"Floating" wood floors over concrete
In New England, a 3/4" hardwood floor installed directly over concrete will absorb moisture from the concrete slab, and the boards will swell and cup. This is the case even if the concrete slab is firred out first (3/4" x 2 1/2" firring strips).
To resolve this problem, a moisture barrier must first be laid over the concrete. A simple and adequate solution is good thick plastic sheeting (say 6 mill or thicker).
Without the moisture barrier, the floor will first swell until it is pressed hard up against the sidewalls. Once it has nowhere to expand further, the individual board joints (lengthwise) will pop up in one or several locations, creating a raised v lengthwise along those two boards. I've seen these rise as high as about 2" above the rest of the floor.
In addition, individual boards will cup more than normal due to moisture absorption, mainly from only one side of the board--the side against the concrete.
I would strongly advise against gluing 3/4" boards directly to the concrete. Nothing is going to prevent wood from absorbing moisture and subsequently expanding. Even the coating of glue from below, and a good multi-coat finish on top will not prevent swelling if you are in an even remotely humid climate. If your location is desert-like, climate wise, I'd say you've got a good chance of gluing the tongue and groove joints as a floating floor working out ok. Just put a moisture barrier down first for a bit of insurance.
From the original questioner:
Doesn't the moisture barrier (plastic sheeting) laid under the concrete prevent rising damp from underneath already?
Yes, that is exactly the purpose of the plastic. However, it then prevents you from cementing the boards directly to the concrete, which is a bad idea, anyway.
However, you will still get moisture penetration through the finish on the top faces of the boards, which will make them swell. Actually, even with the plastic, there will be some moisture absorption from the bottom as well. Theoretically, it will be closer in volume to the top of the board through the finish, and the boards will expand and contract evenly, resulting in less cupping.
I put a hard pine floor down on a small porch on our own home several years ago, without pre-finishing the bottoms of the boards. The deck sits about 16" off the ground, over nothing but dirt. I then put an oil preservative on the deck surface.
Now all the boards have cupped upwards, creating a bit of a roller coaster effect across the porch floor (running across the boards). Took me a while to realize that what had happened was moisture had easily penetrated the bottoms of the boards from the always damp earth below, because there was no finish on them, while the tops had some resistance due to the oil finish. