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Bald, worn-out organic asphalt roof shingles Double vs Single Underlayment
Roofing Felt Installation

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Roofing felt underlayment layers:

Single layer vs. double layer or double-coverage roofing underlayment are defined here. We explain when double-coverage of felt underlayment is recommended and we describe how to figure out if your roofer installed a single-layer or double coverage layer (or maybe no layer) of felt under the roof shingles.

This article series describes roofing underlayment materials, practices, codes & standards.



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Single vs Double Coverage Felt Underlayment on Roofs

Details of installation of double coverage roofing felt on a low slope roof, adapated from Best Construction Practices, Steven Bliss, J Wiley & Sons (C) J Wiley S  Bliss D Friedman InspectApedia.comQuestion: contractor says he used a double layer of felt on a low slope mobile home roof. How do I know?

Great site! 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 a 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.

Without opening up this roof, how do I tell which story is true? - Reader T.L., Tacoma WA by private email 2017/11/06

Reply: Check the gable end edges of the roof

It's possible that your city inspector saw something that you didn't. Perhaps drip edge is installed only on the lower edges and not at the roof eaves.

But it's easy to determine if there is just a single layer vs double-coverage installation of roofing felt by observing the amount of overlap of the roofing felt plies. My illustration above, adapted from Steve Bliss's Best Construction Practices is discussed in more detail at ROOFING UNDERLAYMENT BEST PRACTICES.

[Click to enlarge any image]

To inspect for the presence double-coverate roofing felt you'd need to inspect a representative area of the roof that's accessible such as at a gable end or at the eaves (lower edge).

Working with care with a flat bar (so as not to tear shingles), gently lift the shingles and felt up from the sheathing to see the number of felt layers.

Bald, worn-out organic asphalt roof shinglesMy photo of an asphalt roof installation in process shows that the contractor has installed a peel-and-stick waterproof "ice and water shield" type product along the roof eaves in a 36" wide strip.

Above that is an incomplete-double-coverage layer of 15-pound roofing felt. I've marked in red the lower edge of each course of roofing felt.

You can see that only the first two courses of felt (above the peel and stick ice & water guard membrane) seem to display a roughly half/width exposure. The roofer was not "stretching" his felt in this job, however, and this roofing felt installation is perfectly correct.

That's because on roofs like the one in the photo,with slope of 4:12 or greater, only a single layer of felt is required under asphalt shingles. In a single-coverage felt installation the lower edge of each course overlaps the previous course by a minimum of 2".

[Click to enlarge any image]

How to Recognize that Double Coverage Felt is Actually Double Coverage

Your best view is from the gable end of the roof as there you can see how double-coverage felt would have been installed.

Our photo (below, courtesy of reader T.L.) illustrates the gable end of a doublewide home located in the Tacoma Washington area. The roof is low-enough slope that double-coverage would have made sense.

Gable end of a doublewide home in Washington State gives a spot to check for presence of roofing felt (C) InspecApedia.comIt looks as if there's no drip edge at the gable ends of the house, so if you need to make a determination for sure, your main task is not to fall off the ladder while looking under the shingles at the felt.

Assuming your roofing contractor is using standard 36" wide 15# roofing felt, "double coverage" means that each layer of felt put down across the roof laps 19" over the upper half of the previous course.

(The first course starts with an extra 1/2 wide run). So anywhere along the gable end that you can see the felt you'll see two layers.

You ought to be able to see that with almost no disturbance of the roofing material.

If someone installed the drip edge on top of rather than under the felt along the gable end then you won't be able to see the felt without bending and fouling up the drip edge.

If you encounter that problem you'll need to work carefully with your flat bar to loosen a length of drip edge to pull it out of the way to look at the felt edges.

Watch out: if your peek under the asphalt shingles on your roof displays only bare roof deck (that is you see plywood or OSB with no covering of felt underlayment (or covering of a self-adhesive peel and stick membrane) then that's not the best roofing job.

At ROOFING FELT UNDERLAYMENT REQUIREMENTS we explain when, where and why underlayment is required.

Does Double Coverage Felt Reduce Roof Leaks?

Peel and stick ice and water sheild membrane installed on a new roof (C) Daniel FriedmanIn my view two layers of felt atop roof sheathing would not significantly improve the leak resistance of an asphalt shingle low sloped roof.

I italicized atop because on very low slope roofs some shingle installation applications specify interlaying a course of felt underlayment, perhaps using 30# felt, between every shingle course as part of making a shingled roof on a low slope a successful installation.

In any case, roofing felt alone is can not make a roof waterproof. The felt is punctured by thousands of roofing nails as the shingles are installed and is thus not a waterproof membrane.

For a waterproof roof underlayment the installer would have needed to use an ice and water shield product that seals around every penetrating nail.

My photo at left (with my roofing sandals in photo left side) shows a granule-covered peel-and-stick adhesive-backed "ice and water shield" membrane that the contractor, Eric Galow, installed over the entire roof surface of this home.

Ultimately we planned to install a standing seam metal roof (you can see some of that in the photo as well).

But we needed to make the building water-tight to permit interior work to proceed before the metal roof had arrived at the jobsite. So the whole roof was covered with an adhesive-backed waterproof membrane.

Ice and Water Shield™ is a trademarked name for adhesive-backed waterproof underlayment.

Note: Although its use is nearly generic, Ice and Water Shield™ is a trademarked name for an ice and water barrier adhesive membrane produced by WR Grace.

Owens Corning's Weatherlock Mat Ice & Water Barrier is a similar product. Other adhesive underlayment product names include Buster™ Peel & Stick Underlayment (rubberized asphalt), Titanium Interwrap™, First Step™, and Weatherlock™.

Most peel and stick roofing membranes are designed for use as an underlayment to be covered by a finish roofing material, but some such as MFM Peel and Stick Self Stick white roofing material often used on mobile homes are designed to be left exposed to the weather.

Smaller widths of the same or similar materials are marketed for use as flashing tape around windows and doors and skylights, such as Velux Adhesive Underlayment (around skylights), described at PEEL & STICK FLASHING MEMBRANES - live link given below.

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Continue reading at ROOFING FELT UNDERLAYMENT REQUIREMENTS - home , where we describe methods for application of felt underlayment under asphalt shingles on a low slope roof. , or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see LOW SLOPE ROOFING

Or see PEEL & STICK FLASHING MEMBRANES

Or see STANDARDS for ROOFING

Or see these

Roofing Underlayment-related Articles

Suggested citation for this web page

UNDERLAYMENT DOUBLE vs SINGLE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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