How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manufactured and modular homes
What is the difference between roll roofing and modified bitumen?
Saturday, June 30, 2018
Big difference. Roll roofing is a lightweight, budget roofing material, available in 36” wide rolls, containing 100 square feet (also called a “square” by roofers) per roll. Average lifespan is about 6 to 10 years, with an average of 8 years. It is usually face-nailed in place, which means that the nail heads are exposed and visible on top of the roofing. The combination of being inexpensive to buy and simple to install makes it a popular choice for sheds, porches, and barns.
But every nailhead is a future leak. A spot of mastic at each nail head, like in the photo above, postpones the problem for awhile. A more waterproof installation method uses a strip of mastic to secure the bottom of each new roofing lap over the previous one, along with spots of mastic over the first row of nails. However, the short lifespan of the material makes it a poor choice for re-roofing a house, and we rarely see it used for residences.
Both types of roofing have a surface coating of tiny rock granules for UV-protection, like an asphalt shingle roof, and both appear similar to each other from a distance. The difference is that modified bitumen roofing is a heavier roof material with a longer life expectancy of 10 to 16 years. It is fully adhered with a cold adhesive spread over the underlayment or secured by heating in place with a propane torch. There is also self-adhering version
Roll roofing and modified bitumen are both suitable for low-slope roofs. Modified bitumen is approved for “flat” roofs with minimal slope, but roll roofing is not viable below a 2/12 pitch (two inches of rise for each twelve inches of horizontal run) with exposed nails, or a maybe little lower with concealed nail installation.
Here’s a bar graph that compares modified bitumen and roll roofing life expectancy to other types of roof coverings.
Also, see the blog post Why is my roof leaking?
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Here’s links to some of our other articles about “FLAT ROOFS (LOW SLOPE)":
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