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Question DetailsAsked on 7/27/2016

can you pour concrete over rough slate so you can lay stone on the new concrete floor

refinishing 4x9 area at front door. Slate is old, rough, discolored and bad shape. Easiest thing to do is pour about an inch slab and then after dried and cured decorative patio stones, size of a brick, will be placed on it with mortar up to the height of the door sill.

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2 Answers


Guess you could, assuming the slate is picked at to remove any loose pieces and well cleaned to provide some bonding - if I were going to do this I would be approaching it differently, like a tile job.

First - 4x9 foot area is about 1 hour removal time because all the existing mortar does not have to come clean off the subfloor sheathing because you are going right back on with mortar - so not a big job to just take it out and start with a proper subfloor surface, which is what I would do.

If I was leaving it in for some reason, I would put down a reinforcing metal mesh power nailed with stucco nails with washers (to hold the mesh well) into the subfloor through the grout joints (to avoid cracked and displaced slate pieces lifting the mesh), then put down a mortar bed of correct thickness to ultimately bring the finish floor to proper elevation (usually 1/4-1/2" below the doorsill) - as a single layer scratch-coat mortar bed, not a thin slab then a separate bedding layer for the pavers. Then thinset the stones in and grout them. Always better to have a thicker scratch coat than two separate ones that can separate if something heavy is dropped on it, like a delivery item or furniture or such.

Note I said thinset - I would not normally use a masons mortar for this - in an indoor area like this, why use outdoor materials which are not as good at keeping the pavers in place ? Thinset is MUCH better at adhering to the bottom of the tiles/pavers. Then mortar or grout the joints as desired for appearance - grout again generally being more resistant to wear and staining, though eitehr should be double-sealed in this application.

Answered 3 years ago by LCD


One thing I did not mention is the stair/floor height issue - will have an inch or couple inch transition difference in floor height if you overlay rather than replace, and if you have stairs coming to this entry the stair heights will be off and need adjustment, because the height of step differential between steps will be far more than allowed.

So in long run, probably as cheap or cheaper to just tear out the slate and start from a bare subfloor - especially if your stair treads are not easily removed or have railings mounted in/through them.

Answered 3 years ago by LCD

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