Roofing underlayment - roofing felt requirements:
Is roofing felt required? Who says? Felt details for various roof slopes & conditions
This article series discusses roofing felt, specifically addressing the roofing code or roofing manufacturer's requirements for use of an underlayment, such as roofing felt, tarpaper, or other underlayment products beneath asphalt shingles and other roofing materials.
We review these roofing felt questions: 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?
We also discuss the moisture permeability of roofing underlayments in hot humid climates.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
[Click to enlarge any image]
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.
Above is a peel-and-stick mineral surfaced roofing underlayment shown during installation of a metal standing seam roof. Photo courtesy Galow Homes.
As stated in Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) :
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:
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:
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)"
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.
Best Practices Guide to Residential Construction lists five good reasons to install roofing 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.
As stated in Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) :
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 above).
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.
Side courses of underlayment should run over the valley lining and extend 6 inches past its edge.
Also see ASPHALT SHINGLE VALLEY FLASHING or see page 59 in the printed text Steve Bliss - Best Practices Guide to Residential Construction - (J Wiley)
Continuing from from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) :
Asphalt shingles are permitted on lower-slope roofs with a slope of 2:12 to 4:12 provided that double-coverage underlayment (illustrated above) is installed.
See details
at ASPHALT SHINGLES on LOW SLOPE 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.
Adapted/paraphrased with permission from Best Practices Guide to Residential Construction.
[Click to enlarge any image]
Our photo above illustrates several details of roof drip edge, eaves flashing, and gable end flashing at an asphalt-shingle re-roof job we documented in Dutchess County, New York. From left to right:
See DRIP EDGE FLASHING for ROOFS for complete details about installing drip edge.
(June 28, 2014) Anonymous said:
Hi Dan, a couple of roofing questions:
1) We often see roof felt stopped short of the eaves.
Builders argue its not a problem. I once had a written opinion from NRCA stating felt needed all the way to the eaves but can't find it anymore. Any known reference clearly indicating need to felt to eaves?
2) Asphalt shingles on less than 2:12 pitch,
builder says ice&water shield under it, I know that's not OK but again any reference source you may know of?
Hope all is well in NY land, nice here in Raleighwood. - Steve Smallman - [Mr. Smallman is a professional home inspector in Raleigh NC - Ed.]
Thanks for the important questions Steve:
1. Felt underlayment omission from some roof areas:
this defect - taking a shortcut by leaving off some roofing felt, is an example of a wider problem: building codes & standards writers have given up on trying to anticipate every possible way that people can make a mistake, shifting instead often to "performance" codes that give the general intent of the code. So we'll have trouble finding an explicit citation for your question part 1.
But a reading of roof shingle manufacturer's specifications for using an underlayment (and the article above which is nicely researched) does not find any exceptions that allow leaving out some of the coverage area.
I'd excuse the omission of felt at the eaves and at rake edges of a roof IF instead the roofer is installing a stick-down impermeable barrier such as ice and water shield. Otherwise it's a mistake.
Also see ROOFING UNDERLAYMENT BEST PRACTICES
Quoting from NRCA you'll notice that nothing in NRCA's language refers to an "exception" for some roof areas where underlayment is required:
"Underlayment (or "felt paper" as it is frequently called) is installed over the roof deck before the application of asphalt shingles. An underlayment performs two primary functions: it provides temporary weather protection until the asphalt shingles are installed, and it provides a secondary weatherproofing barrier if moisture infiltrates the asphalt shingles."
2. Applying shingles on low slope roofs (< 4:12)
is discussed in detail
at ASPHALT SHINGLES on LOW SLOPE ROOFS
and also
at LOW SLOPE ROOFING - home
Please take a look. Ice and water shield over the entire roof should work ok and would be an update to the older method described by carson dunlop associate's sketch at inspectapedia.com/roof/0016s.jpg
A Building Sciences Corporation report [at the citations section of this article] lso 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:
11 Sept 2015 Doug Nader said:
When was it made code to have tar paper/felt under shingles?
Doug:
"Code" is a bit vague of a question as to come up with a "first required" date for roofing felt in the building codes we'd need to say what code you are asking about: your local, state, country, or model code or what? Most modern codes require complying with the manufacturer's recommendations even if a specific detail or component is not addressed directly in the code.
The history of tar paper or roofing felt goes back to at least the 1800's.
I have not found an exact citation but I believe you'll find the discussion of felt underlayment in the earliest roofing codes. The first U.S. building codes date from 1859 (Baltimore, MD). - Wikipedia ret. 12 Sept 2015
Building codes in the U.K. obtained impetus with the first significant building regulations passes as the "Rebuilding of London Act" passed by Parliament after the Fire of London in 1666. For a U.S. history of the use of and requirements for roofing underlayment or felt see the following patent (I've edited and cleaned up some typographical errors in the Google patent version of this document to restore Josiah Jowitt to his position of honor).
Below you will find questions and answers previously posted on this page at its page bottom reader comment box.
On 2021-02-06 by (mod) - lap roofing felt over hips & ridges but ... cut through at ridge for ridge vent installation
George
Thanks for the roofing felt overlap at ridge and hip question. I have not seen that described in roofing code specifications, though it appears as a manufacturer recommendation in the article above - and you've reminded us of it once again.
