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I recently finished installing solid hardwood over a new 3/4 plywood subfloor on a concrete slab, and while it looks nice, the installation process wasn't ideal due to variances of up 3/16 to 1/4 of an inch over 8 to 10 feet. I originally wanted the surface flattened, but the concrete contractors were persistent that my installation method of using tapcons to secure the plywood to the slab would fracture the leveling cement. In the end, I decided to just install over the slab as-is.

There's still another area that needs hardwood, and I really want to make sure the area is flattened prior to installation. I spoke to some other concrete contractors who said they could grind down the highest areas, but again, said that using surface leveler is not recommended since it will fracture in some areas if I try to drill and screw through it.

After talking some more, here were my options:

  1. Use surface leveler, and use tapcons to adhere the plywood to the concrete (at your own risk)
  2. Use surface leveler, and float the plywood over the concrete
  3. Don't use surface leveler, and just grind down the high areas (not perfect)
  4. Don't use surface leveler, and use tapcons to adhere the plywood to the concrete. Shim the plywood using 2x4 shims in low areas, and sand the plywood in high areas.

The second option seemed more viable, but there's one issue: I already bought 3/4 plywood for the other room, so I'd have to buy 1/4 and 1/2 plywood to match up to the existing 3/4 plywood I already laid down. I don't know how feasible the fourth option is, but I would be able to use the plywood I already have.

Any suggestions?

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  • a humidity barrier
    – Traveler
    Jun 27, 2022 at 6:05
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    Why is it that option #2 requires 1/4 + 1/2" plywood instead of a single layer of 3/4" plywood? That doesn't make sense to me - what am I missing here? Also, why is that a drawback?
    – FreeMan
    Jun 27, 2022 at 14:49
  • @FreeMan Sorry, I wasn't quite clear here. The 3/4 plywood isn't perfectly flat, where as if I screw 1/2 and 1/4 plywood together, the sheets would lay flat against one another.
    – tripleblep
    Jun 27, 2022 at 20:43
  • plywood is for nail down install? you can use a standard drill bit on SLC and it won't crack. So if you were careful you could first drill through your ply / slc and then hammer drill through the concrete below and likely have the slc hold up. I think the construction adhesive is the better bet though. do you have underlay in your hardwood install or straight nail down? Jun 28, 2022 at 16:20
  • @FreshCodemonger Correct - the plywood is being secured to the slab with tapcons. I'm using rosin paper underneath the hardwood. If I'm understanding correctly, I would first drill through the SLC layer before switching to the hammer drill, correct?
    – tripleblep
    Jun 29, 2022 at 4:20

2 Answers 2

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Use construction adhesive (sub- floor glue) like what is used to glue plywood to floor joists in a typical wood framed floor. It comes in large sized caulking tubes and applies with a large size caulking gun. We typically cut the plywood in half to 4'x4' pieces. Stagger the joints in the plywood and apply good sized dots of glue (about 1 1/2" x 1 1/2" x 1" high) at about 1 foot centers throughout the middle of each sheet and more often at the sheet edges. This will help float/ support the plywood over any low spots in the concrete. If there are any lippage issues at the plywood joints (one sheet higher than the adjacent sheet) you can feather them in with a belt sander.

I agree that self leveling compound on the concrete floor will not hold up.

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  • huh? self leveling compound basically is concrete but with a stronger binder.
    – Jasen
    Jun 28, 2022 at 11:10
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There are a few other options.

You can drill through the SLC before hammer drilling into the concrete. You could do the SLC hole a larger diameter since you'll be anchoring to the concrete. It will help to know the depth of the SLC but you can feel a difference when drilling.

Your plan is concrete subfloor, slc to level, plywood tap conned, rosin paper, nail down hardwood.

You could do concrete subfloor, slc to level, underlay, plywood tap conned, rosin paper and then the nail down hardwood.

If the SLC cracks and you have it sandwitched between a foam underlay and the concrete slab the cracked SLC won't have anywhere to go and even if it did manage to move the foam underlay will prevent any noise. The worst thing is SLC that cracks and moves sandwiched between two hard layers and then it sounds like you are walking on popcorn.

I have a plywood subfloor, SLC, underlay, plywood sleepers screwed through the SLC (I didn't predrill), underlay, engineered hardwood. It has the softer feel of a floating floor but that's what the wife likes.

SLC is painful to install, it won't self level. I used plywood to bring up really low spots and SLC in a couple lifts in certain areas of my install. You need to have the proper primer ( different for concrete vs plywood ) and you should be ready to trowel, screed and spike roll the SLC. Not sure how much of this you are planning to do yourself.

I should mention that I also used a floor edger to remove a high spot in my plywood. In your case I'd use a concrete planner if you have high spots in your slab. I like the Makita 5" concrete planner - it does a nice job of staying flat and the dust collection is excellent with a shop van on the shroud.

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