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Ventilation and Moisture Control for Residential Roofing

The proper ventilation of attic areas is a critical design and performance consideration. If implemented correctly, proper
ventilation methods can help ensure the maximum service life of roof assembly materials, and can improve energy efficiency of
the building. The minimum amount of ventilation required is defined by the building codes for residential construction. In
addition, ventilation is recommended by shingle manufacturers to help ensure the performance of the roof materials.
Overlooking this consideration may result in the following problems:
 Premature failure of the roofing system
 Buckling of the roofing shingles due to deck movement
 Rotting of wood members
 Moisture accumulation in the deck and/or building insulation
 Ice dam formation in cold weather
In cold climates, internal building moisture is often a cause of roofing system problems. Occupancy generated water vapor may
reach an unconditioned space and condense on cold surfaces. This may cause wood to rot in the roof framing, roof decking,
walls and ceilings. Proper ventilation helps to reduce the occurrence of many problems such as expansion/contraction of
decking and ice damming in cold, snowy climates. Ice dams are formed by the cyclical thawing of snow over the warmer
portions of the roof and re-freezing at the cold eave. Refer to ARMA’s Technical Bulletin “Protecting Against Damage from Ice
Dams.”
During the summer months, roof deck temperatures can significantly increase due to the sun’s energy. The heat from the deck
radiates into the attic space, and could reach the living space if the attic floor/ceiling is not well insulated. This will increase the
demand on the home’s cooling system and energy use. Additionally, it will accelerate the aging of asphalt roofing products. By
properly ventilating the underside of the roof deck, heat buildup and its related problems will be reduced.
Refer to ARMA’s Technical Bulletin “Attic Ventilation Best Practices for Steep Slope Asphalt Shingle Roof Systems.” For any given
home, the minimum amount of ventilation required by code is dependent on three primary factors: the size of the attic, the
placement of the vents and the airflow rating of the vents. When considering air movement, there are two categories of vents –
intake vents and exhaust vents. The optimal attic ventilation installation is a balanced combination of properly located, properly
sized intake and exhaust vents (and there are many types within each category).In some cases, a minimum net free ventilation
area equal to one square foot per 150 square feet of attic floor area must be designed and properly installed to provide proper
ventilation.
In other cases, ventilation can be at a ratio of 1 square foot ventilation per 300 square feet of attic floor area. Ventilation
manufacturers recommend that the free-flow ventilation be equally balanced between intake and exhaust vents regardless of
which ratio is used. Because eave and ridge venting provides continuous airflow along the entire roof peak and eave, instead of
localized as is the case with individual vents, it is generally viewed as the superior venting technique (see Figure A).
The manufacturers of ventilation systems and vapor retarders should be consulted for proper use of their products. It should be
noted that the trends continue toward higher energy conservation, air barriers, and generally tighter housing construction
methods. The code requirements are minimums, and as such, make proper ventilation an important consideration for
minimizing energy usage and optimizing roofing system performance. Standard ‘one size fits all’ solutions are not sufficient.

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Variations in Color Appearance on Newly Installed Asphalt Shingle Roofs

