Edit: Resolved. Decorative brick is actually a chase and doesn't need to be completely filled in. Thanks to @Death_Equity@lemmy.world for advice on rebuilding the crown.
I have a double sided wood burning brick fireplace which is honestly just a big collection of code violations. It was converted to propane shortly after -- I'm assuming -- the people who built the house 50 years ago discovered that it smoked badly in the house. I fixed all the major issues with the firebox and smoke chamber and converted it back to wood burning this past winter.
I had a new steel cap fabricated to replace the old brick and concrete cap, which I removed. During this process, I discovered that the brick is basically just a facade. There are huge gaps on either side of the flues which are encased in 4" cinder blocks. My understanding of fire code (at least in the US) for masonry fireplaces is that the material surrounding the flue is not permitted to have any gaps in it. NFPA says all gaps in the brick should be filled with mortar. However, I'm not sure what to do with a gap this large. I'm not sure if filling it with mortar or concrete is an option. I've considered installing a steel flue liner but those are expensive and there are two flues. I'm also concerned about steel liners changing the draft characteristics since the chimney is on the short side.
If money were no object, I would tear this thing out and put a more efficient steel insert in but that is cost prohibitive.
Anyone have any experience fixing something like this?
That makes me feel better. There was no crown initially. The flue tiles terminated flush with the brick inside the chimney cap. The old cap made cleaning and inspecting the flues difficult. It also had a fair bit of water damage.
What would I use as a base for the new crown?
Yeah, those suck. Good call on ripping it off.
I would use a grinder to put rebar down over the gap, set into the bricks, put cement board on top of that which has a 1/2" overhang onto the brick. I am extra and would do a rebar every brick across the short side. From there you can built a mold to pour a rebar and concrete slab with a 2" overhang, or no overhang if your rain cap can accommodate the tile extensions. The overhang will help to protect the mortar joints of the brick, so if your cap has less drip edge, I would just pour with the overhang. You do want to make sure the concrete is shaped to allow for water runoff, water sitting on the crown will fuck shit up faster if it sits. Use some foam seal around the tiles to give some relief for expansion and contraction. Seal over the foam with an exterior polyurethane sealant so water will have a harder time acting on the joint between the tile and the concrete.
If you elect to have the tiles with their own individual rain caps, I would suggest adding a sand cap over the concrete to act as a sacrificial wash. You'll want to slip a tuck pointing trowel up against the tile to give some expansion room. Seal that wash to the tile with polyurethane as well. You don't need more than an inch thickness, tapering to 1/2" at the edge. You will have to replace/patch that sand cap every so often, so make sure to check on it. Most of the time you just need to patch it with the leftover sand cap thinned out and applied with a brush, but you can use mortar as well.
This is great! Thanks so much for the advice.