Plywood support for fireplace

[Mark Raumikaitis]

I found this yesterday; the photo is looking straight up from the fireplace ash dump clean-out door (basement). The hearth area was constructed over plywood. The ash dump was full of ash and charred wood so the owners have been dumping for a long time. I do not see any charring of the plywood around the opening. I called it out as a defect due to risk of burning from embers falling through the clean out trap door. What would you guys have done or said? Thank you

[Larry D. Kage]

I don't see it catching on fire (unless the let the ash build up) but it may have been better to form it with cement board.