Homeowners may look at their newly installed roof and think that the shingle color does not look like
the picture in the brochure. In fact, variations in the appearance of asphalt shingle roofs are not
uncommon, and generally occur for five reasons: color shading, back surfacing transfer, staining,
excessive surface asphalt, and deviation from installation instructions.
Color Shading
Color shading is usually the result of variations in surface reflectance in different areas of the roof. Even
slight differences in shingle texture can make color shading perceptible. This may occur more frequently
with black and other dark-colored shingles since only a very small amount of light will reflect from a dark
surface.
The variations that cause shading of black or other dark-colored shingles are so slight that they are
difficult to detect during the manufacturing process. With white and other light-colored shingles, the
total amount of light reflected is considerably greater, resulting in reduced potential for color shading.
Shingle manufacturers will often use surface granule blends to reduce the potential for color shading by
incorporating a variety of different colors, which help reduce shading by making observable differences
less noticeable. Color shading typically varies with the time of day, light intensity, and viewing angle.
Back Surfacing Transfer
Fine particles placed on the backside of shingles so they do not stick together in the bundle can rub off
onto the colored granules on the exposed shingle surface. This may cause temporary appearance
variation immediately after the shingles are installed. However, natural wash from rainfall will
eventually remove this loose backing material from the shingle surface.
Staining
Staining may occur when shingles are stacked or stored for extended periods. Lighter oils in the asphalt
coating may seep between and migrate onto neighboring surface granules. This is generally eliminated
by natural weathering over time.
Excessive Surface Asphalt
One step in the shingle manufacturing process is pressing the surface granules into a hot asphalt
coating. This can occasionally result in small amounts of asphalt rising between the surface granules and
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affecting the appearance in a manner similar to color shading. Natural weathering may reduce the
variability depending on the amount of over-pressed asphalt.
Deviation from Installation Instructions
Deviations from the manufacturer’s printed application instructions by the roofing contractor may also
result in an unanticipated visual patterning. ARMA recommends that installers follow manufacturer
application instructions to avoid patterning.

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Use of Self-Adhering Membranes as Underlayments in Steep Slope Roofing

Self-adhering bituminous membranes have been used as underlayments in steep slope (greater than
2:12) roofing for many years. When applied as an underlayment, they are primarily used to help
prevent water entry from ice dams at the eave areas of shingled roofs in cold climates. When used as
ice dam protection, the underlayment is typically installed directly to the deck surface from the eave’s
edge to a point at least 24 inches (measured horizontally) inside the exterior wall line of the building
prior to application of the shingles. If the membrane is not wide enough to reach that point, install
additional course(s) of membrane as needed, overlapping the previous course by 2 inches or as
specified by the manufacturer. Self-adhering bituminous membranes are required by building codes
to meet the requirements of ASTM D1970, and newer codes require these products to have a label
indicating compliance with ASTM D1970. Always check local building codes to confirm eaves
protection requirements. The adhesive asphalt component effectively seals the membrane to itself
and seals around the shanks of nails used in the overlying shingles so that any water forced
underneath the shingle layers by wind or ice dams does not reach the deck or attic space below.
These self-adhering underlayment membranes have also been used successfully in other “critical”
roof situations, such as part of a flashing system in valleys or around roof penetrations (skylights, vent
stacks, etc.), and are commonly applied to the entire deck beneath roofing materials on lower-sloped
(2:12 to 4:12) roofs.
Where the roof area of one slope transitions to a roof area of a differing slope, the underlayment
application should extend at least 24 inches up on the steeper slope roof. The transition area between
the steeper slope and lower slope needs special attention due to potential water buildup.
In certain applications, such as lower-sloped (2:12 to 4:12) roofs or in areas where high winds or
hurricanes are prevalent, homeowners and roofing contractors may apply the underlayment
membrane over the entire roof area, not just the first few feet at the eaves. This application improves
roof protection in the event that water gets under the shingles. Check local codes to confirm that a
self-adhering bituminous membrane is acceptable for full-roof application.
When installed, self-adhering membranes restrict the flow of vapor and air through the roof
assembly, and moist air entering the attic from the conditioned space inside the home may condense
on the underside of the self-adhering membrane at the roof deck joints. Condensation may lead to
problems in roofing systems or attics, including but not limited to wood deck swelling, deterioration,
mold growth, and staining on the interior ceilings below the attic. Potential condensation problems
may be reduced by:
1. Confirming attic ventilation is adequate, balanced, and evenly distributed to assure proper
airflow.
2. Installing a proper vapor retarder on the warm side of the attic floor, which can reduce
intrusion of warm, moist air into the attic space.

3. Installing sufficient insulation that covers the entire attic floor.
4. Checking local energy codes for appropriate ceiling insulation R-values and air barrier
requirements.
For more details on ventilation, see ARMA’s Technical Bulletin “Ventilation and Moisture Control for
Residential Roofing.” Check with a building design professional for advice if the home is in a warm,
humid climate, as a different approach may be necessary. Following the four recommendations
described above is sound practice for all steep-slope roofing systems. If your roofing application calls
for applying a self-adhering underlayment or membrane over the entire roof deck, these good
practices will help reduce condensation and the subsequent problems that can occur.

