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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  To learn more about roofs and attics, see these other blog posts:

Why is my roof sheathing sagging between the trusses?

Why is granule loss a problem for an asphalt shingle roof? 

What are the mistakes to avoid when doing attic improvements?

What causes roof shingles to curl up at corners?  

What causes shingles to buckle along a line on the roof? 

• Why is my roof leaking?

What causes leaks at a fake roof dormer? 

What causes a sagging roof ridge line?

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? 

What is "ponding" on a flat roof?

Is an attic required to have a light by the building code? 

How can I inspect my roof for hurricane damage?

Why is premature curl of roof shingles a problem?

How can I tell if a roof has more than one layer of shingles? 

What are the common problems with attic insulation? 

What is the life expectancy of an asbestos cement shingle roof? 

What's the average lifespan of a roof?

Why is it a mistake to replace a roof and not replace its flashings? 

Why is there no attic access hatch in the house?

What is the building code requirement for an attic access hatch, scuttle, or door? 

Does a roof with multiple layers of shingles last longer?

What can I do to prevent roof leaks?

Are roof trusses better than roof rafters (stick framing)? 

Why is a popped nail in a shingle roof a problem? How do I fix it?

What are the most common problems with wood roof trusses?

What causes a lump or dip in the roof? 

If my roof is not leaking, why does it need to be replaced?

How can I be sure my roofing contractor got a permit?

How many layers of roofing are allowed on a home? 

What are the dark lines running parallel to shingles on my roof?

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

How can I make my roof last longer?  

What are the warning signs of a dangerous attic pull-down ladder?

How can I find out the age of a roof? 

Should I buy a house that needs a new roof?

Should I buy a house with an old roof? 

What are those metal boxes on the roof?

What does "lack of tab adhesion" in an asphalt shingle roof mean?

Why do roof edges start leaking?

Why do my dormer windows leak? 

Do home inspectors go on the roof? Do they get in the attic?

Should I put gutters on the house? 

How much of a roof truss can I cut out to make a storage platform in the attic? 

What's the difference between an "architectural" and a regular shingle roof? 

What does a home inspector look for when examining a roof? 

Do stains on the ceiling mean the roof is leaking?

How can I tell if the house needs a new roof?  

 Why does my homeowner's insurance want a roof inspection?

What are the hazards to avoid when going into an attic? 

     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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