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I am remodeling my fireplace. The hearth and mantle were granite/marble slabs (huge ones cut around the firebox). I removed them from the mantle and hearth with a sledge and chisel, expecting thinset or mastic to be used to attach the hearth stone to the subfloor. Unfortunately, I found this instead:

Concrete weirdness

It's very hard and very uneven. I hardly dent it with a chisel or sledge.

In terms of unevenness, it is 1" from flush at the back (where it meets the wall) and 9/16" from flush at the front (where it meets the hardwood). It's also full of dips and divots and high points. It also appears to be set nearly against the hardwood at the edges, with no spacing for a joint or anything. This home is from the 1950's.

What's the best way to remove and/or level this surface out, so that it is a uniform 3/8" deep from flush?

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If there are high points above the desired end level, use a silicon carbide block (you'll find them in the masonry section, with grooves in the face and a big plastic handle on the back - roughly 4x8x2 inches) to grind them down. They work surprisingly well. If the floor you are trying to match is level, then a self-levelling underlayment product (generally a form of gypsum cement) is the typical approach to make the surface level and even. If the floor is not level and you need to match it's non-level-ness 5/16" down you will need to fabricate a screed to run on the floor and level 5/16" below the floor, and use something less self-levelling, like mortar, so it stays put when screeded.

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Thanks for your answer. What about the fact that the self-leveler would make contact with the finished hardwood floor edges. Is that a problem? –  Haney May 27 at 2:00
    
Probably not really (the stone appears to have been, and the base under the stone likewise), but if you desire an extra degree of care you can use a strip of something around the edge - foam sill-seal, weatherstripping, 2-3 layers of resin paper... –  Ecnerwal May 27 at 2:05
    
What if I caulked the edges to create a "lip" in which the self-leveler would do its thing? Level it up to 3/8"... Then when I tile I can caulk those edges again with caulk that matches the grout for a nice clean look. Thoughts? –  Haney May 27 at 2:08
    
Probably fine. Effectively putting in a rubber boundary (assuming a rubbery caulk) - but sill-seal or weatherstripping might do the same thing for you faster and easier. I think either works. –  Ecnerwal May 27 at 2:12
    
Last question before I accept your (awesome) answer: if I ever had to pull and redo the hardwood floor (as hardwood, tile, or even carpet), would it be possible to do that without screwing up the hearth were it connected via caulk or even the self-leveling compound? –  Haney May 27 at 2:28
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