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Understanding Why Proper Product Sampling Provides Credible Results

Sampling is the process of selecting material to be tested. The process used for collecting test
samples directly affects the conclusions that can be supported by the test results. Statisticians
use the term “inference” to describe the extension of test results from a sample to a broader
group, or population, of product. An appropriate sampling process allows the ensuing test
results to be inferred to the desired group. Conversely, inference of the results beyond the
group that can be justified by the sampling process will lead to inappropriate conclusions.
From time to time, roofing industry members collect shingle, roll goods, or other product
samples to have their properties tested and publish the results. This may be for a magazine
article, a product rating on a website, or a blog post, among other uses. The conclusions that
may be reached from those results are dependent upon the process used to collect the product
samples.
Before starting a product test program, consider the intended use of the results. This first step
is essential to creating an appropriate sampling program. If the desire is to make broad
statements about a product, the number of samples to be collected will be much larger than if
a specific statement is to be made about a defined collection of product. Figuring out the
question to be answered is critically important.
A key aspect of any sampling program is the statistical concept of a “lot.” One definition of a lot
is found in ASTM E456 – Standard Terminology Relating to Quality and Statistics which states,
“a definite quantity of a product or material accumulated under conditions that are considered
uniform for sampling purposes.” Examples of a lot might include:
• All the shingles of the same brand and style in a distributor’s warehouse
• All the modified bitumen cap sheet rolls delivered to a job site for application on a roof
• All the packages of a specific ventilation product on a single truckload shipping from a
roofing manufacturer
“Random” is another key concept in sampling. Grabbing the first five bundles or rolls available
is not random sampling. Random selection requires that each unit (e.g., bundle, roll, package) in
the lot has the same chance (i.e., statistical probability) of being selected. This includes the
bundle on the bottom row of the least accessible pallet and the roll in the middle of the pallet.
Proper sampling is essential. If a carefully designed sampling plan is followed, the tester can have reasonable confidence that similar results would be obtained if different bundles or rolls
were selected and tested from the same “lot” of material. Results from testing properly
selected samples can be inferred over the entire lot of material.
Suppose five asphalt ply sheet rolls are purchased at a retail outlet, without being randomly
selected, and are subjected to testing. Results from testing the rolls are applicable only to those
five rolls. The tester cannot properly infer those results to the pallet or pallets from which the
rolls were taken, to the stock of those rolls in the retail outlet’s warehouse, or to the specific
brand and style of roll good, because the randomness requirement of the sampling
methodology was not satisfied. All that can be concluded is that the five-roll sample met or
failed to meet the criteria being evaluated.
As another example, consider a situation in which 10 bundles of shingles are purchased from a
roofing distribution outlet and evaluated for one or more properties. What can be concluded
from the results? As in the previous example, the sampling process determines the limitations.
In this case, the sampling process permits the distributor to select the shingle bundles. If the
distributor selects and ships the most easily accessible 10 bundles, the conclusions of the
testing are constrained to the 10 bundles and should not be inferred to apply to any larger
group or population of shingles. If the distributor selects the shingle bundles randomly from
stock of that product in its warehouse, the associated test results may be inferred to apply to
the stock in the warehouse, but not to a larger group.
Conclusion
When considering test results for shingles, roll goods, or any roofing product, remember to
consider the sampling process. Use of proper sampling techniques ensures confidence in
conclusions drawn from testing of product samples.

