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I have a current gypcrete-over-radiant-over-foamboard-over-slab floor in my unfinished basement. I'm about to frame it out to finish it.

I've already located all the tubing, and I do have spots where I can nail or screw most of the walls down. Three questions for the experts:

  1. If I want to use powder-actuated-nails into the gypcrete, I can't buy nails long enough to go through the gyp and insulation all the way into the concrete slab. Since these are non-load-bearing walls, seems ok if I just get the nail into 1.5" of gypcrete to hold them in place?

  2. Would you apply an anti-fracture membrane (redguard or similar) under the sill plates in order to minimize spalling where the nail goes through?

  3. On the couple walls I need to glue down, I've read that an anti-fracture-membrane first and then construction adhesive will work. Does anyone have products they recommend that work together? (the AFM with the adhesive, like would loctite PL stick to redguard?)

My other option is 5" or maybe even 6" screws which I think will get all the way to the concrete slab, but that seems like a lot of effort for 1300 sqft of walls. If I have to use screws, what screws do you recommend?

I did send email and a phone call to Maxxon, and they never replied, and my heating contractor doesn't have any info either.

Thanks, --Carey

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I will agree with isherwood regarding the nails to jiggle loose rather fast, as the gypcrete doesn't have the compressive strength to hold them. The power-actuated is a no-no, as the holding capacity of those nails relies on 2 main factors:

  1. sintering between the steel in the nail, and the concrete, achieved thanks to the high temperature during the split second when you drive the nail in (it's a sort of "welding" between the steel and the concrete)
  2. friction between the concrete and the nail, as the concrete is trying to "pour" back where it used to be before it was displaced by the nail.

As gypcrete is more brittle and has lower compressive strength, it won't hold the nail at all. Furthermore, powder-actuated nails are design to be driven anywhere between ⅝ and 1" into solid concrete (⅝ into higher PSI concrete, up to 7/8 or a full 1" into softer C15 concrete). In the lower possible setting (using a green cartridge with a tool that allows you to adjust power, such as the Hilti DX351), you would still punch through the track and sink into the soft gypcrete.

I recommend that you bite the bullet, and drive long screws, such as Tapcon concrete screws, or Hilti's Kwik-Con. Probably even Grainger's regular concrete screws would do just fine, as you just need them to hold against shear loads.

Source: was a technical Product Manager at Hilti for 10 years.

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  • 1
    did you mean "sintering" by the way? – ThreePhaseEel Aug 29 '20 at 0:36
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    THanks FrK, appreciate the detailed reply, love the science of the nail holding power! I will get some long screws, and save the powder for spots where there is actual concrete. – Carey Kloss Aug 29 '20 at 18:25
  • Yes, @ThreePhaseEel, excuse the misspelling. English isn't my mother tongue, and I left the business quite a few years ago. :) – FrK Aug 30 '20 at 8:04
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Both. A few nails to secure the location of the walls while the construction adhesive cures. Nails alone are likely to jiggle loose over time due to the soft nature of gypcrete. Penetration doesn't need to be more than an inch or so.

Technically, construction adhesive isn't to be used as a structural fastener, but it'll do fine if augmenting occasional nails. Use more nails near door openings.

I'm not sure about adding a membrane. I'd try a few nails and see what happens. My hunch is that gypcrete is soft enough that it won't be an issue.

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  • Thanks, good point. No reason not to do both :-) I can also attach the boundary walls to the poured concrete basement walls in a few spots to make sure they don't move. – Carey Kloss Aug 28 '20 at 16:47
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Both your answers are right but I think they are missing context.

#1 yes screws are more optimal... But you will damaging the gypcrete as much with screws. So you are basically doing more work for slightly slightly more hold.

#2 "Gluing" isn't a long-term thing for a basement. You should not be relying on glue for keeping walls steady. As the poor substrate and moisture will certainly cause failures.

I would just get some longer nails and pop them 3 to a board as I have done in 50+ basements. The key to keeping basement framing true and to keep it from moving is to make it tight. Nailing to the floor or to the joists should only be a "precaution". The floor should have a few "spikes" in it and if my wall is even a smidge loose I will shim it on the top. Spending time on securing your basement framing footers is fine with infinite time but is probably the least useful.

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  • Thanks DMoore. Just to confirm, I can't get nails that will go all the way through the gypcrete to the slab, but your comment is that as long as it's tight, ie I should be tapping the wall into place with a sledge, and shimming the top if there's any extra space, then a few nails into the gyp with or without glue is fine? – Carey Kloss Aug 29 '20 at 18:23
  • They don't sell 3.5" nails? That's odd. Let me tell you screwing the bottom plate for a basement might take you hours based on the type of concrete and it is hard work and work that could result in chips. I would never do it as there is no pay off. Yes if you can't get long enough nails just go into the gyp... I always try to make sure that 2-3 vertical boards on each section have to be hammered over. You "knock" a bottom plate you should have to hammer pretty hard - before it is fastened. – DMoore Aug 30 '20 at 6:50
  • I think there are 3.5" nails, but only for the super expensive hilti guns. Worse, my gypcrete is sitting on at least 1" of foamboard, so the total depth with the 2x4 is 4" just to reach the concrete. I can nail the boundary walls to the concrete walls instead of the floor, and so I only have a couple interior walls which I will probably start with glue/nails and then finish with a couple screws for safety based on the answers here. Thanks for replying! – Carey Kloss Aug 30 '20 at 18:06

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