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I'm trying to figure out how to put the vapor / moisture barrier under a monolithic concrete slab. Do I just put the plastic down and drape it over the form, and then cut to ground level when done (like the red line in the picture)? Will it stay attached to the concrete?

Or do I just put the plastic across the bottom and leave the perimeter exposed?

I included a picture to try and explain my question. Hope it's clear.

Thanks!

monolithic slab with plastic sheeting

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2 Answers 2

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Whatever's convenient, really. Tack it to wooden forms with staples and trim it below the form top edge. Drape it and trim it as you fill the forms. Or do as you suggest, though that might make finishing the concrete more challenging.

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  • Thanks for that quick response! Am I correct in what the end result should look like (plastic stops at ground level)?
    – Todd
    Commented Jul 11, 2018 at 19:56
  • Is there a ufer ground? If so the footing or other area needs to be exposed a plastic sheet would prevent contact with earth.
    – Ed Beal
    Commented Jul 11, 2018 at 19:57
  • No special grounding. It's just a storage shed with no utilities.
    – Todd
    Commented Jul 11, 2018 at 19:59
  • I can't say where your poly should end.
    – isherwood
    Commented Jul 11, 2018 at 20:00
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We use to worry about that too. Now we omit the moisture barrier entirely. We’ve found it to be harmful because: 1) it’s useless, 2) inhibits the curing process, 3) creates moisture problems later

1) I’ve never had anyone explain how a moisture barrier is suppose to work when the contractor drives grade stakes through it 10’ on center.

2) if it rains the night before a pour or there is Dew in the morning, moisture will lie on the top of the moisture barrier in pockets. This moisture will cause the slab to cure longer on the bottom than on the top. This requires all the moisture to come out the top of the slab only. This differential “drying” will cause the slab to shrink differently from the top than the bottom.

3) When (and if) moisture comes up from below later, it will seep in around the grade stake holes and then lie on the bottom of the slab. Then the moisture will be drawn through the slab ruining any floor finish. Without the moisture barrier, it will recede when the water table goes down. With a moisture barrier it’s trapped there until it can evaporate through the slab.

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