Thickness of concrete base layer for leveling basement floor

Question

TLDR question: How thinly can I feather regular concrete to roughly level my basement floor as a base for self-leveling compound?

Background: I am in the process of re-finishing my basement after addressing the issues we discovered with the PO’s “finished” basement (They slapped bat insulation and paneling directly against the unsealed concrete walls, and over live electrical outlets). Waterproofing, drainage, etc has been addressed.

The basement (23’x17’) has an elevation change of 3.5” from one corner to the other. This is a combination of initial construction slope towards the basement drain (removed during waterproofing) and a more pronounced slope around the drain where some prior plumbing work was done (we assume).

This obviously exceeds the Max thickness for self leveling compound. However, all of the regular concrete lists a Minimum thickness of 2”.

Question: Can I pour a base of regular concrete, about 3” at the thickest, to fill in the majority of the low area and feathering out from there? Then using self leveling compound as a top coat over that?

Also, are there any specific preparations or factors to be considered when layering pours like this?

Thanks!

Answer 1

Old homes sometimes have uneven concrete floors because the concrete was poured in a "U" shape with peaks near the footings. I have heard several versions of the history on this - one is that the floor was poured after the walls and footings, and so the floor is heaped up near the footing to prevent shrinking away as the concrete hardens. It so happens that my 1920s home is built this way, and we found the floor needed to come up about 4" to create a relatively flat floor. Whatever the case may be, there are a variety of opinions on leveling a floor for completing a basement.

a. You could not do anything at all. A 3.5" difference would be dramatic over an 8' span, but over a 23' span it might be negligible. Obviously, you will choose top-of-the-line mitigations for water issues.

b. You could meticulously scribe sleepers to custom fit every segment of flooring. I was curious about that here: Sleepers on unlevel basement floor?

c. A contractor who worked on our basement used rigid foam insulation to bring the floor height up. It was a tedious job and he made masterful work of a knife and level to cut away insulation sections where the floor was high. It was a great solution because it required no adhesion to the slab, they were merely taped to interlock and fitted snugly against the framing. The insulation is inorganic and won't rot if wet, although we put down a plastic vapor barrier anyway to keep water away. I don't recall how the subfloor was adhered to the insulation, I wager it was just construction adhesive.