I can cite NFPA code ref if anyone is interested; temporary wood forms should not remain under the hearth, or the hearth extension. (Sect 801.3, 803.1, or some such; would have to look up.) I've seen it problematic twice. Once, the charred form under the hearth worked its way into the wood under the hearth extension, the part visible in the basement ceiling; it was one continuous piece of plywood, and a charred, scary-looking piece at that. Just a month or so ago, the wood was so charred right under the firebox it could have ignited at the next fire; ash-dump chute was 98.36% full, and a fire waiting to happen; again, the form under the hearth and the hearth extension was one continuous piece of ply. (Easily seen with a mirror into/under the ash-dump doorway in that case.) Older fireplaces have continuous wood planks running from under the visible hearth extension to under the hearth itself; argument that if the wood under the hearth ignites won't be a problem since it's *contained* is bogus; can easily spread to the exposed wood under the extension and burn the house down. Over time, the wood forms suffer pyrolysis from the fire's heat, and the wood can turn into virtual flash paper. Just 'cause "it's been fine for forty years" (or whatever) don't mean squat; the longer the wood has been there, the more the risk. And, concrete hearths can crack, and an ember falling through such a crack can ignite the form. Reason the wood can't remain under the hearth extension as well is the gap that can form where the hearth extension, under its own weight, tends to often rotate down and away from the hearth surface; again, an ember falling through the resulting gap here can ignite the wood below. A "fire" guy at a continuing ed class about 20 years ago in St. Louis asked a roomful of us inspector type folk how many of us checked this. One guy raised his hand (liar). After the fire guy took us on-site to see a recent house burn-down 'cause of this, I suspect most of the other guys in that class now flag this as a hazard.