Using Tiles to Protect the Deck
There is a lot of advice on the Internet, not all of it completely reliable. So how about ceramic tiles. They are fire proof and can reflect heat.
To Conduct this test, I laid out nine 12" square tiles centered on Pressure Treated decking, Capped Composite, and PVC decking.
Then I got a fire going in a Breeo X-24 fire pit. And kept a brisk fire going for 3 hours.
The next day when everything had cooled down, I removed the tiles to see how the decking boards were doing. Part of the tile over the PVC board had heat bonded to the PVC and broke when I tried to lift it. The tile centered over the TimberTech capped composite looked like it was going to take a crow bar to get it off.
I turned over the TimberTech capped composite board with the tile still stuck to it. You can see serious warping of the board. Also the heat from the fire pit had gone through the tile and even through the board to melt the polymer capping on the underside.
With the center board removed you can see the damage to the PVC board in the foreground and the charring of the Pressure Treated board in the background.
CONCLUSION: This was not a lab test. It was a test with a particular set of real world conditions. On a colder day, or with a different fire pit, results would be different. So it does not prove that tiles would never provide enough protection. I think it does show that you are risking your deck if you are counting on tiles for protection. Even substantially less dramatic damage might well be something to avoid doing to your deck.