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Missing Vapor Retarder Under Slab

joe_fb | Posted in General Questions on

Adding on (w/ full basement) to house in eastern MA. Very well draining soil.

Foundation walls are up. Foundation guy poured the slab today and forgot the poly sheet underneath. Probably not a full 4″ slab. Probably only 2″ gravel underneath. Addition is maybe 300 sqft

Main (existing house) is 1960s, no with no poly under the basement slab obviously. But we’d like to finish the new basement.

Should I have him remove the slab and start over? That would entail jackhammering near the new footers. Or is there an alternative?

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  1. bcade | | #1

    Jackhammering is always an option, however an alternative is to do a topical vapor barrier. These are used when converting existing garages with no vapor barrier into living space. One example is Redguard

    1. joe_fb | | #4

      Thanks for this. Any idea what the long term durability of redguard is?
      We were going to paint the floor and throw down an area rug for the next couple of years. My son is thinking indoor soccer. Would this really hold up to foot traffic? From teen boys?

      1. bcade | | #16

        I've never seen it left as the finished surface so I couldn't say. Per the manufacturer: "Do not use as a wear surface; the membrane must be covered with tile or other permanent flooring."

  2. steve41 | | #2

    I'm not sure which path I'd take, though it would be somewhat dependent on the relationship I had with the slab contractor. I would have to believe they could grid the floor off, drill some holes for eyebolts/lifting, and diamond saw it into easily removable chunks.... without any jackhammering.

  3. Expert Member


    It's maybe not the way you intended to do it, but there a good solutions that incorporate above slab poly, and the foam I would have added too.

    1. joe_fb | | #5

      The new sump pit for the exterior drain is in this area. So I'm not sure I can build up the floor near that - do I just sort of ignore that small area?

      I hadn't planned on finishing this right away - it was a down the road project. I think the key question for me is will ripping out the slab potentially cause more problems than not dealing with it.

      1. Expert Member
        BILL WICHERS | | #6

        I don't think the sump pit is a problem. The built up floor will just look like higher walls on the sump pit. Just make sure the float switch on the pump is set to turn the pump on before the water level gets up to level of the original floor and you should be fine.

        If your intent is for the interior basement floor to be able to drain into the sump pit, then something like the Dricore basement subfloor tiles may be a good option for you, because the air space they create under the subfloor would allow for a drainage plane that would drain into the original sump pit without issue.


        1. joe_fb | | #7

          Makes sense.
          The sump is for the exterior drain which is required by code. The existing basement is dry. The entire neighborhood is basically on gravel. I don't anticipate groundwater problems so the basement floor shouldn't need a drain path to the pit.

  4. Patrick_OSullivan | | #8

    Congratulations. You just got a free slab.

    I'm slightly kidding, and I would be willing to work with a contractor whom I had an otherwise good relationship with, but it's a big-ish mistake in that it's a code required item that can't be easily retrofitted. If it's not being replaced, it needs significant compensation in lieu thereof.

    That being said, lot of us (including you) have existing basements without poly. With a finished floor, it's mostly a non-issue, because you can add a vapor barrier, either via poly on top of slab, or by using a vapor impermeable finished floor. In the less finished portions of my basement, I have PVC floor tiles that are typically used in garages, but they have a nice side effect of being a vapor barrier. The particular product I have, for reference:

    Side note: Where's the permit and inspection process in all of this? I think of MA as having pretty diligent code enforcement. In NJ, you need an inspection right before pouring the slab, and effectively the only thing that inspection confirms is subslab poly.

  5. gusfhb | | #9

    No poly and maybe not 4 inch?

    What is wrong with people?

    Mass has better code enforcement than say North Dakota, but many small towns have limited resources and basically only pay attention to the skilled trades.

    Look this guy owes you, a real inspector would make them jackhammer it up
    Not like this is a new concept. I put poly underslab in my garage floor in 1987

  6. Expert Member
    Michael Maines | | #10

    It's an egregious oversight for a trade professional to "forget" the vapor retarder, and not code-compliant:

    Code also requires slabs to be at least 3.5" thick:

    It sucks for them but I would insist they tear it out and do it properly. I would also add sub-slab foam insulation while I was at it.

    1. joe_fb | | #11

      Agreed, it's terrible.
      Michael, if I have them tear it out, I'm concerned the jackhammering could damage the new foundation and footers or (worse) the old foundation and footers.

      What should I do/ask for to ensure the foundation will be safeguarded? Meaning, how will I know nothing has been damaged in removal? I'm concerned the cure for this might be worse than dealing with it.