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Self-Adhering Underlayment Removal Prior to Steep Slope Re-Roofing

Self-adhering underlayment is generally applied to the roof deck on the eaves, rakes, and valley
areas of steep slope roofs and as flashing around roof penetrations. Self-adhering
underlayments are installed on critical areas of the roof to minimize the likelihood of water
penetrating the roofing system. In order for self-adhering underlayment to perform well, it
must adhere firmly to the roof deck. As a result, it can be difficult to remove without damaging
the deck material.
Removing self-adhering underlayments is always recommended in situations where it can be
removed without damaging the deck. Removal will facilitate examination of the deck for
deterioration and damage, reduce buildup of material that could interfere with proper
drainage, and eliminate unevenness that may create an aesthetic issue with the newly installed
roof covering. Removal of self-adhering underlayment becomes more important when more
than one layer of self-adhering underlayment is present.
If only one layer of self-adhering underlayment is in place, and it is not possible to remove it
without damaging the deck, check with the underlayment manufacturer’s installation
instructions and local building codes to determine if installation of a second layer of
underlayment is permissible. If a second layer is permitted, offset end and side laps in the new
and existing underlayment to minimize thickness buildup and “feather in” the new
underlayment by extending the new material a minimum of 8” up the slope onto the bare deck.
This will reduce the likelihood of problems with drainage and aesthetics.
If two or more layers of self-adhering underlayment are in place, all layers should be removed.
If removal of the existing material cannot be accomplished without damaging the deck, then
the roofing contractor may have to remove and replace the decking in the areas covered with
self-adhering underlayment.

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Effects of Roof Color and Solar Reflectance on the Accumulation of Moisture in Membrane Roof Systems

Introduction
Moisture content within a roofing assembly may fluctuate significantly over the life of the roof
depending on a variety of factors including, but not limited to moisture in the existing roof
assembly at time of installation; interior and exterior temperatures; interior and exterior
humidity conditions; deck type; under-deck ventilation; amount and location of insulation; and
presence of vapor retarders/air barriers in the roof assembly.
The potential for condensation and moisture buildup in a membrane roof system from interior
moisture sources has always been and should continue to be an issue that must be accounted
for in the roof system design. Furthermore, the color, solar reflectance, and thermal emittance
of the roof surface can affect a roof system’s drying potential and, therefore, the buildup of
moisture in a roof system.
Moisture buildup in the roof assembly can result in deck deterioration, including rotting wood
decks and corrosion of metal decks, growth of mold and other organisms, deterioration and
reduction of the effectiveness of thermal insulation, premature failure and deterioration of the
roof system, and re-emulsification of certain water-based adhesives.
Effects of Roof Color and Reflectance
The use of light color/reflective roofing is increasing, driven in part by requirements such as the
California Building Standards Commission’s Title 24, LEED, and local code requirements across
the U.S.
Changing the color of a roof membrane from a dark or non-reflective surface to a light color or
highly reflective surface both reduces the amount of time the roof spends in a “drying” mode
and the roof temperature when the roof is in a “drying” mode. When there is a source of
interior humidity, a light colored or highly reflective roof surface can allow moisture and liquid
water to build up in the roof assembly with less opportunity to evaporate or dry. Accumulation
of moisture within roof systems can be exacerbated in buildings with elevated humidity or
periods of excessive moisture generation and if often not addressed in the design of the
building envelope. Some examples of moisture generators include:
• Apartment/condo buildings (showers, cooking, air humidifiers, etc., produce high levels
of interior moisture)
• Swimming pools, food processing, paper mills, and foundries • New construction with high interior construction moisture (i.e., from freshly poured
concrete, space heaters, wet insulation installation, drywall installation, etc.)
• A compact ceiling assembly where there is typically drywall, batt insulation, roof deck
and membrane with little or no insulation above the deck, no vapor retarder or air
barrier in the system, and little or no ventilation below the deck
• Reroof conditions where moisture may be present in the existing system
Things to Consider
In new construction projects, the design professional must evaluate the anticipated interior and
exterior conditions and design the proper water vapor control (including considerations for
transfer of water vapor via diffusion and air flow). This evaluation should include the necessary
calculations to ensure there will not be a condensation problem and a determination regarding
whether a vapor retarder, air barrier, or underside roof deck ventilation is necessary. If
adequate water vapor control measures cannot be integrated in the design, use of light colored
or highly reflective roofing may create condensation issues.
Regarding tear-off, recover, and coating applications, a roofing professional should evaluate the
existing roof assembly for signs of water infiltration and/or condensation issues (water stains,
wet or deteriorated insulation, deck deterioration, organic growth). The professional should
also determine whether there are interior vents (such as bathroom exhaust fans) and, if
present, confirm that they are all properly ducted to the outside and in good condition so they
do not allow moisture to enter the roof system. A roof design professional or climate control
specialist should be consulted to evaluate the existing conditions and to develop a plan to
address moisture issues within the existing roof assembly.
Some things that can be done to help control moisture accumulation in the roofing assembly
include:
• Remove wet areas within the existing roof system prior to recovering the system with a
new assembly
• Provide insulation above the deck to shift the location of the dew point
• Use at least two layers of insulation with staggered joints to prevent moisture migration
through the joints between the insulation boards
• Use an adhered membrane system to minimize moisture migration within the roofing
system
• Provide a vapor retarder and/or air barrier to the system at the proper location within
the roof assembly and seal roof deck penetrations, terminations, and transitions
• Provide adequate ventilation below the deck to remove moisture before it enters the
roofing system (always check with local codes to confirm below-deck venting
requirements are met)

