Steep Roofing: Underlayment Upgrades That Sometimes Aren’t

May 15, 2006

Self-adhering, modified bitumen
membranes are often
installed as continuous waterproofing
layers below steeproof
systems to enhance
weather protection. Sometimes
these “bullet-proof” roofs develop an
unexpected problem – condensation. This
article explains why and offers suggestions
on how to enhance weather protection and
reduce the risk of condensation.
This author acknowledges that selfadhering
“ice and water” type underlayment
membranes for steep roofing are useful in
the marketplace. He has personally specified
their installation as part of steep-roofing
systems, including copper, slate, clay
tile, concrete tile, and asphalt shingles.
These products offer excellent waterproofing,
and some possess a “self-sealing” characteristic
around fastener penetrations.
They are particularly useful to protect
against ice dam conditions along eaves and
to help achieve weather protection at penetrations,
complex transitions, and terminations
where rigid materials are inherently
difficult to make weathertight on a longterm
A problem can be created, however,
when these products are installed without
considering how they might foster the
Photos 1 (left) and 2 (above) – Steep
roofs without conventional attic spaces
(e.g., enclosed rafter spaces).
36 • I N T E R FA C E DE C E M B E R 2006
potential for condensation.
Self-adhering, modified bitumen, sheettype
products are waterproof and quite flexible,
but they are virtually impermeable to
water vapor transmission. The manufacturer
of one of the “ice and water” type products
publishes a “maximum” water vapor
transmission value of about 0.05 perms. In
contrast, “15-lb asphalt felt” underlayment
is listed in the ASHRAE Fundamentals
manual as having a water vapor transmission
rate (permeance) of about 1.0 perms
(dry cup method) – or about 20 times more
vapor permeability than the “ice and water”
type product. Materials with perm ratings
below about 1.0 or 0.5 perms are usually
considered vapor retarders.
It is important to keep in mind that recommendations
included in well-known
manuals such as Copper and Common
Sense by Revere Copper Products, Inc.;
Slate Roofs by Vermont Structural Slate
Co., Inc.; and Architectural Sheet Metal
Manual by Sheet Metal and Air Conditioning
Contractors National Association, Inc. discuss
conventional felt underlayments but
are generally silent in regard to “ice and
water” type waterproof membranes, except
along eaves with potential ice dam conditions.
Shake Roof Replacement
During an investigation of “leaks”
reported the first winter after a wood-shakeroof
in the San Francisco Bay Area was reroofed
with concrete tile, this author
observed literally thousands of water
droplets covering the underside of the roof
deck in the attic (see Photo 3). The owner
reported no leaks (condensation-related or
otherwise) from the previous wood shake
roof that had been installed over spaced
sheathing. The roofer reported that he had
installed new plywood throughout and had
even “upgraded” the underlayment from a
conventional #30 felt to a self-adhered,
modified bitumen membrane. Therefore, he
was convinced the leaks were not his problem.
Although attic venting was about 50% of
the code-required minimum, the old wood
shake roof installed over spaced sheathing
had been air- and water-vapor permeable
just enough to avoid noticeable condensation.
Installation of new plywood sheathing
and a waterproof membrane changed this.
The new plywood and waterproof membrane
greatly increased the condensation-producing
conditions by virtually eliminating any
water vapor migration and “supplemental”
attic air exchange that previously occurred
directly through the wood shakes.
It is important to recognize that “ice and
water”-type underlayment membranes are
excellent vapor retarders; as such, if they
are installed on the cold (exterior) side of the
ceiling insulation rather than on the warm
(interior) side, this can sometimes lead to
Other projects where this author has
observed very significant condensation conditions
created or at least contributed to by
the installation of self-adhering “ice and
water”-type waterproofing membranes
include several non-vented cathedral ceiling
assemblies and copper barrel-shaped roofs
(see Photo 5).
Condensation Basics
During the winter, the air inside a heated
and occupied building typically has
water vapor (and vapor pressure) in
amounts well above that present in the air
DE C E M B E R 2006 I N T E R FA C E • 3 7
Photos 3 (left) and 4 (below) – Literally thousands of water droplets form on
the underside of a plywood and spaced sheathing roof deck despite
conventional attic eave vents on a project where a self-adhered membrane
underlayment was installed as part of a roof replacement project.
outside. This difference in
water vapor pressure works to
equalize itself by some of the
higher pressure interior water
vapor diffusing or migrating
outward through the walls
and ceilings. (It’s like air slowly
escaping from a balloon.) As
the water vapor migrates, it
also cools. And, if it cools to
its dewpoint, it will condense
on a surface of one of the
components within the roof or
wall assembly.
