How To Look At A House

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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)":

Do flat roofs always leak?

What causes bubble-like blisters in a built-up and gravel roof?  

Why does it cost so much more to replace a steep roof than a low slope roof? 

Why are most house roofs slanted instead of flat?

What is "ponding" on a flat roof?

Why is there no attic access hatch in the house? 

 What is the purpose of the gravel on a flat tar roof? 

Can metal roofing be used on a low slope/pitch roof? 

What is a TPO roof? 

What is the stuff you paint an old mobile home metal roof with to extend its life?

• What is the average life expectancy an EPDM roof? 

 I'm buying a '50s modern house with a "gravel" roof. Is the roof going to be a problem? 

Is "ponding" (standing puddles of water) normal on flat roof?

What is a low slope roof?

• What is the average life expectancy of a built-up roof? 

What is the average life expectancy of an elastomeric roof coating?

 • Can you coat a TPO roof to extend its life? 

What causes ponding (standing water) on a flat roof?

• What are the signs my flat roof needs to be replaced? 

What is the average life expectancy of a PVC roof?

    Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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