      Presumably the ground under the slab is no longer virgin ground after this and will have to be compacted, yes?

      1. Expert Member
        BILL WICHERS | | #12

        Have them saw cut a deep line parallel to the footers and slightly inside of the innermost edge of the footers. That will allow the majority of the slab to be removed first, then the perimeter can be removed in chunks "with care" ("care" is always relative with concrete demolition) from the immediate area of the footing. This should protect your foundation walls and footings from any potential issues from destruction of the slab.

        I agree with Michael too, it's best to have them remove the incorrectly installed slab and do it properly. The general rule in construction (and engineering too) is: "if you build to the plans, and the plans are wrong, it's the designer's/owner's problem, and fixed at their expense. If the plans are correct, and you fail to follow the plans, then it's your responsibility to fix, at your cost.". I would withold payment from your concrete contractor and have them correct this at their expense. If they offer any resistance here, you could politely mention to them that they didn't build it to code, in two different ways (lack of a vapor barrier, and insufficient thickness), which pretty much guarantees they would lose a lawsuit against them. Use that threat as a last resort though, try the friendly approach first, but be strong and insist they do things right.


      2. Expert Member
        Michael Maines | | #13

        Do you know if they included reinforcing in the slab? How recently was it poured? How recently were the footings poured? Concrete typically reaches about half to two thirds its ultimate strength in 7 days and by 28 days it's at close to full strength, so the sooner you can break up the slab, the better, and the longer the footings have been in place, the better.

        1. joe_fb | | #14

          There is no reinforcement in the slab (rebar, mesh, etc). It's just concrete.
          New slab is 24 hrs old.
          New walls are about 2 weeks old.
          New footers are 3 weeks old. Lots of rain has spaced these out.
          The old stuff is 60+ years, but the footer on the old part is almost non existent.

          1. Expert Member
            Michael Maines | | #15

            In that case, I'd follow Bill's suggestions and get it done ASAP--the sooner the better. I hate wasting the carbon emissions but you still have a chance to get it done right.

  7. walta100 | | #17

    Just to be 100% clear this was not an accident. This is their standard operating procedure bid low install 1/2 of the code required concrete and if anyone happens to catch then claim it was a mistake. My guess is they got paid upfront and theirs a snowballs chance in hell that they will remove and replace the slab.


    1. joe_fb | | #18

      GC is removing it. Not sure why not the foundation guy. I will let y'all know how it goes. Looks brutal.

      There are some saw kerfs that look like they must go into the footer, so I hope we aren't turning this into a structural problem.

      1. Expert Member
        DCcontrarian | | #19

        That's the GC life.

        My bet is he's done with the foundation guy, he'll have someone else pour the replacement floor.

      2. Expert Member
        MALCOLM TAYLOR | | #20


        "Not sure why not the foundation guy"

        Probably doesn't have any of the necessary tools. His inventory is likely just a power trowel, a bull-float, a wheelbarrow, and some shovels and rakes.

      3. Patrick_OSullivan | | #21


        Side question: How did you find out there was no poly underneath?

        It's a funny situation in that if it was realized right at pour time, there is a chance it could have been fixed on the spot. On the flip side, if it was discovered after the fact, it's somewhat surprising that someone was honest enough to admit the omission.

        1. joe_fb | | #22

          Caught the tail end of the pour. They admitted they didn't add one.

          They didn't say this, but my understanding is omitting it allows the slab to cure faster and they can get home sooner. Lovely.

          1. Expert Member
            DCcontrarian | | #24

            Concrete mixed properly is pretty stiff. If you add extra water it becomes easier to pour and work, but isn't as strong. When it cures the extra water isn't absorbed, and puddles on the top. Unless there is some way for that water to drain into the soil -- which a vapor barrier would block.

            They knew exactly what they were doing.

            In the scheme of things, a half-thickness pour of overwatered concrete is probably adequate for a basement floor that's only going to see residential use. What I'd worry about is them cutting corners elsewhere on the foundation where it really matters.

  8. joe_fb | | #23

    So - slab removal is in progress, but several cuts have been made into the footer. Probably 1-2" deep saw kerfs in a 10" thick footer. This seems bad.

    I think I need to bring in a structural engineer now to say what to do next, and probably the building inspector.

    What should I imagine for recourse and remediation here?

    A vapor retarder under the slab and a damaged footer seems like a bad trade.

    1. Expert Member
      BILL WICHERS | | #25

      Where are the cuts? If the cuts are parallel to the foundation, you may have a problem. If the cuts are perpendicular to the foundation and spaced periodically (such as every four feet), they probably aren’t an issue.


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