Always refer to roofing manufacturer published requirements and consider local building and
energy code requirements. Consult a roofing professional when questions and decisions are to
be made on condensation and refer to ASHRAE for design guides and standards.

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Decking Recommendations for Built-Up Roofing and Modified Bitumen Membranes

Introduction
The Asphalt Roofing Manufacturers Association (ARMA) recommends that the structural roof
deck meet certain minimum requirements to be an acceptable substrate for the specified
roofing system. Generally, all decks should be clean, dry, and securely fastened to the building
structure with no abrupt level changes exceeding 1/8”. Roof deck deflection should never
exceed 1⁄240 of the span under total design loads — including rooftop traffic. Requirements of
the roofing system manufacturer, insurer and local building code must also be met when
designing and applying the roofing system.
The following are suggested for various roof deck types:
I. Steel Decks
Steel decks should be installed in a way that allows the rib spacing to be uniform and straight so
that: (1) the roof insulation boards may be laid with side joints parallel (with end joints
perpendicular to the ribs); and (2) the roof insulation edge is supported by the flanges.
Deck specifications shall comply with all applicable code requirements. They should also meet
the requirements of the Underwriters Laboratories “Building Materials Directory” and, when
applicable, the requirements found in Factory Mutual Global’s RoofNav and “Property Loss
Prevention Data Sheet 1-28.”
II. Wood Decks
(A) Wood Planks: Wood deck material should consist of kiln-dried, tongue and groove, shiplapped, or splined boards. All boards must have a bearing on rafters at each end and be
securely fastened. They should have a minimum of splits and knotholes, and under no
circumstances can these boards be warped or cupped. All holes over 1⁄4” should be
appropriately covered. Individual boards must not exceed 8” width and must be no less
than 1” thick (nominal). There should be a 1⁄8” space between boards to allow for
expansion. Wood deck preservatives and/or treatments must be compatible with the
type of bitumen used.
(B) Plywood and Oriented Strand Board (OSB) Decks: Individual roofing manufacturers
approve either or both plywood and OSB Performance Rated Panels for use as
sheathing. These panels should be a minimum thickness of 7⁄16” for OSB and 15⁄32” for
plywood. The panels must be manufactured with a water-resistant adhesive and should be labeled “Exposure 1”. There should be a minimum of holes and voids within and on
the surface. The panels should also be marked properly as “Performance Rated Panels”
by either APA – The Engineered Wood Association or another recognized testing agency.
Install so that all edges are supported or clipped to the adjacent sheet. *Fire treated
plywood and particle boards are not recommended.
III. Concrete Decks
(A) Poured Structural Concrete Decks: These decks typically vary from 4” to 12” in thickness
and must be properly cured prior to application of a roofing system (normally a
minimum of 28 days). Curing agents must be checked for compatibility with the roofing
system to be installed. After installation, the underside of these decks must continue to
remain unobstructed and should be exposed, or they should be poured over vented
metal forms to allow the escape of water vapor. A primer compatible with the bitumen
or adhesive used to install the roof system should be used on these decks.
(B) Pre-cast Structural Concrete Decks: Joints must be filled with a masonry grout to correct
imperfections between slabs and feathered to provide a slope of not greater than 1/8”
