Flat Roofs

There are several common types of flat roofing materials, although no roof should be truly flat. All roofs need some pitch to allow water to drain. Generally, a roof with less than a 4-in-12 pitch (4’ in height for each 12’ in length) is considered a flat roof. Greater than that is considered a pitched roof. The reason asphalt shingles are not normally used on lower pitched roofs is due to the ease of lifting shingles by strong winds and water entry from wind-driven rain.

The most common flat roof materials are rolled asphalt (aka modified bitumen), rubber membrane (EPDM), and metal.

Rolled Asphalt Roof

Rolled asphalt roofing materials are similar to asphalt composite shingles except that they are manufactured and installed in rolls, whereas shingles are cut into various shapes and orientations. The general overall makeup is the same. Rolled asphalt roofs typically last approx. 8~15 years depending upon manufacturer of the material, pitch of the roof, and grade of the material. This also assumes proper installation and regular maintenance. Rolled asphalt roofs are generally the cheapest type of flat roof. A UV paint can be applied over a partially worn rolled asphalt roof to help prolong its life. The UV paint is not meant to make the roof water-tight but instead help reflect the sun’s UV energy which deteriorated asphalt roofs (shingles and rolled) over time.

When a roof has a low pitch, sunlight and rain water can sit on the roof for an extended time. This can cause faster material deterioration. This material is essentially a felt backing with asphalt and mineral granules. Since roofs generally will first leak where there is a joint or penetration, it is essential to properly seal the seams where one roll meets the next as well as where chimneys and roof vents penetrate the roof. As part of regular maintenance, these same areas also need to be regularly maintained. Over time, as a wear characteristic, the granules can come loose and expose the felt. Once this happens, water and UV light can speed up the remaining material’s deterioration and lead to leaks. Regularly resealing rolled asphalt roofing can extend its life and help seal out water.

EPDM/Rubber Roof

Rubber membrane (EPDM) roofing is a very good water sealing material, but as mentioned above, seams and roof penetrations need to be properly installed and maintained. Instead of long narrow rolls, rubber membrane is often supplied in large squares or rectangles. This material will generally last 10~20 years, although some EPDM materials (a special type of rubber membrane) can last much longer.

Most often where parapets, vents, or chimneys meet rubber roofing, the installer will roll the roofing material up the penetration maybe 12”. This prevents a termination from being at a point where water can easily enter.

EPDM is a black, single ply rubber roofing membrane. EPDM stands for ethylene propylene diene monomer, which describes its chemical makeup. EPDM is considered the premium single-ply rubber membrane. EPDM is also very durable. It is very resistant to UV damage and needs no other sealant over it. This material typically wears about 0.001”~0.0015″ per year and, since it comes in 0.045” and 0.060” thicknesses, theoretically, it should last for several decades assuming regular maintenance. EPDM does not like petroleum products (oil, grease, etc.), however, as these can damage the rubber material, although it is rare that these two will meet.

As you can see, there are a wide array of choices when covering a low pitched roof and costs, life expectancy, and appearance can vary. As with any roofing system, regular maintenance now can provide many years of protection from the elements and expensive repairs down the road.

As part of a home inspection, the home inspector could report on the accessible roof covering(s) such as their type(s), condition, any damage found, etc. Many homes have only one type of roof whereas some homes have 3 or 4 types of roof coverings. In some cases, the roof may not be safely accessible or visible, such as 3 story flat roofs or in times of rain or snow cover.

“This Old House” also has a good Flat Roofs article.

I also wrote a Common Roofing Types article.

Matthew Steger is a Certified Level 1 Infrared Thermographer, an ASHI Certified Inspector (ACI), and an electrical engineer. He can be reached at matthew@thehomeinspectorsnotebook.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Matthew Steger.