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I'm having a new concrete floor poured in. its going to be 4 inches.

The drywall that will be in contact with the cement is green board(moisture resistant).

Will that be ok? or should I cut the bottom 6 inches of drywall and put pressure treated boards?

closed as unclear what you're asking by isherwood, Tester101 Jun 30 '16 at 14:19

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  • Images would help... – tahwos Jun 30 '16 at 20:27
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Assuming that this is a new 4 inches of floor on a ground level structure...

No this is definitely not ok.

Greenboard is mold resistant. It is, however, still just gypsum with paper over it.

Neither is using pressure treated wood as a barrier.

Why?

Concrete wicks water. Any water near the area will be distributed to be brought into direct contact with the gypsum or wood. Gypsum and paper, when left exposed to moisture, falls apart. Wood, even pressure treated, will rot eventually.

This is why sill plates, which sit on top of foundation walls that can be several feet above ground, are protected by moisture barriers.

You need to put a waterproof (PROOF, not resistant) substance between the new concrete and existing walls, OR lift the walls up (not likely feasible).

  • I agree about the drywall, but in my part of the world it's routine to use PT lumber directly on concrete. We don't have houses rotting away. After 20 years or more they're fine. – isherwood Jun 30 '16 at 13:30
  • It's also code in NA to use a moisture barrier (such as poly, or other impermeable materials) between the sill plate and the concrete. – Chris Jun 30 '16 at 13:42
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    @isherwood just because it is done that way, doesn't make it the right way to do it. – Tester101 Jun 30 '16 at 13:42
  • @Chris What is "NA", and what source are you citing? I've been out of the business for a handful of years and may have missed some recent changes. – isherwood Jun 30 '16 at 13:48
  • @Isherwood NA as in North America (well, US, and Canada) PT is also code, but as this would suggest, technically, PT is worse than untreated directly on concrete (in terms of absorption, not mold growth, it's unquestionably better than untreated). – Chris Jun 30 '16 at 14:49