Is roofing felt needed under asphalt shingle roofs?
Roof shingle warranty requirements for roof shingle underlayment
NRCA recommendations for roofing felt underlayment
Permeability of felt underlayments in hot humid climates
What are the benefits from using roofing underlayment or felt?
What problems can occur when using a roof shingle underlayment?
This article series discusses roofing felt: the requirements for use of an underlayment, such as roofing felt, tarpaper, or other underlayment products beneath asphalt shingles and other roofing materials. We also discuss the moisture permeability of roofing underlayments in hot humid climates.
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Our photo (page top) shows felt underlayment in good condition as a worn out roof was being stripped. It looks as if the installer of asphalt roof shingles (still visible at below right but now worn out) may have placed roofing felt on top of a still older wood-shingle roof.
The roof deck should be sound and level before laying the
underlayment. Fifteen-pound or heavier felt underlayment
is required by code in some areas. Whether or not it is
required, underlayment is cheap insurance against problems.
There are several good reasons to install underlayment:
It protects the roof deck from rain before the roofing is
installed.
It provides an extra weather barrier in case of blowoffs
or water penetration through the roofing or
flashings.
It protects the roofing from any resins that bleed out of
the sheathing.
It helps prevent unevenness in the roof sheathing from
telegraphing through the shingles.
It is usually required for the UL fire rating to apply
(since shingles are usually tested with underlayment).
Our OPINION is that on new asphalt shingle, tile, fiber cement, slate, and many wood roofs, most roofing contractors apply an underlayment membrane of roofing felt ("tar paper"), or fiber-reinforced roofing felt. A special underlayment may be recommended by the manufacturers of specific roofing product and hybrid products.
Our photo (left) shows new felt underlayment in place during an asphalt shingle roof installation in New York.
While the requirement for felt underlayment beneath asphalt roof shingles seems to be a topic of almost timeless argument subject to much arm-waving and little reading of manufacturer's instructions and warranties, various sources recommend or require installation of a felt underlayment over the roof deck before asphalt roof shingles are installed.
Booth & Roberts reported at length on the uses of underlayments on asphalt shingle roofs, citing (quoting):
Asphalt shingle underlays are commonly used throughout North America. The National Roofing Contractors
Association’s (NRCA’s) definition of underlay(ment) includes some important functions:
To separate the roof covering from the roof deck
To shed water
To provide secondary weather protection
The authors point out that
"Resins from green lumber roof decks have been known
to interact with roof shingles, and a separation layer of
underlay can be useful. Underlays are quickly applied and
they can provide temporary roofing before the roof shingles
are installed. Underlays have remained intact after
shingles have blown away, thereby providing temporary roofing [7]. Underlays provide secondary weather protection
in ice-dam situations, when water backs up over the
head-laps of shingles.
Other functions of underlays have been reported [8].
Two of the more important ones are air-flow resistance and
moisture storage. Air-flow resistance reduces windblown
water penetration from the outside and moist air penetration
from the inside of a roof. Moisture storage provides
temporary storage for small amounts of leak water, which
subsequently evaporates in dry weather. We know of no
experiments that quantify these advantages.
There are disadvantages to underlays. Felt materials can
wrinkle and buckle, and these imperfections can telegraph
through the finished shingle layer [8,9]. Peterson reported
that underlays reduced the life of roof shingles in
California due to heat build-up [10]. Explaining how an
underlay will act as secondary weather protection when it
has shingle nails through it every 8 inches (203 mm) or so
can be a challenge.
The liquid water transmission test in
ASTM D 4869 for asphalt felt underlayments for shingles
[11] includes the following note: “Take care to ensure that the
staples do not protrude at the front surface of the plywood board so
as not to puncture the test specimen,” this is hardly realistic.
Slipperiness and poor ultraviolet (UV)"
NRCA, Building Code, & Manufacturers Recommend or Require Felt Underlayments on Shingle Roofs
Booth & Roberts and other sources also report that underlayment is required or recommended below shingles. [Reference numbers are to references in the cited document.]
NRCA: NRCA recommends underlays on all shingle roof systems
without exception [1]. A 1990 survey indicated that
79 percent of U.S. roofing contractors always used underlays
[12].
Roof Warranty: The absence of underlays can invalidate some
manufacturers’ warranties and roof system fire classifications
[13].
Manufacturer's Requirements: typical requirements found in roof shingle manufacturer's installation instructions specify the use of underlayment: Non-perforated, [Type I, No. 15] [Type II, no. 30], asphalt saturated felt complying with ASTM D 226, ASTM
D 4869 or ASTM D 6757. - Oakridge Pro30™ shingle instructions.
