This article defines low-slope roofing and describes low-slope roofing materials, choices, installations, inspection, defects, roofing repairs, and product sources.
We discuss the following: Definition of low-slope roofing. Low slope roofing inspection, design, leak detection, repair procedures. Recommended materials to use on low-slope roofs. How to install asphalt shingles on low-slope roofs. Low-slope Roofing Material Choices.
A "low slope roof" is one that slopes 2" in 12" to 4" in 12" of run.
Because low slope roofs also drain water more slowly than moderate or steep sloped roofs, systems such as slate or asphalt shingles that rely on mechanical drainage for successful performance, are not used.
Instead we use sealed or membrane type roofing systems similar to those used on "flat" roofs.
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According to ARMA, the Asphalt Roofing Manufacturers Association, "The two broad categories of asphalt roofing systems for commercial, industrial and institutional buildings are Built-Up Roofing (BUR) and Modified Bitumen Systems (MBS). "
But a wider range of materials has been used on low-slope roofs, as we describe in the articles below.
Summary of Definitions of Roof Slope Types: Flat, Low Slope, Steep Slope Roofs
The illustration at left, courtesy of Carson Dunlop & Associates, summarizes the ranges of roof pitch or slope for flat, low-slope or conventional or "steep slope" roofing.
What is the Difference in Slope Between Low Slope & Flat Slope Roofs?
Actually most flat roofs are not dead flat and in good design also include slope towards their drains.
Flat roofs (0" to 2" in slope) are flatter than low sloped roofs and pitch just enough to drain water. In our photo (left) the roof slopes less than 1" per foot - notice that dark ponding area at the center of the photo.
Most roof coverings can be applied on roofs as shallow as
2:12 as long as a fully waterproof membrane is installed
over the decking.
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In this case, the finish roofing material,
whether asphalt shingles, wood, or tile, functions mainly
as a decorative element but also helps protect the underlying
membrane from UV radiation and physical damage.
At slopes lower than 2:12 on residential structures, the
primary roofing options are built-up roofing (BUR), often
called “tar and gravel,” modified bitumen, and EPDM (see Table 2-17 above).
In addition, a handful of proprietary single-ply
roofing membranes designed for easy application to small jobs
have entered the market and offer a few new choices. While
some of these products look promising, how long a new
product will perform over 20-plus years is uncertain.
Why Low-Sloped & Flat Roofs Become Leaky
Even if a low slope roof was properly constructed when new, a combination of age, sunlight exposure (UV-radiation), temperature variations, a history of leaks, and gradual sagging of the supporting wood structure increases rooftop ponding, drainage defects and thus the frequency of leaks.
Flat and low slope roofs and the roof drainage system should be inspected annually and any leaks or drain clogs repaired.
That's my Saab at the right, parked on the flat roof of a private residence located on the West bank of the Hudson River immediately south of the Tappan Zee bridge. Like the TappanZee, this older roof may be due for replacement soon. Certainly unless a building was designed to carry the extra weight, the added loading of vehicles parked on the roof would be expected to contribute to ponding and sagging.
For a roof-recover that is keeping the low slope structure, a roofer may add tapered insulation and then a new roof covering of EPDM or other roofing material.
But in some climates such as areas of deep snow cover, the property owner may prefer to convert to a more steeply-sloped roof structure - a procedure we describe at LOW SLOPE ROOF CONVERSION.
Minimum Slope for Low-Slope Roof Systems
With any roofing material, a slope of
1/4 inch per foot is recommended to promote
drainage and minimize ponding. Where deflection from
snow or other live loads is a concern, a greater slope will
be needed to prevent any ponding.
Most manufacturers of
low-slope roofing products specify a minimum slope of
inch per foot in their warranties.
While membranes, such as vinyl or EPDM, are unaffected
by standing water, it will shorten the life of asphalt based
materials, such as BUR and modified bitumen.
any roofing material, ponding of water increases the likelihood
of leakage, increases deflection in the roof framing,
and contributes to rooftop growth of mosses, algae, and
other plant life. Also, the freezing and thawing of ponded
water can harm most roof surfaces.
My roof pitch is 3/12 but the roofing company use PABMF001-34192, laminated shingles, a product of Pabco Co. Now my roof be blow back leaks occur when a shingle rain seal is breached.
Are they re-roof with this kind of shingles above correct for my roof pitch less than 3/12?
Please help me to research there defects.
Use of conventional roof shingles on a low slope roof invites leaks from wind-driven rain or similar weather problems unless the roofer takes additional, special steps to prevent the problem using extra layers of felt and roofing cement.
The answer to "can we use shingles on a low slope roof?" is Yes, ... and No. Or as Mark Cramer says, "... It depends."
NO: We do not apply asphalt shingles nor other shingle types on low slope or flat roofs using ordinary shingle installation methods because lacking adequate mechanical drainage, such a roof installation will be leaky and short-lived.
YES: But Carson Dunlop's sketch (left) illustrates a low-slope asphalt shingle application method permitted by some roofing manufacturers. You will note that this approach will be labor intensive.
My neighbor is having a new roof installed and not by an established company, more like friends of a friend.
Because the pitch is a low slope roof, the installer is installing the 3-tab shingles in reverse of the normal way, leaving the black solid part of the shingle exposed, instead of the 3 tabs (which are being covered by the above shingle.
This does not seem right at all. He said he is a roofer and because of the low slope roof, this is the way he installs shingles. Opinions wanted, thank you.
The roof installation you describe is fundamentally incorrect and will give a short life and a leaky roof.
1. The upper portion of roof shingles are not intended to be exposed to the weather. Rather they are expected to be covered by successive upper roof courses of shingles. This portion of the shingle is not finished with the same weather protection as the exposed shingle tabs.
2. The self-sealing properties of the roof shingles may not work properly when installed in this upside down orientation.
3. When installing shingles on a low slope roof, other methods are required to avoid a leaky short-lived roof.
Asphalt shingles can be installed on roof slopes of 2:12 to 4:12 if special procedures are followed for underlayment (see “ ROOFING UNDERLAYMENT BEST PRACTICES” or see page 54 in the printed text Best Practices Guide).
Question: is there a double layer of felt on my low slope mobile home roof ?
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?
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 as we explain in detail at UNDERLAYMENT DOUBLE vs SINGLE, it's usually pretty 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 at the gable end of the roof.
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: email@example.com
"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: firstname.lastname@example.org. 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.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
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.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones