What is the average lifespan of a built-up and gravel roof?
Saturday, August 25, 2018
The expected lifespan of a built-up and gravel roof, often called just a “gravel roof,” is 20 to 30 years in Florida, with an average of 25 years. The material is used for roofs with a very low slope to nearly flat. The roof is literally “built up,” like prepping a pan of lasagna, with alternating layers of roofing felts and hot liquid asphalt (instead of tomato sauce), and the black tar paper felts being pressed into each previous layer of asphalt before it cools and hardens.
The roof layers are called “plies,” and 3 to 5 can be applied, but the average roof has 4 plies and is called a 4-ply built-up roof. The final “cheese” topping is a thick flood coat of asphalt, with gravel raked over and embedded into it while still hot. The layers of roofing felt and asphalt are the actual waterproofing, and the gravel topping protects the black asphalt from the sun’s UV-rays, which would otherwise accelerate the deterioration of the roof surface.
This graph compares average lifespan of a built-up and gravel roof to other roof types:
The 20 to 30 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.
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.
As a gravel roof reaches the end of its lifespan, it looks like the picture below, with areas of missing gravel and exposed roofing felt.
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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