Please someone help me!
I want to install non-engineered oak hardwood floors over an above-grade cement slab, but there's a problem with the slab.
The exterior walls sit on cement footings. The slab floor was poured within the footings and is "floating".
Here's the problem. After 50 years, the slab has settled so that it is about 3/4" lower than the footings. Inside the house, next to each exterior wall, there is a 6" wide area of cement that is 3/4" higher than the rest of the floor. Of course, I could level the floor by pouring another 3/4" of cement, but I'd prefer not to do this.

1 - would it be possible to install a plywood subfloor over the slab but have no subfloor over the 6" ledge? This means that a few top oak floorboards along the edge would be directly on the cement (with a good moisture barrier). Could they be floated? glued? Remember this is not engineered wood.

2 - If this is not possible, could I used engineered wood on top of the 6" ledge, and non-engineered wood over plywood everywhere else?

3 - What if I didn't put any wood on the 6" ledge at all? I would put wood only over the slab and put a tile border on the 6" ledge. But, then I would be unable to leave a gap where the wood meets the tile. Would I have a problem with expansion?

Any suggestions or assistance would be greatly appreciated.

- Paul

You have a very unique situation.

My first suggestion is to not put solid wood over concrete.

My second suggestion is to hire someone that has dealt with your situation, in the past, with references, insurance and deep pockets.

If it is above grade you are in a much better position than if it where on or below grade. Though it is the golden rule that a good installer never glues a solid to concrete the rule does allow for variances. If it where me I would do one of the following:

I would find 1” plywood and use it to bring me 1/4 higher than the concrete barrier and then put ¼ over the higher border area, blind nail to the 1” and urethane glue to the remaining 6” area that has the ¼ on it.

The second option would be to put in ¾ in the lower floor and use a moisture barrier product like Bostik’s Moisture Vapor Protection and a urethane adhesive like Bostik’s Best. If you chose this route be sure to check your slabs moisture content to make sure it is within allowable range in the 6” concrete area that will be exposed.

Finally, make sure you contact both the wood manufacture and Bostik and explain the proposed job and the product that will be used in the 6” area mainly Bostik MVP and Best glued directly to concrete. If they say go for it, get some documentation from them and go for it (I would email it so I have return email documentation). If there is a warrantee issue you can go back to the document as proof of warrantee for special application.

Personally, I would expect little problems with either method due to the fact that it is above grade and only a small 6” area is directly exposed to concrete as long as the slab is dry and MVP is employed it should not present a major problem, though gluing solid oak directly to concrete is not something a person should make a habit of this is more of an exceptional case. If I where to walk into your job my first recommendation would be 1” on the floor and ¼ over the 6” as it eliminates more variables.

thanks for the information. I said above grade, but should I have said on grade? it's a slab on the ground floor. The top of the slab is about 1-1/2 to 2 feet above the outside ground level. There have never been any moisture problems or wet spots. The slab is ~50 years old. I performed the "poor man's" moisture test by taping down sheets of plastic in several places and after 4 days they were still completely dry.

If I do glue down over the 6" area, should I worry about expansion problems?

Other possible solutions:
1 - pour 3/4" of leveling cement - I'd rather not lose so much height or wait long for curing.
2 - feather the edges with concrete patch - not for 3/4"! the floors wouldn't be level.
3 - one suggestion very similar to yours was to make a floating subfloor with two layers of 1/2" plywood nailed or screwed to each other. Let the top layer extend 6" beyond the bottom layer, sitting on the ledge. The person didn't realize that the edge was a full 3/4" high. I was thinking of doing this and filling the extra 1/4" with a thicker membrane type moisture barrier, or, instead putting some kind of sleeper system to take up the 1/4" and make the floors more comfortable to boot. The problem is, I still end up losing an extra 1/2" of height.

Also, If I do not float the subfloor, do you have any recommendations as to how to attach? Some suggest shooting down. Others suggest using split drives. Still others suggest drilling and using hit anchors or screwing down with inserts... and, of course, whoever suggests one method says the others do not know what they are talking about.

Lastly, any suggestions as to the choice of moisture barrier? I live on Long Island, New York (not near the water). Thanks.