Filling and capping gap around round chimney liner

Question

I would like to fill and cap the last few inches (is there a standard how deep?) between this r6" ound chimney liner and the bricks around it. I was planning to make a base with plywood that matches the currently empty space between, essentially a square with a 6.5" hole (leave 1/4" spacing) in the middle, suspend it a few inches below the top on 4 pieces of wire from two pieces of rebar sitting across the top of the chimney, them pour concrete on top.

Since the plywood base would be permanently trapped inside, my question is is plywood the correct material considering the general fire hazard of a chimney?

Answer 1

Disclaimer: I'm no chimney pro

My brief research indicates that cement backer board seems to be pretty fire resistant, I'd suggest using that instead of plywood. While plywood might be sufficient, I'd think cement board would be better.

The strength of the cement board may be questionable when laid horizontally and expected to support weight, so maybe a doubled layer. If you're going to suspend it, maybe use eye bolts through both pieces of cement board with large fender washers on the bottom to spread the load and help keep it from breaking.

Answer 2

You can corbel bricks or tiles mortared in place to reduce the gap, and you can support mortar or concrete on hardware cloth (galvanized or stainless steel wire mesh), so long as it's not too wet.

I would check the instructions for your chimney liner, as direct contact with mortar/cement may not be advised, (due to potential corrosion) and a metallic cap may be preferred or specified as a result.