Roof tear off exposes felt underlayment (C) Daniel Friedman

Roof Underlayment Requirements & Recommendations
Is roofing felt required? Who says? Felt details for various roof slopes & conditions
     

  • UNDERLAYMENT REQUIREMENTS on ROOFS - home - CONTENTS: what are the requirements for roofing felt or roof underlayment & what are the installation specifications for roofing felt? 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?
  • POST a QUESTION or READ FAQs about when & where roofing felt underlayment is required
  • REFERENCES

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Roofing underlayment - roofing felt requirements: 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.

Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.

Is Roofing Felt Underlayment Needed Under Roof Shingles, Tiles, Slate, Wood Shingles or Shakes?

Peel and stick roof underlayment (C) Daniel Friedman Eric GalowOur 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.

At left 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:

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 blowoff's 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).

Bald, worn-out organic asphalt roof shinglesOur 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

Best Practices Guide to Residential Construction lists five good reasons to install roofing underlayment:

  1. Roofing underlayment or felt protects the roof deck from rain before the roofing is installed.
  2. Roofing underlayment or felt provides an extra weather barrier in case of blowoffs or water penetration through the roofing or flashings.
  3. Roofing underlayment or felt protects the roofing from any resins that bleed out of the sheathing.
  4. Roofing underlayment or felt helps prevent unevenness in the roof sheathing from telegraphing through the shingles.
  5. 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.

Underlayment On Standard Slopes where Asphalt Roof Shingles are Installed

/BestPractices/Figure 2-2 (C) J Wiley, S BlissAs stated in Best Practices Guide to Residential Construction:

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.

Side courses of underlayment should run over the valley lining and extend 6 inches past its edge. (See ASPHALT SHINGLE VALLEY FLASHING or see page 59 in the printed text Best Practices Guide)

Underlayment Application of Asphalt Shingles on Low Slope Roofs

/BestPractices/Figure 2-3 (C) J Wiley, S BlissContinuing from from Best Practices Guide to Residential Construction:

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.


/BestPractices/Figure 2-4 (C) J Wiley, S BlissIn 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).

/BestPractices/Figure 2-5 (C) J Wiley, S BlissWhere 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.

...

Roofing Felt Installation Details - omitted felt & felt specifications for various roof pitches or slopes

Reader questions on roofing felt installation specifics

(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.]

Reply:

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 inspectapedia.com/roof/low_slope_roofing.htm

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

  • for asphalt shingles, NRCA recommends a single layer of no. 15 asphalt-saturated underlayment be used with roofs having slopes of 4:12 (18 degrees) or greater.
  • for roof slopes between 3:12 (14 degrees) to 4:12 (18 degrees), NRCA recommends a minimum of two layers of no. 15 underlayment. if you are installing a heavier-weight shingle with a projected long service life, using no. 30 underlayment instead of no. 15 would be appropriate.

Inadequacy of Roofing Felt as a Vapor Barrier for Asphalt Shingles in Hot Humid Climates?

Roof underlayment peel and stick (C) InspectApediaA 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?]

 

 

Continue reading at ROOFING UNDERLAYMENT BEST PRACTICES or select a topic from the More Reading links shown below.

Suggested citation for this web page

ROOFING FELT UNDERLAYMENT REQUIREMENTS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

More Reading

Green link shows where you are in this article series.

...




Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References