Photograph of  really worn out asphalt roof shingles Low-Slope Roofing Types, Product Sources, Installation, Defects, Repairs

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Low slope roofs, inspection, installation, troubleshooting:

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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Low Slope Roofing Materials, Choices, Costs, Life Expectancy, Characteristics, Installation Guidelines

Flat roof over a residential building (C) Daniel Friedman

Definition of Low Slope Roofing

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.

[Click to enlarge any image]

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

Roof slope definitions and illustrations (C) Carson Dunlop Associates

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.

Details about how roof slope is measured or calculated are at ROOF SLOPE DEFINITIONS.

Table of Low-Slope Roofing Options: Pros & COns of Roll Roofing, BUR, Modified Bitumen, & EPDM Roof Systems

Table 2-17: Low Slope Roofing Options (C) J Wiley, S Bliss

The following low slope roof system discussion is adapted/paraphrased with permission from Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

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

Parking cars on a low slope residential building (C) Daniel Friedman

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.[3][4][5]

Minimum Slope for Low-Slope Roof Systems

Low slope or flat roof seen from the Poughkeepsie Railroad Bridge (C) Daniel Friedman

With any roofing material, a slope of at least 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 between 1/4 and 1/2 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.

With 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.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Can We Apply Asphalt Shingles on Low Slope Roofs?

Asphalt shingle application method for low slopes (C) Carson Dunlop Associates

Reader question: 4/21/2014 over at our article BEST ROOFING PRACTICES, Anonymous asked:

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.

As noted at ASPHALT SHINGLE INSTALLATION and in the printed text Best Practices Guide to Residential Construction:

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).

Eaves flashing to a point at least 24 inches inside the interior wall is recommended if there is any possibility of ice dams or water backup from leaves or pine needles.

A conservative approach is to run self-adhering bituminous membrane over the entire lowslope area. Once the underlayment is complete, shingles are installed in the standard fashion.

In cold weather, manual sealing may be required as wind uplift will be greater on shallow roofs (see “WIND DAMAGE to ROOFS).

Reader Question: roof shingles installed "upside down" on low slope shingle roof

(Oct 21, 2014) MaryS said:

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.


Mary S

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.

In the article above we explain that

As noted at ASPHALT SHINGLE INSTALLATION and in the printed text Best Practices Guide to Residential Construction:

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 ?

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 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.

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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