Figure 7 shows severe
deterioration caused by water
vapor migrating through the
ceiling of a laundry room and
condensing on “cold,” underside
surfaces of a plywood
roof deck above enclosed and
insulated (but not vented)
rafter spaces in Minnesota.
Throughout the winter, water
vapor in the air in the laundry room (68˚F,
50% RH) migrated upward and outward
through the gypsum board ceilings and into
rafter spaces filled with fiberglass batt insulation
and covered by the plywood roof
deck. Whenever the underside of the plywood
was at a temperature less than the
dewpoint of the water vapor in the air inside
the rafter spaces (for this example, assumed
to be about 49˚F), it condensed. Eventually,
the plywood became wet enough (and warm
enough) to support growth of wood decay
Mold (A Fungus)
In addition to decay, toxins produced by
some molds have been linked to adverse
health conditions. Since, among other
things, mold requires moisture to grow, limiting
condensation-producing conditions
(including high-humidity) can help control
the growth of mold in roof and wall constructions.
Condensation in Steep Roof Assemblies
Unfortunately, the mechanics of condensation
in many situations are more
complicated than illustrated in the above
example. This is true for condensation in
many steep-roof assemblies, with and without
conventional attic spaces. (Note: Attic
venting helps control condensation in at
least two ways: first, by mixing moistureladen
interior air that enters with drier outside
air before it contacts a surface below its
dewpoint; and secondly, by facilitating drying
of moisture that does condense on surfaces
in the attic).
Condensation depends not only on how
temperature changes along the path of
water vapor migration, it also depends on
Photos 5 (left) and 6 (below) –
Saturated plywood sheathing
below self-adhering membrane
underlayment in a non-vented
barrel roof with a standing seam
copper covering.
38 • I N T E R FA C E DE C E M B E R 2006
how the water vapor pressure changes along this same path. In fact, if a vapor
retarding material is installed on the warm (interior) side and the other components
of a non-vented roof assembly are sufficiently vapor permeable toward
the cold (exterior) side, condensation will not occur, even though the “dewpoint”
temperature is reached (i.e., the dewpoint temperature as calculated from the
temperature and relative humidity of the interior air).
This is not so hard to believe when considering that the water vapor arriving
at such a theoretical “dewpoint” location is “filtered” by each layer it pass-
Photos 7 (left) and 8 (below) – Severe
plywood deck deterioration with fungi
growth caused by condensation over a
laundry room in multi-family apartment
DE C E M B E R 2006 I N T E R FA C E • 3 9
es through. Each layer – especially the
vapor retarder layer – serves to hold back
some of the water vapor and then, at the
theoretical dewpoint location, the air and
water vapor mixture isn’t at a 100% saturation
level. (Note: The ability of some materials
to absorb water vapor is another reason
why many steep-roofing systems over woodframe
construction in mild climates avoid
noticeable condensation, even though simple
dewpoint calculations would suggest
Wall Cladding
Condensation considerations for walls
are similar to those for roofs. As with roofs,
if impermeable membranes are installed on
the cold side, excessive amounts of condensation
can sometimes accumulate and fuel
deterioration and fungi growth.
Consultants are often involved with reroofing
over existing enclosed and insulated
rafter spaces that are not vented (e.g, vaulted
or cathedral-type ceiling assemblies) or
over existing attic spaces that are minimally
vented. Local building officials often do not
require upgrading to comply with current
code venting and vapor retarder requirements.
If, in such cases, the new system
includes an impermeable membrane above
the deck, significant condensation conditions
can be inadvertently created where
they did not exist before.
When installation of a continuous
waterproofing membrane beneath a steeproof
system is desired to enhance weather
protection as part of a reroof project, this
author strongly suggests checking and
enhancing existing roof ventilation, even if
not required by local codes. (Note: Also consider
asking the owner to confirm that all
flue, bath, and kitchen exhaust vents are
operational and discharge humid air and
combustion products (containing large
amounts of water vapor) to the outside – not
into attic areas.] Non-Vented Assemblies
If the existing steep-roof assembly
includes enclosed and insulated rafter
spaces, and the reroof project does not or
cannot include ventilation between the roof
deck and the insulation, this author suggests
proceeding with caution or considering
not proceeding at all. Installation of
insulation above the roof deck and installation
of continuous vapor retarders on the
warm (interior) side would be, in this
author’s opinion, a prudent practice in such
cases in most climates.