Canadian building codes do not require general (full
roof) underlays under roof shingles. Many Canadian roofing
contractors use general underlays, while others do not.
Typically, underlays are used more over wood plank roof
decks and less over plywood. Underlays are more likely to
be used on lower-sloped roof systems than on steeper
sloped roofs. There also are provincial preferences, for
example, Ontario roofing contractors tend to use underlays
less than their neighbors in Québec.
Local building code requirements: here is an example from Coon Rapids, Minnesota
All dwellings, structures attached to dwellings, and all conditioned (heated and/or cooled) structures, require eave
flashing to be installed at the eaves and extending on the roof to a point even with 24” past the inside wall line. This
barrier shall consist of at least two layers of underlayment cemented together, or be a self-adhering polymer modified
bitumen sheet (many times referred to as ice dam protection or ice and water shield). It must be installed the full length
of all valleys. On roof slopes of 2/12 to 4/12 the remainder of the roof shall be two layers of 15# felt (applied in shingle
fashion), or 1 layer of ice dam protection. On 4/12 or greater slopes, the balance of the roof shall be one layer of 15# felt.
Five Best-Practices Reasons for Using Roofing Felt Underlayment
Roofing underlayment or felt protects the roof deck from rain before the roofing is
installed.
Roofing underlayment or felt provides an extra weather barrier in case of blowoffs
or water penetration through the roofing or
flashings.
Roofing underlayment or felt protects the roofing from any resins that bleed out of
the sheathing.
Roofing underlayment or felt helps prevent unevenness in the roof sheathing from
telegraphing through the shingles.
Roofing underlayment or felt is usually required for the UL fire rating to apply
(since shingles are usually tested with underlayment).
The roof deck should be sound and level before laying the
underlayment. Fifteen-pound or heavier felt underlayment
is required by code in some areas. Whether or not it is
required, underlayment is cheap insurance against problems.
On roofs with a slope of 4:12 or
greater, use a single layer of 15 lb. asphalt-saturated felt,
starting at the eaves and lapping upper courses over lower
by a minimum of 2 inches. Vertical joints should lap a minimum
of 4 inches and be offset by at least 6 feet in successive
rows (see Figure 2-2 at left).
Secure each shingle course along seams and edges with
enough corrosion-resistant nails to hold it in place until
the roofing is installed.
In high-wind areas, apply fasteners
a maximum of 36 inches on-center along overlaps.
For best protection against leaks, run felt 6 inches
over ridges and hips, from each direction, and 6 inches
up any adjoining walls. Valleys should be lined with a
full width of roofing felt (or bituminous membrane)
pushed tight into the valley so there is no slack.
Asphalt shingles can be used on roofs with
a slope of 2:12 to 4:12 if double-coverage underlayment is
used.
Start with a 19-inch strip of 15 lb. asphalt-saturated
felt along the eaves, and lap succeeding courses by
19 inches as shown in Figure 2-3.
Wherever there is a possibility of ice or snow buildup
or the backup of water from leaves or pine needles, install
a self-adhering bituminous membrane along the eaves that
extends up the roof to a point at least 36 inches inside the
interior wall line.
An alternative approach, not widely used
anymore, is to seal all laps in the lower courses of roofing
felt with lap cement or asphalt plastic cement.
In areas with extensive snowfall or windblown rain,
the best protection against leakage is to cover the entire
low-slope roof area with a bituminous membrane, as
shown in Figure 2-4.
Vertical end laps should be at least 3 inches and horizontal
laps 6 inches. If the roof changes to a steeper slope,
for example, where a shed dormer joins the main roof, extend
the membrane 12 to 18 inches up the main roof slope.
Bituminous membranes are self-healing around nail holes,
and because they bond fully to the sheathing, any leaks
that occur cannot spread. As a safeguard against expensive
callbacks, many roofers now apply membrane to the entire
surface of any roof with a slope of 4:12 or less.
Eaves Flashing Recommendations for Asphalt Shingle Roofs
The best defense against ice dams in
cold climates is a so-called “cold roof,” consisting of high
levels of ceiling insulation separated from the roof surface
by a free-flowing vent space (see “Preventing Ice Dams,”
page 97 in the printed text Best Practices Guide to Residential Construction).
Where a cold roof cannot be achieved due to
complex roof shapes, unvented roofs, or retrofit constraints,
ice dams may form during severe winters, in some cases,
causing pooled water to wet wall cavities and interior
finishes.