per foot. When the membrane or roof insulation is adhered directly to the deck, use a
concrete bituminous primer that meets the membrane manufacturer’s requirements
and is compatible with the type of bitumen used.
(C) Pre-stressed Concrete Decks: Because of variation in camber and thickness of prestressed concrete decks it is recommended that a minimum 2” lightweight concrete fill
be installed over these decks.
(D) Lightweight Structural Concrete Decks: Refer to ARMA’s Lightweight Structural Concrete
Roof Decks Statement, which can be found here.
IV. Lightweight Insulating Concrete Decks
Lightweight insulating concrete decks, which are placed as a slurry, contain more moisture than
many other roofing substrates. Retained moisture may contribute to problems with the roofing
systems installed over such decks when proper precautions are not taken.
When these decks are used as a substrate for built-up or modified bitumen roofing, the
following should be considered:
• When lightweight insulating concrete is poured over a galvanized metal deck, the metal
deck should be perforated to provide underside venting. Topside pressure relief is also
suggested.
• The base ply of the roofing system should be attached using appropriate mechanical
fasteners.
• Pull-through resistance for fasteners should comply with the membrane manufacturer’s requirements.
• The deck applicator and deck manufacturer should certify, in writing, that the roof deck
was installed in accordance with the deck manufacturer’s recommendations and is
satisfactory to receive the roofing system.
• The roofing contractor should install the roof in accordance with the roofing
manufacturer’s recommendations for application over lightweight insulating concrete
decks.
V. Cementitious Structural Wood Fiber Decks
These decks should be bonded by binders that are not affected by water. The units should be
attached to the building structure with mechanical fasteners to prevent movement and to
provide the required uplift resistance. The base ply should be attached using fasteners
recommended by the structural wood fiber and base ply manufacturers.
Typically, manufacturers do not recommend that insulation or a membrane be fully adhered to
these decks. The roofing contractor should install the roofing system in accordance with the
roofing manufacturer’s recommendations.
VI. Poured Gypsum Concrete Deck
Gypsum is a mineral, calcium sulfate, which is initially heated to remove hydrated water which
it readily reabsorbs when being made into panels or being poured in the field. A more common
name is plaster. Upon adding water, rehydration occurs and the gypsum sets up into a
monolithic nailable substrate. Reinforcing wood chips and shavings are usually added for
additional strength. Gypsum is noncombustible and nailable when fresh. Gypsum decks were
manufactured as precast panels or poured in place in the field. Gypsum decks are typically a
minimum of 2″ thick. Most gypsum deck projects will be reroofs or tear-off.
The precast panels are generally designed with tongue and groove edges that reinforce
adjacent panels and accommodate bulb-T roof truss construction. Precast panels harden
significantly upon aging.
Poured deck systems contain some excess water; provisions must be made for moisture to
escape so as to avoid related moisture problems for roofing systems. Roofing systems should
be attached to this deck using a nailed base sheet. Special fasteners are available for this
purpose. Direct solid attachment of the roof membrane to this deck is not recommended;
occasional shrinkage cracks in the deck could result in splits in the roof membrane.
Typically, manufacturers do not recommend that insulation or a membrane be fully adhered to these decks. The roofing contractor should install the roofing system in accordance with the
roofing manufacturer’s recommendations.

