Filled ash dump chute with concrete: Can I do concrete all the way up?


New Member
Jan 8, 2018
Dayton, OH
Hi folks,

The plywood forms were never removed from under the hearth when the home was built :eek:, and major demolition would be required to access the ash pit to remove the plywood. (The pit is roughly 2x13 feet of floor area and a couple feet high, making it almost impossible to reach the top except in one or two areas.)

I want to make the fireplace safe to use and ultimately install an insert. The original construction was 12" concrete followed by two courses of CMU (no mortar), a layer of common brick, mortar, and then the firebricks. To prevent embers from igniting the wood, or the construction debris left in the pit that I can't easily reach, I placed a CMU cap at the bottom of the ash dump chute and poured all the way up to the base of the common bricks with solid concrete--about 28" deep.

In hindsight, it probably would have been better to use gravel or sand--something with a lot of entrained air--for part of the fill to improve the R-value. If I simply replace the existing firebricks and ash dump door, there's a good 2" air gap above the concrete. If I were to install a firebrick instead, I'd need to fill up those last few inches. I have plenty of common bricks and quick-setting cement as well as some Rutland Fireplace Patch mortar. Other possibilities include sand or pea gravel which would provide more of a thermal barrier between the firebrick and concrete. The easiest thing to do would be to stick a brick in there and slather on some mortar on top, then place the final firebrick.

Is there anything I can do to improve the safety of this setup as I finish? I have no idea how hot the base firebricks get in operation. Does using an insert improve the picture by removing the need to have the entire system up-to-code as an operational fireplace? (An insulated liner attached directly to the insert exhaust port removes the need to restore the surrounding masonry in the chimney, so do I need to fully bring the firebox up-to-code to use an insert?) #1 concern is making sure sufficient heat can never get down to that plywood to start a fire. I am overthinking this? (Wouldn't be the first time.)