Should I use glue or powder-actuated nails when framing over gypcrete?

Question

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

Answer 1

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.

Answer 2

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.

Answer 3

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.