Self-adhering, modified-bitumen membranes
can enhance weather protection
below steep-roof systems but can also inadvertently
increase the risk of condensation
by serving as vapor retarders on the “cold”
side of roof/ceiling assemblies. Before specifying
“ice and water” or other types of
impermeable membranes that will completely
cover steep roofs, this author suggests
designers use caution and, as a minimum,
comply with code-stipulated venting
and vapor retarder requirements.
For steep roofs that do not have conventional
attics (e.g., enclosed and insulated
rafter spaces) or that have relatively high
interior humidity conditions, this author
suggests designers be conservative and, if
needed, consider retaining suitably qualified
individuals to consult regarding options
(e.g., framed ventilation spaces, above-deck
insulation, vapor retarders, mechanical
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Phone: 800-828-1902 • Fax: 919-859-1328 •
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40 • I N T E R FA C E DE C E M B E R 2006
ventilation systems, and interior humidity
control systems) for controlling the accumulation
of condensation moisture.
Future articles may address other important
considerations such as interior air flow
into insulated and non-vented air spaces
(e.g., through unsealed lighting fixtures),
complications associated with fire-controlrelated
“draft stops” and “wrap backs,” and
the potential benefits promised by the new
generation of watershedding, yet highly moisture-
vapor-permeable underlayments.
EDITOR’S NOTE: This article is a revised
version of an article of the same title
originally published in Western Roofing,
January/February 2001. Reprinted with
42 • I N T E R FA C E DE C E M B E R 2006
Phil Dregger, RRC, FRCI, PE, is president of Technical Roof
Services, Inc., a DNG Group Company, in Concord, California,
specializing in finding solutions to difficult building
moisture problems. Phil received his Masters of Science
degree in civil engineering from the University of Minnesota in
1978. He is a past director of RCI, a former faculty member
of RIEI, and serves as RCI’s representative to the Roofing
Industry Committee on Weather Issues (RICOWI). Mr. Dregger
has designed roof and waterproofing systems to meet the
requirements of a wide variety of clients, including the University of California, SBC
Communications (formerly Pacific Bell), Kaiser Hospitals, Mervyn’s, and Disney. He has
investigated numerous roof and waterproofing problem conditions, including damages
sustained after major hurricanes and earthquakes. Phil is the author of several articles
on roof technology and has lectured to industry groups, sharing lessons learned from
his many investigations.
Phil Dregger, RRC, FRCI, PE
U.S. roofing demand is projected to expand less than one percent per year through 2010 to 278 million square feet, with
value expected to rise to $14 billion. The nonresidential construction market will provide the best opportunity for gains in the
roofing industry, assisted by new office, commercial, institutional, and industrial segment expansion.
Among the various roofing materials, plastic and metal will see the fastest growth in the U.S. through 2010. Thermoplastic
polyolefin (TPO) and spray-applied roofing will continue to make inroads. Metal roofing will continue to increase in popularity
in commercial applications as well as residential markets, where metal panels, tiles, and shingles are being used as alternatives
to roofing tile and asphalt shingles.
In 2005, asphalt shingles accounted for nearly 70% of the total installed square footage and will maintain the lead position
through 2010. Demand for asphalt shingles will be constrained, however, by the weak outlook for new residential roofing.
Products designed to mimic asphalt shingles, roofing tiles, wood shakes and shingles, and slate will post gains, as will environmentally
friendly products such as recycled roofing materials and composite shingles.
(million squares)
% Annual Growth
2000 2005 2010 05/00 10/05
Total Roofing Demand 235.0 268.0 278.0 2.7 0.7
Asphalt Shingles 138.1 160.4 161.5 3.0 0.1
Bituminous Low-slope Roofing 34.3 34.5 36.0 0.1 0.9
Metal 18.5 19.9 23.0 1.5 2.9
Elastomeric 17.9 18.4 20.0 0.6 1.7
Other 26.2 34.8 37.5 5.8 1.5
Interface wants to know what projects consultants have seen or worked on that had the
most individual layers of tear-off.We want to find the thickest roof out there.The roof that
wins the Princess and the Pea contest: layer after layer after layer after layer – and still, it
leaked! Send your experiences to Kristen Ammerman at