I've just finished the framing for a low deck that's partially over a concrete slab (see pics below). As you can see, there is minimal clearance under the deck for air circulation.

I'm now thinking I should've laid a vapor barrier before building the framing over the concrete. I'm installing IPE 5/4x6 decking, which I'll face-fasten with 2x SS screws per joist. I'll 'kerf cut' three grooves along the underside of the decking to help relieve swelling in an attempt to avoid cupping.

Do you think it's also worth laying plastic vapor barrier sheeting between the joists to cover the concrete and further mitigate cupping? If so, how would you suggest I fasten it/keep it in place?

P.S. There will be a roof over this, so water from above should be limited/minimal. P.P.S. Any other suggestions before I lay decking would be appreciated!

Thank you!

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  • 1
    I would have advised pouring more concrete (or laying pavers) rather than what you have chosen to do. Given what you have chosen to do, make sure there's plenty of gap between your decking boards to permit some sort of ventilation. And I agree with Isherwood regarding the plastic being worse than useless.
    – Ecnerwal
    Commented Apr 17 at 16:51
  • Thanks for confirming.
    – Texaus_76
    Commented Apr 17 at 19:23
  • Welcome, and please take the tour so you know how to respond to answers. "Thanks" comments are discouraged.
    – isherwood
    Commented Apr 17 at 19:50

1 Answer 1


I would not. Your lumber is treated against rot anyway, and poly will mostly serve to trap moisture and encourage mold and odor.

Let it breathe.


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