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Different ways to level existing concrete basement slab for new floating floor assembly?

adepaoli | Posted in General Questions on

Seattle, WA 4c

Trying to figure out a good way to keep an existing basement slab, and build a floating floor assembly above. The current slab (if you would even call it that) is of various condition and build – when the house was first built in the 20’s there was no slab, and between now and then different sections have been filled in with different thickness/mix of concrete, and there is even one small bare patch of dirt. the slab/floor also has a variance in elevation/height by some 2-3 inches. Originally we were thinking of taking this slab out and replacing it with a new slab (level grade, gravel base, poly, 6″ foam, and new concrete). But now I’m trying to save a few bucks and use as little of new concrete as possible so I thought this could potential be a good place for a floating assembly (gravel base, vapor barrier, less foam [save some head height], 3/4″ subfloor, and 1/2″ uponor quick trak panels[radiant heat]).
Two main questions:
1.) Am I right to think this is a possible assembly in this situation? No load bearing walls will fall on the new floor assembly, just some new partition walls, and some appliances/furniture. One concern of mine is I’ve read that for a floating subfloor to work it is important to have well compacted soil below – since I don’t know what the condition of soli is below the mess of existing slab. 

2.) What is a good way to establish a level base for this assembly –  can gravel be levelled and built off of, or should I pour some concrete over the existing slab to get a level base?


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  1. Expert Member
    Michael Maines | | #1

    I understand the appeal of working with what you already have, but in this situation it's almost certainly a good idea to tear it out and start over.

    1. adepaoli | | #2

      I was worried this would be the response haha - thanks Michael

      In that case, should I still make the argument for a new concrete free assembly? The client seems set on wanting a concrete slab given it's the conventional approach, and that we are either going to install in-slab hydronic or other form of radiant floor heat. But if we did remove the existing slab, regrade, then add gravel, vapor barrier, rigid foam, and floating subfloor, that could be a better alternative (in terms of cost and less embodied carbon, not necessarily client relations) than to install a new concrete slab, no?

      1. Expert Member
        Michael Maines | | #3

        Just a few days ago a builder convinced our client and me that a garage foundation and slab was too shoddy to work with, which in their case means the oversized 3-car garage will have to come down, so the issue is painfully close at the moment!

        On the same project, we are removing the basement slab in the house and plan to add drainage and radon mitigation, insulation, a vapor retarder and a floating slabless slab, with in-floor heat. If you have a builder and code enforcement on board with the slabless slab idea, it can save money and carbon, but it's not a hill I choose to die on--on another project, the builder is not excited about something that novel so we are keeping it simple with a conventional concrete slab and floating finished floor, with otherwise good detailing. Pick your battles!

        1. adepaoli | | #4

          Thanks again. Curious as to what in-floor heat you are spec'ing and why?

          1. Expert Member
            Michael Maines | | #5

            I'm not sure yet, I discouraged it but my clients want it and my mechanical engineer thinks it will be good for the space. It's a walkout basement with a 14' sliding door so in-floor heat will probably be a good thing.

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