In Mikes picture, you can see the cut-out for the ash-dump flapper assembly that's put in the bottom of the hearth. Over time, the wood dries-out from the fires, and the wood undergoes pyrolysis, and the wood's kindling temp lowers. A spark or an ember falling down the chute could touch the edge of that cut-out area and readily ignite that wood. And, since that same wood form typically runs/extends into the basement or crawl area, under the hearth extension, the entire house can burn down. I've seen it. Guy teaching a *fire* class in St. Louis about 15 years ago took a group of us to a nearby house to see the remains, and explained how they knew the wood form was the culprit. I also saw scorching like Mike mentioned, but much more wide-spread and definitely heat/fire related scorching; scorching had extended to the wood under the hearth extension. Wasn't caused by the ash-dump chute location, though; the concrete hearth had cracked (like most all concrete cracks), and an ember had fallen though crack and atop wood form. After looking at this for a while, it was obvious how the wood got scorched. I always call it out, and I use NFPA 211, Section 8-3.1 as my code reference. Same reads: "Masonry fireplaces shall have hearth extensions of brick, concrete, stone, tile, or other approved noncombustible material properly supported and with no combustible material against the underside thereof. Wooden forms used during the construction of hearth and hearth extension shall be removed when the construction is completed."