Hearth extension

IRC 2009 R1001.9.2 exception says that

When the bottom of the firebox opening is raised at least 8 inches above the top of the hearth extension, a hearth extension of not less than 3/8" thick brick, concrete, stone, tile or other approved noncombustible material is permitted.

Does the 3/8" thickness mean that the hearth extension should be that much above the floor surface or is that merely the thickness of the material?

This homes hearth had no change in thickness.

Thickness of non-combustible material…

so then it’s simply saying as long as non combustible floor is 3/8" thick then we are good. No reading into it any more than that?

nope that’s it…

That’s it.

Additionally, if this is on a slab, you can use 1/4" tile and still be compliant. The 3/8" minimum-thickness is assuming there’s a combustible material beneath the extension.