InspectAPedia®   -   Search InspectApedia

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

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.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

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 (Steve Bliss, J Wiley & Sons) , 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.

[Click to enlarge any image or table]

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 (Steve Bliss, J Wiley & Sons) .

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

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.

Reply:

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 above illustrates a low-slope asphalt shingle application method permitted by some roofing manufacturers. You will note that this approach will be labor intensive.

Asphalt shingles can be installed on roof slopes of 2:12 to 4:12 if special procedures are followed for underlayment.

Please read details about how to install asphalt shingles on low slope roofs now found

at ASPHALT SHINGLES on LOW SLOPE 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.

Reply:

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 InspectApedia.com 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.

[Click to enlarge any image]

Low Slope Roofing Material Choices

Below we list some low-slope roof systems not already discussed in detail.

See our complete list of low-slope roof types

at LOW SLOPE ROOFING More Reading

Thermoplastic Polyolefin (TPO) Roof Systems

TPO Roofing or Thermplastic Polyolefin Roofing is an example of a single ply or membrane roof covering currently widely used on low slope commercial buildings.

Below you'll find more detail about TPO roofing by looking at these IO Manual(s). Firestone's IO manual below also includes TPO roofing inspection advice.

On 2019-12-30 by George Wilson - example roofing sample TPO Roofing Test Cut

TPO roofing test cut for lab analysis - Wilson G (C) InspectApedia.com Wilson

We are looking for lab to send samples from time to time. I teach a roofing course for industial flat roofing membranes.

This [photo above] is a sample of a TPO cut test.

I show the class cut tests every course.

Please see photo attached. Would your lab consider this a pass or fail? George Wilson, Cindott Inc, 416 606 3270, gwilson18@cogeco.ca. www.roofingtrainingcourse.com

This Q&A were posted originally

at TEST LABS - ROOF SHINGLE

On 2019-12-30 by (mod) - looking for roof test lab to send samples from time to time- TPO test cut

George

We're not a roofing test lab, though we do refer our readers to them.

In my opinion your tape shows a damaged section that would certainly not give a reliable bond.

InspectAPedia.com provides building and environmental diagnostic and repair information. In order to absolutely assure our readers that we write and report without bias we do not sell any products nor services, nor do we have any business or financial relationships that could create such conflicts of interest.


For other readers:

TPO Roofing or Thermplastic Polyolefin Roofing is an example of a single ply or membrane roof covering currently widely used on low slope commercial buildings.

See

TPO ROOFING OPTIONS found in

LOW SLOPE ROOFING - where you'll also find an index to our articles on low slope roof installation, inspection, repair

Low Slope Roofing Manuals & Guide

 


Reader Q&A

On 2020-01-13 by (mod)

There is no true single that will work correctly on a very low slope roof.

However you might be able to nail shingles through a complete membrane of ice and water Shield or you might laminate shingles together to form a sealed surface. My view is that neither of those is as proof as a membrane type roof on that surface. Let's both look further to see if there is presently a manufacturer offering a membrane roof that looks like shingles.

If for cosmetic reasons you must install shingles on a low slope roof, you'd do it over a waterproof membrane using ice and water shield that'd seal around the many nails.

On 2020-01-13 by Joe

What kind of shingle can I use on a carport that fors not allow shingles, because the roof pitch is really low.

On 2018-12-18 by (mod) - plastic roof skates ?

Sorry I don't know what plastic roof skates are.

If you are asking about snow guards that keep snow from sliding down off of a roof, find SNOW GUARDS in the ARTICLE INDEX

On 2018-12-14 by Ken

Can I use an a adhesive rather than screws or nails for plastic skates my roof is 15 degrees

On 2018-05-11 by (mod) -

The article above on this page list and describe several roof covering options. On Residential Properties I've had really good luck with rubber or EPDM roofing.

However the key for success in any of these Roofing Products is that the material must be correctly installed with careful attention to seams and flashings.

On 2018-05-11 by Jamie

I've got a a flat slope roof with TPO material on it now I'm looking to replace it but not sure what to go with

On 2017-07-30 by David

I have a barn that there is only one side of a roof by barn is 26ft long an goes from 0 to 2 ft is that a low slope roof

On 2017-02-18 by (mod) -

"Code" isn't the answer here. For very low slope roofs such as 2/12 shingles will leak or at best have a short life unless laid over a specially-sealed membane.

On 2017-02-18 by David rogers

My roof is 2/12 but there are two valleys 1.5/12 will code allow asphalt shingles

On 2016-12-06 by Patrick Ferris

this is really very informative blog

On 2016-11-04 by lcs

Can you please let me know HOW I can verify a pitch has been added on a new roof IF there are no inspection notes for a TAPERED insulation on file?
In case you are wondering,Thehome is in Miami and I live in another state. I have NO PROOF that a pitch/slope has been added to my roof.

I have contacted the building dept and the chief inspector sent me a copy of the permit application - which verifies NOTHING. I cannot contact the roofer directly since he is the one I am questioning. Thank you

On 2016-02-08 by (mod) - Leaks caused by ice-blocked roof drainage can occur on any roof type.

George:

Leaks caused by ice-blocked roof drainage can occur on any roof type. What you report your insurance company said is technically correct but misleading: you want to mince words, a physical "dam" of ice forms at the lower edge of a sloped roof, causing water to back-up and leak into the structure.

On a flat roof the analagous situation occurs if the roof drain system becomes blocked for any reason: ice freezing in or around the drain, for example. The result can be the same: as weather or other conditions allow on-roof ice or snow to melt but not to drain (as the drain remains frozen or clogged), water may find its way into the building.

Normally a flat or low slope roof will tolerate brief standing water or ponding but deep water accumulating on a roof surface may cause water level to rise above flashed or sealed penetrations and thus to leak.

Your most productive approach may be to drop the "ice dam" term and instead note that leaks and water damage occurred because of frozen or blocked roof drainage. If that damage is covered by your policy that ought to be made clear.

On 2016-02-08 by George

Does ice damming occur on commercial low slope roof? I had 2 drains freeze over. As a result water backed up the I tire length of the roof. This caused many areas to pitch up. These areas then split open letting water into the envelope.

My property is insured for ice damming. The insurance company said that ice damming does not happen on flat or low sloped roofs. I do not agree with their reasoning. The drains clogged and ice started to build up until it reached the other end of the roof. Any thoughts on this would be helpful. Please post a response or email me at theriv310@aol.com. Thanksgiving any help George

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

Reply:

Steven air barriers such as house wraps are used on building exterior walls but not on roofs.

For underlayments used under roofing please see ROOFING UNDERLAYMENT BEST PRACTICES

On 2015-09-29 by David

Any recommendations on creating a steeper pitch on a flat roof?

On 2015-08-19 by Anonymous

roof is at very end of useful life



...

Continue reading at Xspan class="LI_Spaced">ASPHALT SHINGLES on LOW SLOPE ROOFS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

Suggested citation for this web page

LOW SLOPE ROOFING at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING ROOFING

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.
Our Comment Box is provided by Countable Web Productions countable.ca

Comment Form is loading comments...

Citations & References

In addition to any citations in the article above, a full list is available on request.


ADVERTISEMENT