Where adequate insulation and ventilation cannot be
assured, self-adhering bituminous eaves flashing should
be installed.
The membrane should go from the lower edge
of the roof to a point at least 24 inches inside the interior
wall line (Figure 2-5 at left).
Where two lengths of eaves flashing meet at a valley,
run each across the valley, starting with the length from
the roof with the lower slope or lesser height. The valley
flashing should later lap over the eaves flashing.
Inadequacy of Roofing Felt as a Vapor Barrier for Asphalt Shingles in Hot Humid Climates?
A Building Sciences Corporation report also elaborates the usefulness of placing a vapor barrier on the roof deck below shingles in hot humid climates. BSC points out that: [some paraphrasing -DF]
Unvented roofs with asphalt shingles in hot humid climates require a vapor
barrier between the asphalt shingles and the roof deck. This is because asphalt roofing materials
store water from dew or rain. Thus asphalt shingles
form a water reservoir not unlike wood shingle or shake roofs. The report argues that this stored moisture is driven inwards [presumably as water vapor, not liquid water] when sun strikes the damp or wet roof surface, and it continues to argue that moisture is driven through vapor-permeable roofing paper, felt, and plywood or OSB roof decking, thus ultimately into the attic space
But unlike an asphalt shingle roof nailed [over felt] directly to a roof deck, a wood shingle or shake roof that has been installed using best practices includes a disposal path for water absorbed in the roof surface: an air space between the wood roofing and the roof deck, or the installation of wood roofing over spaced nailers or "skip sheathing".
In cool or temperate climates this does not present a problem because the combination of heavy wetting from due or rain i snot combined with solar heating at high outdoor temperatures, say the authors who go on to argue that that buckled roof shingles observed in the morning (caused by moisture migrating back up from the roof deck) relax during the day. But on an un-vented roof moisture driven inwards [through the shingles, roofing felt, and OSB or plywood roof decking] in hot humid climates, needs to be addressed.
This phenomenon can
typically be ignored in climates other than hot humid climates because the combination
of extensive dew formation and solar heating at high outside ambient temperatures is not
common. In vented roofs, this is often manifested in the buckling of shingles early in the
morning as the moisture migrates in to the roof deck sheathing and the joints close. This
is followed by relaxation and opening up of the roof sheathing later in the day—the
buckling disappears.
But in un-vented roofs in hot humid climates, the authors argue that water from the roof surface is drawn upwards in liquid form, by capillary action, between plies of overlapped shingle courses where it passes ultimately through the vapor barrier and through the roof decking to the roof cavity interior.
The driving force of moisture through the roof and into the building is by solar heating according to the authors.
[OPINION-DF: from exterior roof inspections at all times of day and seasons, we have not observed this time-related morning roof shingle buckling in the Northeastern U.S. nor in Florida, nor the Southwest, though the authors report the phenomenon. It is possible that the authors are not quite correct that daily buckling and relaxing of roof shingles can be ignored on a vented roof as harmless, since certainly the product is expected to remain flat, and flexing daily might reduce its anticipated wear life.]
With unvented roof assemblies, this inwardly driven moisture must
be addressed. The preferred method is to prevent the moisture from entering the roof
deck material via the installation of a vapor barrier.
[OPINION-DF: we argue at ROOF VENTILATION SPECIFICATIONS that un-vented roofs are not a best building method in any climate.]
Asphalt shingles are quite impermeable to the passage of liquid water directly through
them. However the geometry of their installation allows wicking at overlaps. This inwardly driven capillary water is the source for the wetting of the
roofing underlayment and roof sheathing. The material properties of shingles change
under elevated temperatures and moist conditions due to their hygroscopic nature. The
large vapor pressures resulting from incident solar radiation and the changed material
properties are sufficient to drive moisture inward through the shingles.
Roofing felts or underlayments vary greatly in their permeability to water vapor; the
typical underlayment used under asphalt shingles in residential construction is quite
permeable.
[QUESTION-DF: we note that the test chamber constructed by BSC was itself in an enclosed, air-conditioned space, and that the underside of the test chamber roof was at least in part exposed to the air conditioning. It seems possible that the reduced humidity and lower temperatures on the "interior-side" of the test roof may have contributed to moisture behaviors that vary from what occurs in the field. Attics and under-roof spaces such as in an un-vented "hot roof" cathedral ceiling are certainly not exposed to cool dry conditioned air. BSC may have addressed this concern but we did not find it in the referenced article.
