How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
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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?
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To learn more about roofs and attics, see these other blog posts:
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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