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We're doing some renovations and got to the flooring and I'm unsure how to proceed, so I'd love some input. A few things for context:

  • House was built in 1961
  • Single story
  • Raised foundation
  • 3/4 plywood subfloor
  • Subfloor isn't particularly flat, and has ridges in some places where joints butt up against each other
  • Asphalt mastic (or asphalt cutback adhesive, whatever it's called) containing asbestos is applied directly on the plywood subfloor

I would like to encapsulate the mastic/cutback that contains asbestos. We are planning on putting down LVP flooring, and there's a slim chance we'd maybe install engineered hardwood in the future. Because the floor isn't flat, I can't put LVP directly on top of the subfloor, and because the mastic/cutback contains asbestos I can't (shouldn't) sand it down and not many products are approved to stick to it with a plywood subfloor.

I came down to a couple of options for my situation after quite a bit of research:

  1. Purchase (very expensive, and the only product that I can find that says it can do this) PerfectPrimer to paint/encapsulate the mastic. Then pour self leveler on top of that. My concerns: expensive, my understanding is that floor leveler should have lath which means it's going to be a lot thicker, and I'm not confident of my ability to do this at this scale.
  2. Put down plywood underlayment over the entire floor. My concerns: 1/5" plywood underlayment stapled down seems to be an okay solution, but I'm concerned about the ridges/unevenness telegraphing through and still not quite sure how to handle flattening it out, but at least I'd have a clean surface to work with that I might then be able to throw self leveling compound on here and there instead of the entire thing. (It occurred to me that maybe going with a thicker plywood and screwing it down might help solve this issue).

If this was on concrete, it might be a little simpler, but the fact that it's plywood is throwing me for a loop. Any suggestions out there, or anything I haven't considered? I've been going around in circles about the best way to tackle this. Thanks!

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    The good thing about asbestos is if you don't bother it by doing something like sanding/trying to remove it, it is quite safe. Depending on how high the ridges are, thick(1/2 or 3/4) enough plywood might be enough to have a flat floor.
    – crip659
    Commented Apr 23, 2023 at 19:41
  • How much flattening does the floor really need? The new flooring itself counts as an encapsulation layer. I'm looking at dropping a floating floor directly on top of a layer of suspect linoleum (to replace the ancient carpet in that hallway); and calling it good enough.
    – keshlam
    Commented Apr 23, 2023 at 19:42
  • @keshlam That's a good question. I haven't gotten a straightedge yet to actually gather that data, just have been eyeballing it. Perhaps a thick enough LVP would suffice, and would do enough for encapsulation, but I like the idea of another layer.
    – Matt
    Commented Apr 24, 2023 at 23:24

1 Answer 1

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Floor leveler does not need lath and is super simple to do. The drawback is it can be expensive depending on how many bags you need.

If any ridges or dips are no thicker than a nickel, you can put down an underlayment such as this: Underlayment

There are other products that can be used as well. I chose the QuietWalk as an example.

Remove the baseboards. Install the underlayment with seams taped. Then the flooring. Reinstall the baseboards and the floor is good.

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  • Regarding floor leveler, thanks for the feedback. I thought I had seen that installation instructions required lath for one of the products, but perhaps I was mistaken. That underlayment looks like an interesting option, I think I need to do some more digging rather than just eyeballing it to get a better idea of what needs to happen.
    – Matt
    Commented Apr 24, 2023 at 23:25

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