As part of a basement remodel, we tore up the old (1925) wood flooring. It appears they built it by pouring concrete, then setting 2×4 floor joists in the concrete. The subfloor and finish floor were then built on the 2×4’s. The joists had a lot rot. I plan to pour concrete on top of the existing concrete for the new floor. So my question is..would it be better to cleanup and etch the existing concrete and pour directly on top or to level the floor with pea gravel, lay a moisture barrier and then pour a thin (2-inches?) slab on top of that? Or is there a better way?
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We'll see what some of the more experienced people say. Since no one has answered yet I'll get you started.
It is not a good idea to put pea gravel or anything else that can hold water between two basement slabs. It can fill up with ground water and create a permanent reservoir of water. Lay a moisture barrier on the existing concrete and place new concrete right over it.
Pea gravel anyway should never be used under concrete of any kind, since it won't compact and keeps moving (like a layer of marbles) it forms a very soft and poor support.
I can't say for sure if 2" of concrete is sufficient over an existing slab, but my impression is that if the existing floor is fully settled, even with the wood in it, it would work fine. Probably you should use wire mesh which would reduce the chance of cracking where bridging the wood. However, in a slab that thin you'd have to take great care for the mesh to end up centered in the slab.