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Slab-on-Grade Retrofit

blakeedward | Posted in GBA Pro Help on

Hi, I’m about to undergo a garage conversion.   Including a slab on grade retrofit, where PVC drain pipe will be added beneath the slab.

The plan is to saw and then jack hammer the path where the drains are to be added.  There is a taped and sealed Stego radon barrier as well as 2-4″ of XPS beneath the slab.

My question is if any one has done anything like this.  And has techniques that might be used for repairing the Radon barrier and foam when reassembling and patching the concrete.  Or alternate strategies to have a functional radon barrier when all is said and done.

I should also note there is an existing passive radon ventilation system already in place.  And no historical issue with high levels in the space.

Additionally if anyone wants to weigh in on the debate regarding untreated wood being laid direct on a slab.  With a 15 Mil vapor barrier and 2-4″ of foam in place.  It would be appreciated.

Thanks !

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  1. Expert Member
    PETER G ENGLE PE | | #1

    Depends on where the radon barrier is located. If it's under the foam, you can probably save most of it and simply patch and tape it when you button everything up. If on top of the foam, it's going to get pretty beat up when breaking up the slab. Still, your contractors can do their best to try to protect it at least at the edges. Lay in a new piece and use liquid flashing to seal the two together if necessary. Same flashing to seal around the penetrations in the membrane for utilities.

    For the foam, simply cut pieces to fit approximately in the trenches, then seal the gaps with canned foam. Easy stuff.

    I'm a bit of a water Nazi, so I still prefer PT lumber in contact with a slab, even a mostly dry one. I say "mostly" dry because all slabs will get wet sometime. If not from outside, then from inside. PT plates give you a bit more resiliency and time to discover and correct the leaks before damage accrues. Then again, I do believe the risk of using untreated plates is relatively low, in the big scheme of things.

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