How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manfuactured and modular homes

What is the average lifespan of a modified bitumen roof?

Saturday, August 25, 2018

 The expected lifespan of a modified bitumen roof, often called a “modified bit” roof, is 10 to 17 years in Florida, with an average of 15 years. The material is used for roofs with a low slope or nearly flat. It is a roll roofing that is applied with liquid mastic and has a surface finish of small rock granules similar to a 3-tab shingle roof. As the material approaches the end of its lifespan, there is granule loss and cracking of the surface as shown in the photo above. 

   The 10 to 16 year average lifespan estimate is based on “average” conditions. Many factors contribute to a longer or shorter life of the roof; so a particular modified bit roof’s life can vary—sometimes significantly—from the average. 

    This graph compares average lifespan of a modified bitumen roof to other roof types:

    Here’s a list of conditions that affect roof longevity:

•• Color of roof - A dark roof absorbs more heat, which shortens the lifespan.

•• Orientation of roof surface - A roof slope facing south will get more sunlight, and have a shorter life.

•• Attic ventilation - An unventilated or poorly ventilated attic reduces roof lifespan.

•• Quality of roof material - “Economy” roof materials have a shorter life.

•• Installation - Sloppy or improper installation shortens roof life. Manufacturers typically blame the roofing contractor for a roof that has a short life, and they are sometimes correct. 

•• Accumulation of leaf debris on roof - Because this type of roof is often nearly flat, rain does not fully wash away pine needles and leaves, and they tend to build up over time if not removed regularly. Their decomposition creates acidic areas that speed up the aging of the roof. In the photo below, the roof surface is buried under decomposing leaves.

••Trees near roof - Tree branches rubbing on a roof or the acidity from the accumulation of leaf debris on a roof shortens its life. 


•• Ponding - When there is an area of a roof that has a puddle of water that does not drain after a rainfall, it is called “ponding.” Although a gravel roof is watertight when correctly installed, ponding tends to deteriorate the roof in the ponding area over time. Even a “flat” roof is designed with a slight slope for drainage, and ponding is always the result of settlement or sagging of the roof framing, or poor installation of the roofing.

•• Harsh climate - Severe weather, both harsh winters and hot summers, along with big temperature swings within a 24-hour period, also shorten lifespan because of the expansion and contraction of roof materials.

    Go to our blog post What is the average lifespan of the parts of a house? for rating of other house components. To understand the basis, potential use, and limitations of lifespan ratings, see our blog post How accurate are the average life expectancy ratings of home components? Are they actually useful?

• • • • • • • • • • • • • • • • • • • • • • • • • • • •  

  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  and LIFE EXPECTANCY pages for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

NOTE: These life expectancies are based on data provided by InterNACHI, NAHB, FannieMae, and our own professional experience. Because of the numerous variables that can affect a lifespan, they should be used as rough guidelines only, and not relied upon as a warranty or guarantee of future performance.

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