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Coating of Asphalt Shingles After Installation

Many types of roofing permit the application of a coating for a variety of reasons, such as
increasing solar reflectivity, resisting biological growth, improving impact resistance, or
increasing roof life. However, ARMA strongly advises against the application of any type of
field-applied coating over installed asphalt shingles.
There are many types and formulations of roof coatings, so it is always important to consult the
shingle manufacturer before proceeding with any type of coating. Many asphalt shingle
manufacturers specifically do not recommend field coating of their shingles. Additionally, state
or local building codes may prohibit this practice, as the field-applied coatings may negatively
impact the performance characteristics (including the fire classification, algae resistance,
impact resistance, etc.) of the roof assembly.
Some of the problems reported after asphalt shingle roofs have been field coated include
shrinking of the coating, which may result in unsightly curling and/or cupping of the shingles or
loosening of the granule surfacing of the asphalt shingles. In addition, non-permeable roof
coatings may create a vapor-retarding layer by sealing the voids around and between the
shingles. If this occurs, it may contribute to moisture accumulation within the roofing system.
It has been suggested by some that the use of field-applied coatings over existing asphalt
shingles will produce overriding benefits to the homeowner, such as longer roof life, energy-use
reduction, or remediation of small roof leaks. There is limited available documentation showing
the extent to which the field coating of asphalt shingles provides any of these benefits, but the
risks and concerns mentioned above remain very real. Further, many coatings need regular
maintenance reapplications to provide a consistent appearance.
In summary, the application of a coating may be detrimental to asphalt shingles. Be sure to:
• Check with the asphalt shingle manufacturer before determining a specific roof
coating.
• Check with the local building and zoning department and, if appropriate, your
homeowner’s association to determine whether this application is allowed.

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Application of Asphalt Shingle Systems to Decks Installed over Insulation or Radiant Barriers

The Asphalt Roofing Manufacturers Association (ARMA) has established the following
recommendations for applying asphalt shingles and/or asphalt-based underlayment directly
over insulation, insulated roof decks, and radiant barriers.
Asphalt Shingle and/or Asphalt-based Underlayment Application Directly over Insulation
Applying shingles and/or asphalt-based underlayment directly over insulation is not
recommended for several reasons.
• Asphalt shingles and/or asphalt-based underlayments are designed for attachment to
deck surfaces such as plywood and oriented strand board or other surfaces acceptable
to the asphalt shingle or underlayment manufacturer.
• Continuous free-flow ventilation is impossible to achieve when applying shingles and
asphalt-based underlayment directly over insulation. Heat build-up, a typical result of
inadequate ventilation, may accelerate weathering and reduce the anticipated life of
the products.
• Asphalt shingles and/or asphalt-based underlayment may be damaged or punctured
when nailed onto a non-rigid surface such as roofing insulation.
• Insulation does not have adequate nail-holding ability. Consequently, shingle damage
and/or blow-off may occur if shingles are attached to insulation. Wind classification of
the installed roofing system may be affected.
The fire classification of asphalt roofing products may be adversely affected when applied
directly over insulation. Individual asphalt shingle and/or asphalt-based underlayment
manufacturers should be consulted to determine the effects on such classifications. Fire
classification installed roofing system may be affected.
Asphalt Shingle and/or Asphalt-based Underlayment Application Directly over Insulated Roof
Decks
Applying asphalt shingles and/or asphalt-based underlayment to insulated roof decks is not
recommended unless the following factors are considered.
• Direct installation over insulated roof decks is not recommended unless an adequate
continuous ventilation space, free of obstructions, is provided between the top of the
insulating material and the underside of an acceptable roof sheathing, Proper
ventilation must be provided to dissipate heat and humidity build-up under the roof sheathing. More information on this can be found in ARMA’s technical bulletin,
Ventilation and Moisture Control for Residential Roofing. Factors influencing the
minimum ventilation requirement include type of construction, roof pitch/run,
temperature, humidity, etc. Consult the deck manufacturer, deck system designer, and
asphalt shingle/underlayment manufacturer for specific requirements.
• Asphalt shingles and/or asphalt-based underlayment should only be fastened to deck
surfaces such as plywood and oriented strand board or other surfaces acceptable to the
asphalt shingle manufacturer.
• Application of asphalt shingles and/or asphalt-based underlayment directly over
insulated deck systems without providing adequate ventilation may affect the asphalt
shingle and/or asphalt-based underlayment manufacturers’ product warranties. Consult
individual product manufacturers for details and refer to local building codes.
Asphalt Shingle and/or Asphalt-based Underlayment Application over Deck Systems
Containing Radiant Barriers
Applying asphalt shingles and/or asphalt-based underlayment over deck systems containing
radiant barriers is at times acceptable, but several considerations should be noted.
• Radiant barrier sheets that are fastened between or beneath the roof rafters should
have proper ventilation between the radiant barrier and the decking so heat and
humidity build-up can be dissipated.
• Radiant barriers require a minimum 1-inch air space between the metallic surface and
the next nearest surface. Otherwise, thermal conduction will override the reduction in
radiant heat transfer. See the US Department of Energy’s bulletin on Radiant Barriers
for more information (found here).
• Radiant barriers installed directly beneath and in contact with the roof deck sheathing
may interfere with proper deck ventilation. The asphalt shingle and/or asphalt-based
underlayment manufacturers’ product warranties may be affected, so consult individual
manufacturers for details. Refer to local building codes for specific project requirements
that may apply.
Ventilation Considerations
Most vent system manufacturers recommend a soffit/ridge (inlet/outlet) venting ratio of
between 50 and 60 percent. An air space of 3/4-inch (19 mm) is suggested as a minimum
ventilation space; a 1.5-inch (38 mm) or wider space is preferred. Factors influencing this
measurement include type of construction, roof pitch/run, temperature, humidity, etc. Larger
roof expanses, such as those on commercial buildings, may require a much larger air space to
move heat and moisture from the system because of their longer run. Adequate intake airflow
must also be provided for proper ventilation dynamics. Consult the deck manufacturer, deck system designer, and asphalt shingle/underlayment manufacturer, as well as local building
codes, for specific requirements. Some methods for creating a continuous air space for proper
ventilation are shown in Figures A, B and C.

