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plywood over cement slab floor extension

tcleveen | Posted in Construction Techniques on August 15, 2011 02:35am

I’m pushing out a wall two feet beyond existing wall. The existing room is 50’s construction era with plywood underlayment and is carpeted. The extension floor will be this 24 inch wide slab out onto an existing concrete patio (no crawl space) and will have no weather protection as I’ll lose the eve overhang. This slab will be pretty much at outside ground level and water does sometime pool on the patio not far from this new 10 foot long extension wall. Your thoughts on water issues around where the wall ends (can’t elevate slab much or the height won’t match existing room floor level). Also will want to recarpet this two foot by ten foot area of floor just as the rest of the room is already wall to wall carpeted. Want to put plywood over new concrete slab and carpet over it. How should plywood be attached to concrete ( don’t want to wait weeks before concrete  cures for either screw fastening or concrete nailing). Is there any way to prep slab for plywood overlay when pouring this new slab? Is there something else I can use than plywood? Do I have to use pressure treated plywood ( I don’t like the idea of introducing a somewhat toxic material into the family room area where a lot of time is spent). I coluld just leave it a slab but that change from plywood in the rest of the room to concrete doesn’t appeal.

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  1. Pinkyringz | Aug 16, 2011 11:06pm | #1

    I hate to be the bearer of bad news but laying plywood over a concrete slab is never a good idea. I would recommend pouring your slab 2 - 1/8" lower then your existing floor. Frame a floor over the slab using 2x4's laying on their side. To this affix 5/8" tongue and groove plywood subfloor.  This makes repairs and venting easier and broadens your range of floor covering options.  Extra steps are required for proper venting and moisture barriers but overall you get the idea.  Best of luck!!

    1. tcleveen | Aug 17, 2011 02:50am | #2

      thanks for that suggestion though means a lot more digging than planned. Tjought pressure treated plywood could be appleis on concrete with little risk of dry rot over the long haul

      1. calvin | Aug 17, 2011 05:57am | #3

        tcleveen

        How do you plan to isolate your new slab from the patio slab and the rest of the water that will want to wet the slab, wick the moisture back in under your new wall and create a perfect spot for mold growth under your carpet?

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