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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.
Single vs Double Coverage Felt Underlayment on Roofs
Question: 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.
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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.
My 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.
It 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.
In 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.
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.
Question: proper location of air barrier for Hot Climate Roofs
(Sept 25, 2014) Steven said:
Location Central Florida. Inquiring about installing 1.5" foil faced closed polyiso insulation over the top of the existing 6/12 sloping OSB decking. The intention is to take the heat load off the attic space. With concerns of humidity and condensation in a vented attic space, My Question - where should the Air Barrier membrane be installed - on the bottom (original OSB deck) OR above the new decking on top of the insulboard? Any further conserns or comments to be aware of? Thanks
Steven air barriers such as house wraps are used on building exterior walls but not on roofs.
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Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: firstname.lastname@example.org
"Choosing Roofing," Jefferson Kolle, January 1995, No. 92, Fine Homebuilding, Taunton Press, 63 S. Main St., PO Box 5506, Newton CT 06470 - 800-888-8286 - see http://www.taunton.com/FineHomebuilding/ for the magazine's website and for subscription information.
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
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).
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume.
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Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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