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Community and Q&A

Drainage mat below basement slab, to replace stone

Matt Berges | Posted in GBA Pro Help on

I tore out my damp musty basement floor, and we have excavated down 16″ to gain some head room, and install 4″ foam, below the new slab. I would typically use 4″ of gravel as the base, but am trying to find information about the use of drainage mat, or soil gas mats instead (to save labor and head room).

I realize the stone serves two purposes (capillary break and air space for soil gas control). The sales people I have talked to from a drainage mat site, says I would just need 12″ of it around the perimeter, but I was thinking I would need it under the entire slab (to replace the stone).

So I would potentially lay this drainage mat all around the perimeter (right in the sand), with a 4″ t fitting for soil gas exhaust pipe, then lay 4″ of foam, then 6-10 mil plastic over the foam (for extra air sealing of radon I hope), and then the new 3-4″ floor.

Any thoughts?

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  1. Audetat | | #1

    Unless you have high ground water, a vapor barrier and more than enough insulation should be good.

  2. Riversong | | #2

    A drainage mat around the perimeter may be effective if there's an inside drain pipe to receive the flow, but it does not take the place of subslab crushed stone (not gravel) as a capillary break and radon mitigation medium (with vent pipe through roof).

    I would recommend 2" of pea stone if you don't have enough room for a full 4" of 3/4" washed stone, and the use of reinforced vapor barrier material such as Tu-Tuf.

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