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Unread 12-01-2019, 09:50 AM   #1
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Granite slab hearth spanning concrete and subfloor

For a wood stove installation, I'd like to put a 1.25"thick 25" deep granite slab over an existing 17" deep concrete hearth extension base. The extra 8" of depth will extend over a 3/4" plywood subfloor. The subfloor surface is roughly 3/16" lower than the concrete. It is sturdy and well framed below. The slab will be the same width as the concrete (55").

Would it be adequate to simply put down a layer of thinset on the concrete, and a significantly thicker layer on the subfloor to make up the 3/16" gap... assuming I put a couple hard points/shims there so it wouldn't 'sink' and force the thinset out and become unlevel before it has a chance to harden?

Would appreciate any advice or alternatives (short of reframing and repouring the concrete - it would be invasive of a project underneath)... or specific details of what to use (type of thinset etc).

Attached a picture (prior to cutting back the flooring in front, but showing one peice of flooring removed to the side to show the subfloor).

Much obliged!
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Unread 12-01-2019, 10:51 AM   #2
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So, I assume you still have to remove some of the hardwood. I would use filler to fill the lower area to get the whole surface flat. Once it's set, stick the whole thing with thinset.

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Unread 12-01-2019, 07:25 PM   #3
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Yes that's right - just haven't removed the flooring yet in the pic. What would you use specifically for a filler? Does thinset not act as a filler itself, or would you use some thicker type of mortar?
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Unread 12-01-2019, 11:10 PM   #4
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The problem with using thinset mortar is shrinkage. It will shrink more in the deep areas. So, if you filled everything in one shot so that it was perfectly flat across everything and you dropped the slab on it, the thicker areas shrink more than the rest and you’d lose support in those areas.

There are plenty of floor patching compounds that would work well. Fill the deep areas to get everything flat. Then use a modified thinset mortar of your choice to set the slab.

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