Hi there,

I tried the search function and didn't seem to see anything that specifically addressed this, so here it goes. I am in looking to float 5" solid oak over a concrete pad. This is a DIY job, and my 3rd hardwood install (1st involving a concrete pad).

To prepare the concrete pad, I have:
- Installed a 6mil poly vapour barrier
- Intend to lay down 1/2" ply
- Intend to glue and nail 3/8 ply perpindicular to the 1/2" with construction adhesive and 3/4" 18-guage brad nails
- Intend Staple roofing paper underlay to the ply
- Nail oak flooring using 1 1/2" cleats

My main question is making some minor adjustments to the flatness of the concrete pad using wood shims on top of the vapour barier to ensure a flat plywood subfloor. I have a 2' wide by 4' long section against one of the walls that seems to dip about 3/8". I assumed (perhaps incorrectly) that since I would be installing a subfloor anyway, that I could obtain the same flatness by shimming the plywood instead of leveling the concrete pad with some type of leveling compound. I have been even considering using some rigid foam formed into a shim in lieu of the wooden shims. Considering I would ideally be floating the floor, do you see any issues with this procedure? Any recomendations in general on the installation technique that I have proposed?
Any advice and recomendations would be greatly appreciated.

Regards,

M

I don't see anything wrong with using wooden shim shingles to take up that irregularity along the wall.
Also though, you could use 1 3/4 inch cleats since the sub-floor penetration using 3/4 inch flooring is less than the thickness of the sub-floor you are installing

Thanks Dennis,

Thanks for the comment on the shims. As for the cleats, are you saying that since I am using nail-in wood anyway, it would be redundant using the brad nails because the cleats would tie the hardwood and plywood anyway? Also, any opinion on rigid foam as a shim instead of the wood shingle shims?

My only concern about using shims, foam, cardboard or what have you, to fur up a low spot on this particular installation, is the possibility of the furring material shifting during or after placement of the plywood. There is also a possibility of that area sounding more hollow than the rest, since there is a void under the area. My preference would be to use a cement based patching compound. Since you are installing a floating subfloor, the patched area does not need to be finished perfectly smooth. All that is needed is to be flat to the rest of the slab.

You do need to attach the two plywood panels together so they work in conjunction with each other. And you do not want places where the top layer isn't solid to the bottom. That could cause a lumpy substrate. Some construction adhesive and short screws seem to be the favored approach.

In reference to the cleat length, it was merely an observation that you should (or can) use the maximum length cleats short of full penetration when installing over plywood on concrete, and 1 3/4 inch will give you better holding power