I was reviewing an inspector’s report due to a claim, and I realized that there was a little confusion about what a hearth is.
For masonry fireplaces, a hearth includes both the floor of the fire box and the projection in front of it. The hearth should be made of masonry or concrete. Should be supported by non-combustible materials. And a hearth should be reinforced to carry its own weight and all imposed loads.
There should not be any combustible materials remaining, installed up against the underside of the hearth and hearth extension after construction. That would be a potential fire hazard that ought to be reported IMHO. I look for that piece of plywood under the hearth extension, as seen from the basement or crawlspace ceiling.
The hearth should be at least 4 inches thick. The hearth extension should be at least 2 inches thick. But there’s an exception.
The hearth extension must be 2 inches thick unless the bottom of the fireplace opening is raised at least 8 inches above the top of the hearth extension. In that situation, the hearth extension thickness can be reduced to 3/8-inch thick. Brick, concrete, stone or other approved noncombustible materials can be used.