The conclusion of the BSC report is an argument for installation of an impermeable moisture barrier underneath roof shingles, perhaps in place of the traditional and permeable roofing felt.
[QUESTIONS-DF:
What are the differences between the test roof and a roof's behavior in the field?
How does the impermeable moisture barrier under roof remain impermeable when perforated by shingle nails or staples? What will be the market effects of suggesting entire roofs be underlaid first with a moisture barrier unaffected by roof shingle fasteners such as ice and water shield?
Why not recommend that all roofs and roof cavities include ventilation in their design, not only solving the possible driven-moisture problem discussed by BSC but also providing for longer shingle life (cooler roof surface) and in cooling climates, possibly reduced building cooling costs?]
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"Unvented Roofs, Hot-
Humid Climates, and
Asphalt Roofing Shingles
Research Report - 0306
Feb-2003
Building Science Corporation", Building Science Corporation, 30 Forest Street,
Somerville, MA 02143
Quoting from the article abstract:
"When constructing unvented roofs with asphalt shingles in hot-humid climates, a vapor barrier must be
installed between the asphalt shingles and the roof deck." - Web Search 5/21/2010
NRCA Roofing & Waterproofing Manual, 5th Edition, National Roofing Contractors Association, 10255 W. Higgins Road
Suite 600
Rosemont, IL 60018-5607
Telephone: (847) 299-9070 Toll Free: (866) ASK-NRCA (275-6722), e-mail: info@nrca.net. - http://www.nrca.net/rp/pubstore/details.aspx?id=243&c=9
Asphalt Shingle Roofing [Inspection & Installation Guide], Coon Rapids Minnesota, Coon Rapids City Hall
11155 Robinson Drive
Coon Rapids, MN 55433
763-755-2880, - Web search 07/12/2010
City Website - http://www.ci.coon-rapids.mn.us/ Contact the City of Coon Rapids at 763-755-2880
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
Architectural elements: the technological revolution: Galvanized iron roof plates and corrugated sheets; cast iron facades, columns, door and window caps, ... (American historical catalog collection), Diana S Waite, available used out of Amazon.
Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design
Construction methods for new and existing buildings
Information on insulation, drainage, detailing, irrigation, and plant selection
Details on optimal soil formulation
Illustrations featuring various stages of construction
Best practices for green roof maintenance
A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality
Tips on the LEED design and certification process
Considerations for assessing return on investment
Color photographs of successfully installed green roofs
Useful checklists, tables, and charts
Problems in Roofing Design, B. Harrison McCampbell, Butterworth Heineman, 1991 ISBN 0-7506-9162-X (available used)
Roofing The Right Way, Steven Bolt, McGraw-Hill Professional; 3rd Ed (1996), ISBN-10: 0070066507, ISBN-13: 978-0070066502
Slate Roofs, National Slate Association, 1926, reprinted 1977
by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It
has gone in and out of print on occasion.)
Roof Tiling & Slating, a Practical Guide, Kevin Taylor, Crowood Press (2008), ISBN 978-1847970237, If you have never fixed a roof tile or slate before but have wondered how to go about repairing or replacing them, then this is the book for you. Many of the technical books about roof tiling and slating are rather vague and conveniently ignore some of the trickier problems and how they can be resolved. In Roof Tiling and Slating, the author rejects this cautious approach. Kevin Taylor uses both his extensive knowledge of the trade and his ability to explain the subject in easily understandable terms, to demonstrate how to carry out the work safely to a high standard, using tried and tested methods.
This clay roof tile guide considers the various types of tiles, slates, and roofing materials on the market as well as their uses, how to estimate the required quantities, and where to buy them. It also discusses how to check and assess a roof and how to identify and rectify problems; describes how to efficiently "set out" roofs from small, simple jobs to larger and more complicated projects, thus making the work quicker, simpler, and neater; examines the correct and the incorrect ways of installing background materials such as underlay, battens, and valley liners; explains how to install interlocking tiles, plain tiles, and artificial and natural slates; covers both modern and traditional methods and skills, including cutting materials by hand without the assistance of power tools; and provides invaluable guidance on repairs and maintenance issues, and highlights common mistakes and how they can be avoided.
The author, Kevin Taylor, works for the National Federation of Roofing Contractors as a technical manager presenting technical advice and providing education and training for young roofers.
The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com,
143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).
Solar heating, radiative cooling and thermal movement: Their effects on built-up roofing (United States. National Bureau of Standards. Technical note), William C Cullen, Superintendent of Documents, U.S. Govt. Print. Off (1963), ASIN: B0007FTV2Q
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
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