Masonry fireplace hearth is both floor and projection

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.

Hmmmm…Ya think wiring is considered a combustible material? Panel was installed directly under the hearth extension for a fireplace on the first floor. Hearth and extension both had multiple cracks in the mortar and bricks. You could see light from the first floor when loking up at the underside of the fireplace. There is a fire waiting to happen!


Good post. Thanks.

Thanks for the info.

I have found that most houses have this piece of plywood under the hearth extension. It is important to note where that piece of plywood is in relation to the top of the joist, and therefore the top of the extension. For example, if the plywood is supported by a 2x4 frame at the bottom of a 2x12 joist, there is roughly 7 or 8 inches between the plywood and the top of the fireplace extension (including the brick if it is a brick extension). If it is a brick extension, there is roughly an additional 4 to 6 inches of support material, usually concrete, before the plywood comes into play.