Looks like an undercut saw is one way to go when meeting a fireplace.

My fireplace has a tyndall stone slab about a foot above the ground. The slab is held up by three plywood boxes each with tyndall stone facing.

Because of the boxes, there are four inside corners in the spaces between the middle and side boxes. It looks to me like a round undercut saw blade wont be able to do these corners. Also, the stone facing is of varying depths with a jagged appearance.

If I use an undercut saw, wont I cut through the stone facing and the plywood box? If so, what holds up the fireplace ledge?

Since the stone is only facing, I assume glued to the plywood boxes, what about using a chisel to pry the facing pieces off, then lay the wood right to the now square plywood (I guess leaving a 1/4 inch for expansion), then cut the stone facing pieces the required space shorter on a rock saw table, then glue them back on above the wood?

My Crain angle grinder style undercut saw, will do it.

You were kind enough to help me with an earlier inquiry and I purchased an angle grinder with a 4.5 inch blade. I think it came with a masonry blade.

I am not sure how I prevent it from cutting too deeply and damaging the plywood. Do I just, "be very careful"? or do I "don't care" because whatever plywood I cut off I can replace with a similar sized piece?

Nor what I do about those inside corners between the boxes.

My Crain angle grinder style undercut saw, will do it.

OK, I see from the product information that it does do inside corners.

1. If I cut through only the facing stone, the stone under the cut line will still be glued to the plywood. So, is the plan to cut right through the plywood box too? If so, then do I later repair the plywood boxes by replacing the strips cut out by this saw? The plywood boxes appear to be what hold up the stone ledge.

Or does the thing somehow eliminate everything from the floor to the cut line. If so, then I would just go only as deep as the stone but not as deep as to cut into the plywood.

2. How tall is the 812 assuming the blade height is 1"?