The "code" requirement for overlapping felt at the ridge (during construction) derives from general building code statements requiring that the manufacturer's roof installation recommendations be followed.
Where I've installed asphalt shingle roofs, because usually we are going to cut out a vent opening along the ridge, there was no point in lapping felt over the ridge top except when the roof and building below are going to be left exposed to the weather before the shingle and ridge vent installation have been completed.
We don't, of course, install ridge vents on hips.
I would be grateful if you could point me to the source where you read that lapping felt over ridges and hips is desirable - that way I could look for more-authoritative citations on the topic.
We should not forget however that the felt at a ridge is going to be cut away for the ridge vent opening. So it won't be continuous over the ridge as the roof is completed.
In the original article above on this page we agreed with your point, as Steve Bliss put it at "Run Felt over Ridges & Hips"
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.
Side courses of underlayment should run over the valley lining and extend 6 inches past its edge.
The blocking of the ridge vent opening above by roofing felt is improper. So we can see that even if the installer complies with the GAF instructions, following additional instructions to install proper roof exit ventilation at the ridge, that felt has to be cut away.
Really? well yeah but ...
The Atlas Roofing felt underlayment instructions specifically state:
When you reach the top course, lap about 6 inches of felt over the ridge top. The longer the deck will be exposed to the weather before shingles are applied, the more important it is to lay the felt so that the deck is watertight.
The GF RoofPro felt application instructions (cited above on this page) require "Apply double coverage to all valleys, hips and ridges."
The GAF instructions for the company's Timberline shingles and for GAF FeltBuster high-traction synthetic roofing felt provided in PDF format above make no mention of this "lap over the ridge" detail.
Bottom line: So where does this "run the felt over the hip and ridge" requirement come from?
I'd be grateful to see the specific reference that you found, George as that would be helpful to other readers too.
On 2021-02-06 by George M - GAF specifies lapping felt over roof ridge
I researched this in manufacturers instruction manuals and found that GAF, Owens Corning and Certainteed require the 6" lap over the ridges and hips.
This is Direct quote from GAF:
For ridges, the underlayment should … be lapped over the ridge at least 4” (102 mm) to 6” (152 mm) as would be done on a new or complete tear-off installation.
I read that as a good practice is to run felt 6 inches over ridges and hips, from each direction, and 6 inches up any adjoining walls. Is this mentioned in IRC or any other publications?
On 2020-10-26 - by (mod) -
That's most likely a layer of insulation, probably a wood fibre or fiber board product. Search this website for
FIBERBOARD SHEATHING IDENTIFICATION
to see examples
On 2020-10-25 by Lee
I know about 15# and 30# felt but what is half inch thick sheet OVER the plywood on my near flat 1954 rolled roof?
On 2020-09-20 - by (mod) -
We need continuous solid decking
On 2020-09-19 by Trent
Can you install modified bitumen over spaced decking in Oregon.
On 2020-09-14 - by (mod) -
Sounds as if you want to remove the lower courses of shingles and install ice and water shield, then re-shingle.
On 2020-09-14 by Robert Parker
New asphalt shingles installed over 3:12 roof using synthetic felt as per mfg instructions: eg, double layer a eves with 22" overlap on
succeeding layers.
Drip edge under underlayment on eves and over on gable ends. Massive capillary action from heavy fog condensate
leaking under both layer of underlayment and starter shingles. What's the cure?
On 2020-09-07 - by (mod) -
Charles
I have not found a building code that requires "double underlayment" for roofing.
On 2020-09-06 by Charles
Does anyone know around what year did double layer underlay become standard code? Not looking for exact dates just around about time frame
On 2018-01-21 by (mod) - don't use felt as a basement ceiling cover
No.
On 2018-01-21 by donna
can this felt underlyng material be used to cover insulation in a basement ceiling?
will it be safe we live in rhode island
On 2017-12-01 - by (mod) -
DK
Use of felt on a flat roof would be less common. On a flat roof the surface is waterproof membrane
You would indeed find heavier 30# felt used in multiple plies in a traditional built up tar and gravel roof
That's a different system.
On 2017-12-01 by dj
Is this underlayment recommended for flat roofs?
On 2017-11-07 - by (mod) -
RE-posting some lost comments and replies
On 2017-11-07 by (mod) - bad roof job, leaking, where do I get an inspector
I would start by focusing on identifying what needs to be fixed on the roof and having those problems corrected as quickly as possible in order to avoid expensive damage to the building. In the experts directory at the top of any of our Pages you may find a home inspector who is able to help you.
On 2017-11-06 y carole brinkert
New roof installed, an have had nothing but leaks, no drip edge either...water is wicking up into the roof!! Were can l get an inspector or do l need a lawyer too?
On 2017-11-06 by (mod) - how many layers of felt were actually installed under the shingles?
Tom,
A detailed answer to your question about determining whether or not the roofer used single or double felt underlayment is given at
UNDERLAYMENT DOUBLE vs SINGLE
at https://inspectapedia.com/roof/Roofing_Underlayment_Requirements.php
On 2017-11-06 by Tom
I have a doublewide mobile (low slope) that the contractor says he put double felt on. I was not on site that day. Roof is now coverded in 30 year composite shingle. The city inspector says it's single layer of felt.
With flashing all around the edge of the roof I'm wondering where he's looking? Contractor is standing by his statement. How do I tell?
...
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