Bald, worn-out organic asphalt roof shingles Roof Underlayment Installation
Requirements & Best Practices

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Requirements & recommendations for use of underlayment on asphalt shingle roofs & on low slope roofs.

This article explains the benefits of and best practices use of roofing felt or other roofing underlayment products beneath asphalt shingles and other roofing materials.

We discuss underlayment or felt installation details for standard slope roofs, low slope roofs where shingles will be installed, and low slope roofs in freezing climates.

We also will describe the following: Underlayment for Standard Slopes. Underlayment on Low Slope Shingle Roofs. Flashing in Freezing Climates. 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?

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Best Practices Guide to Uses of Roofing Underlayment

Low slope roof underlayment specifications (C) J WileyOur page top photograph shows 15# roofing felt underlayment being installed in preparation for placing a new roof on an older home in Poughkeepsie, NY.

This article includes excerpts or adaptations from BEST ROOFING PRACTICES, adapted from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

[Click to enlarge any image]

Article Contents

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.

Note that building codes or roof product manufacturers also require using roofing felt in many circumstances including to protect the shingle warranty -


Guide to Roofing Underlayment or Felt Installation for Standard Roof Slopes

Underlayment on standard slope roofs (C) Wiley & SonsAccording to 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).

On roofs with a slope of 4:12 or greater, use a single layer of minimum No. 15 asphalt-saturated felt, starting at the eaves and lapping upper courses over lower. Run the felt 6 inches over ridges and hips from each direction, and 6 inches up any adjoining walls.

Secure each 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 Best Practices Guide to Residential Construction “Valley Flashing” page 59.)

Guide to Roofing Underlayment Installation on Low Slope Roofs with Asphalt Shingles

Low slope roof underlayment specifications (C) J Wiley

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.

Guide to Eaves Flashing by Roofing Underlayment Installation on Low Slope Roofs in Freezing Climates

Cold climate eaves flashing using self adhering membrane (C) J Wiley

For low slope roofs in cold climates where ice dams and ice dam leaks are likely, Bliss recommends a double layer of No. 15 felt underlayment as shown in Figure 2-3above.

With slopes from 2:12 to 4:12, use a double layer of No. 15 underlayment as shown. Where water may back up from ice or debris from trees, protect the lower portion of the roof with a bituminous eaves flashing or fully cemented felt, as shown.

OPINION-DF: use a self-sealing self-adhering bituminous membrane such as Ice and Water Shield in these locations to provide maximum leak protection.

These products seal around nails used to secure the membrane in place. Mr. Bliss makes this recommendation in text that follows and in Figure 2-4 (left).

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 on Roofs,” or see page 97 in the printed text Best Practices Guide).

Eaves and skylight sealing in freezing climates (C) J Wiley

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).

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 with permission from Best Practices Guide to Residential Construction.

Roofing Felt Underlayment Installation Position

Reader Question: is it ok for roofing felt to run at right angles to the roof eaves?

2/10/2014 - Jenn Bullington  said:

I live in Albany, New York and recently had a new roof put on my house. It has come to my attention (after the fact) that the roofer installed the tar paper/underlayment material vertically rather than horizontally. Is this against code? Is this something that should be addressed with my roofer and should I pursue action before a problem occurs? I'm worried. Thanks!


Roofing underlayment is specified in manufacturer's installation instructions and thus is indirectly require by building codes (stating that the manufacturer's installation instructions should be followed). Quoting from UNDERLAYMENT REQUIREMENTS on ROOFS - home

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.

But it is worth noting that some building codes such as the Canadian CBC do not require full roof underlayment below asphalt shingles. So at least in those cases the felt positioning question does not arise.

I'm not sure building codes describe underlayment positioning, but indirectly that installation probably is in violation of the best practices guides & probably the roofing manufacturer's instructions in that at least all the refs we have show placement parallel to eaves and with upper courses lower edges overlapping lower course top edges.

That said, if the finish roof has been installed this IMO not a sufficient defect to warrant re-roofing or a tear-off. The felt on a normal asphalt shingle roof installation is not intended to serve as the waterproof barrier - that's the shingle's job.

The jobs of roofing felt underlayment on an asphalt shingle roof are described in the article above. Of these, most of the functions are position independent. By "position independent" I argue that installing felt underlayment parallel or orthogonal to the house eaves does not affect those functions.

If there were NO felt installed we'd be missing an extra level of "insurance" against water damage from roof leaks following a partial shingle blow-off in a storm. With felt installed orthogonal to the eaves, the efficacy of felt as "leak insurance" would depend on wind direction as well as size of the blow-off: both conditions that could also affect the efficacy of felt even if installed properly, parallel to the eaves.

And in my OPINION, because the felt is punctured thousands of times by the roofing nails that fasten down roof shingles, its ability to protect a home from leaks after the shingles have been installed is reduced significantly anyway.

In fact a worse mistake than running felt at right angles to the eaves is probably running felt lapped "wrong way" with upper course lower edges underneath rather than on top of the upper edge of the courses below.

If some dope makes that mistake and if there is ever a shingle blow-off and water runs underneath the remaining shingles, a horizontal felt edge lap mistake will do a great job of sending nearly ALL of the leaked water underneath the felt. Still nothing much may happen until that water hits an open butt joint between two sections of roof sheathing. Then it may come pouring in at that opening.

If we have a high-risk roof for which we want to add real leak protection extending beyond that afforded by the roof covering shingles, we'd cover the roof with an ice-and-water-shield type barrier - a membrane that seals around the nails.

Watch out: What you should do before any arguing with your roofing contractor is give the manufacturer of your roofing shingles a call, speak to the warranty department, and ask if the position of roofing felt as we've discussed affects the shingle warranty. It might. I suspect that roof covering manufacturers are thrilled when a product failure claim can be blamed on improper installation - which is in my opinion often the case.

Put another way, if the roofer was inexperienced or uninformed enough to do something funny with the felt, one wonders what other mistakes were made: improper nailing for example.


Roof Underlayment Requirements for Wood Shingles, Shakes, or Clay Tiles

Please see our separate discussions in the recommended reading links below.


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