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Asphalt Shingle Reroofing: Replacement vs. Recover

When the time comes to reroof an existing asphalt shingle roof, a decision must be made whether to remove the old shingles or
apply new shingles directly over the existing layer. Most building codes define the options as follows:
Reroofing: The process of recovering or replacing an existing roof covering.
Roof Recover: The process of installing an additional roof covering over a prepared existing roof covering without
removing the existing roof covering.
Roof Replacement: The process of removing the existing roof covering, repairing any damaged substrate and installing
a new roof covering.
In some cases, local building codes will limit the available options—most do not allow more than two roof coverings on a
building. However, there is no easy, universal answer if only one roof is in place. Although in many cases it is not necessary to
tear off old shingles before installing new shingles, some roofing professionals will insist on replacement because it ensures that
a completely new roofing system is installed.
Although each roof must be evaluated individually, general guidelines can help make an informed decision whether to replace or
recover an existing asphalt shingle roof.
 If a roof has only one layer of shingles that lay flat and the decking is in good condition, a tear-off may not be needed.
Not only will the existing layer provide a secondary back-up roof for the new shingles, but it will also save the cost and
inconvenience of removing and disposing or recycling the old shingles.
 Before making a final decision to tear off or recover, check that local building codes are being followed.
 Adequate roof ventilation should be provided (See ARMA Technical Bulletin, “Ventilation and Moisture Control for
Residential Roofing” for additional details and information).
The existing shingles will probably have to be removed if:
 An inspection of the roof deck reveals rotted or warped wood or large gaps between the deck boards. Any rotten or
damaged boards must be replaced before applying new shingles. [Note: for best roof performance, consider re-decking
“board” roof decks with a layer of APA (The Engineered Wood Association) Grade ½” plywood before installing new
shingles.]
 There are more than two layers of existing shingles on the roof. Note that the local building codes may require removal
of more than one layer.
 The roof structure shows signs of sagging across the ridge or truss lines. If the roof does not look straight and feel solid,
have the structure inspected by a licensed structural engineer to check for structural defects.
 The condition of the existing shingles is so uneven and distorted that it would not be practical to flatten all raised areas
enough for the new roof to lay flat.
Many factors may play into whether a roof can be recovered or replaced, so it is important to discuss the options with your
roofing professional. Your decision can impact the curb appeal of your home and the performance of your roof.

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