” is a discrete portion of the total snow pack, occurring near the outlying
28 • Interface January 2002
Figure 6—Mold/mildew in a humid
southern climate. While attic insulation
was properly held back at the edge and
openings were present in the soffit
cladding, the eaves carpentry was continuous
along the entire perimeter. The
mold/mildew colonies were fostered during
a wintertime condensation scenario,
drying to the powder during the hot
season as shown.
1) Chapter “Ice,” McMillan’s Encyclopedia of Physics, 1996.
2) R.L. Corbin, “Water Dams: Up North, It’s Ice—Down
South, It’s Pine Needles,” Interface, January 1997, pg.7.
3) R.L. Corbin, “Proper Fastening of Self-Sealing Shingles,”
Contractors Guide, December 1992.
4) R.L. Corbin, “Mechanics for Properly Sealing Fiberglass
Reinforced, Asphalt Roofing Shingles,” Interface, April
2001, pg. 13.
5) MRCA, “The Theory and Practice of Steep Roof
Ventilation,” Oct. 1998, pg. 3.
January 2002 Interface • 29
Lyle Hogan is a senior engineer
with Geoscience Group, Inc., working
out of the firm’s Greensboro,
North Carolina office. He is a registered
engineer, a Registered Roof
Consultant, a licensed home
inspector, and a Fellow of the Roof
Consultants Institute. Mr. Hogan’s
technical articles have been published
in numerous technical journals
and conference proceedings.
He is a recipient of RCI’s Horowitz Award for outstanding
contribution to Interface journal.
Raymond L. Corbin is Director
of the Better Understanding of
Roofing Systems Institute (BURSI),
sponsored by Johns Manville. He
holds several United States roofing
shingle design and application patents.
Corbin is a faculty member of the
Roofing Industry Educational Institute
(RIEI), an Industry member of RCI,
and has served as Chairman of the
Code Committee for the Asphalt
Roofing Manufacturers Association (ARMA).
According to a recent article in ENR magazine, schools
nationwide are enrolling increasing numbers of students
specifically studying for academic degrees in construction
education. Construction education is no longer viewed as
merely vocational. An estimated 170 universities currently
offer bachelor-level construction education programs under
various names, from construction management and building
science to architectural design and construction and
construction technology.
The Associated Schools of Construction lists 69 varying
programs among its 96 members. Of 88 schools responding to
ENR’s survey, some 17,500 students were enrolled in the current
school year, and approximately 3,400 graduated in the
spring of 2001. Of these, only five percent are minorities and
seven percent are women. There are 53 university programs
accredited by the American Council for Construction
Education (ACCE).
Institutions enrolling the largest number of students in
their construction management programs: Colorado State
University, 630; Texas A&M, 520; Purdue University, 485;
Auburn University, 479; Brigham Young University, 465;
Louisiana State University, 402; East Carolina University, 400;
University of Cincinnati, 385; California State at Chico, 340;
Iowa State University, 320. Kansas State University, 320;
Arizona State University, 300; Indiana University/Purdue
University, Indianapolis, 300.
Construction Degree Programs
Coming of Age
Britain has experienced a 34% increase in fatalities on the
worksite. In 2000-01, there were 295 such deaths, with more
than one-third occurring in the construction industry. Of
those, 73 (68%) were caused by falls from heights.
—Roofing Cladding & Insulation (RCI)
Workplace Deaths in Britain