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I'm getting ready to install subflooring in a residential basement, which currently has exposed concrete slab. The concrete is well-cured (50 years) with no moisture problems. The plan is to lay down 6 mil polyethylene sheeting, followed by 3/4" AdvanTech subflooring. I plan to use Tapcon screws to fasten the subflooring to the concrete.

Some of the installation guidance I've received seems like overkill. Can anyone with more experience weigh in?

  1. The fastener pattern for each 4'x8' sheet of AdvanTech requires 52 screws, if my math is correct. Do I really need that many?

  2. Someone mentioned that the poly sheet should be glued to the concrete slab. The only reason I can think that would make sense is to keep it from sliding around during installation. (If I really do want to glue it down, any suggestions on a good adhesive?)

  3. Depending on who you ask, there should be a gap of at least 1/4" to 3/4" between the edge of the subflooring and the nearest vertical obstacle. But if the fastener pattern is every 6"-8" along the edges of each subfloor sheet, how could the AdvanTech ever shift/swell enough to require that much clearance?

  4. After I drill the holes for the Tapcon screws, some people suggest squirting roofing caulk (or similar) in the hole before inserting the screw. I think the idea is that it helps restore the integrity of the moisture barrier system around the screw hole. Would that be overkill in this basement?

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  • I'd argue that the subfloor itself is overkill. I put both carpet and tile down on a slab of the same vintage, in Minnesota, and had no regrets. What's your motivation?
    – isherwood
    Commented Dec 11, 2019 at 18:49
  • Multiple motivations: (a) cover old cutback that may contain asbestos; (b) provide flatter surface for future flooring project. The AdvanTech is already onsite, so even if alternative solutions to (a) and (b) look viable, I'm pretty biased towards the current approach.
    – Christian Convey
    Commented Dec 11, 2019 at 18:57
  • Update: questions #1 and #4 above overlap this discussion: diy.stackexchange.com/questions/26714/…
    – Christian Convey
    Commented Dec 11, 2019 at 19:04

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  1. Do you care about the manufacturer's warranty? If no, no. Do you need the moisture barrier, or can you use a small-bead adhesive bond directly to concrete? If no, and previously no, no.

  2. I agree. It would serve no real purpose otherwise.

  3. I agree, but see point one, sentence 1. You'll want to shoot for 1/4" just for ease of fitment anyway, but much larger in a fixed subfloor seems pointless.

  4. Caulk splooshed haphazardly into a dusty hole will do nothing that the concrete itself and the screw threads aren't already doing. You won't end up with anything like a waterproof seal. That suggestion was made by someone who considers "seems like a good idea" as a threshold for viability. I don't.

Disclaimer: This answer is entirely opinion, based on extensive experience, but opinion nonetheless. The question itself is borderline off-topic for that very reason.

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  • Manufacturer's warranty isn't important to me. But if we agree that 52 tapcons per sheet is overkill, the question becomes: what fastener pattern DOES make sense? For the field, I would guess the standard is: at least a few, and as many more as needed to avoid bounce. Not sure about the edges though, especially if I'm already going to glue the tongue-and-groove joints.
    – Christian Convey
    Commented Dec 11, 2019 at 19:12
  • That answer to that depends on whether you're using adhesive, and exactly how uneven the concrete is. With no adhesive, I'd do a 16-24" grid with a few extra on the edges and check for noise when you walk. Add as needed.
    – isherwood
    Commented Dec 11, 2019 at 19:14
  • Have you had a chance to see how well that (approximate) fastener pattern works out in the long-run? I'm grateful for the advice; just trying to understand how thoroughly it's been vetted in the field.
    – Christian Convey
    Commented Dec 11, 2019 at 19:18
  • Actually, I guess this raises a meta-question: Suppose I use a somewhat sparse fastener pattern, like you're suggesting. If it turns out that it was TOO sparse, how long would it take me to realize that, and would there be permanent damage to the subfloor as a result? Because with the flooring not going on right away, perhaps I could go back and add more tapcons later as needed.
    – Christian Convey
    Commented Dec 11, 2019 at 19:25
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    That's my thinking. If there's reliably so little moisture that warpage isn't a problem, you're mostly concerned with noise due to flex over depressions in the slab causing tapping or crunchy sounds. Those would be apparent right away. I'd seriously consider using heavy-duty construction adhesive in a 1/8" bead though, heavier in troublesome areas. It's cheap insurance.
    – isherwood
    Commented Dec 11, 2019 at 19:39

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