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Test: How a Fire Pit Actually Affects Composite Decking

There are many references to the fact that using a fire pit on a composite deck can damage the deck, but we could not find any test results that show exactly what using a fire pit on a composite deck will actually do. So we conducted tests of our own on two very different days. We are not a certified testing facility, and our test was probably not strictly scientific. We bought composite deck boards at Home Depot, placed them under a fire pit, and burned a vigorous fire for over an hour. And of course, we did not place a DeckProtect under the fire pit.
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The fire pit for the test (shown at left photographed unused in our studio) was a very typical fire pit we purchased at Home Depot for $89. The feet raise the fire pit bowl about 5 inches above the surface.

Also at Home Depot we purchased Veranda® composite deck boards for the test. Veranda is similar to other brands, but may not be exactly the same as other brands. It is made of "80% pre- and post-consumer recycled wood and polyethylene."
The polyethylene is a very thin layer about 1/32" thick that has the color and texture. It is this layer that is most vulnerable to heat damage.
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Undamaged: The photo above shows a section of the board from our test that was about 10 inches away from the fire pit and therefore remained undamaged.
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Test One - Slight Damage: Our first test was conducted in November in New England on a cold day. I believe the temperature in the air was between 25 and 32 degrees Fahrenheit. We burned a heck of a fire for more than hour to see what damage we could do to the composite deck board directly underneath. The feet of the fire pit were sitting on the board, so the fire bowl was raised a few inches, and there was room for cool air to flow under it. If there was a breeze, it was light. The result shown above was less than dramatic. You can see a smoothing of the surface texture. It is possible that using a fire pit under the same conditions multiple time over the same section of deck would result in a build up of the effect and a more damaged deck. But we did not run that test, so we don't know what using a fire pit on a cold day many times would do.
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Test Two - Visible Damage: Our second test was the same as the first test, but on a very warm afternoon in July. The air temperature was about 75 degrees Fahrenheit, and the composite boards had already been warmed up just by being in the sun. (in the first test, we started with very cold boards.) The board above was under the side of the fire pit, not directly under the center.
Result of using a fire pit on a composite deck
Test Two - Visible Damage: This board was under the center of the fire pit. The result is not as dramatic as the board catching fire, but the blistering from the heat is severe.

Conclusion:
Again, our tests were less than strictly scientific. There are so many variables, I am not sure how a controlled and measured test would be done. Obviously, there is a big difference between a cold day and a very warm day. Heat of the fire is a factor. We used hardwood logs and got a brisk fire going. Duration of the fire would be a factor. There are several other variables including the space between the fire bowl and the deck, whether or not there is a breeze that would replace hot air under the fire pit with cooler air. The cumulative effect of using a fire pit more than once on the same section of your deck is something our tests did not address. Some composite decking has a polyethylene layer, as in our experiment, and some brands have polypropylene or poly vinyl chloride. Melting temperatures vary, not just between different types of plastic but within each type with different formulations.

Therefore we do not pretend that our test is somehow definitive. We wanted to see for ourselves what a fire pit will do to a composite deck and share what we found with our readers.
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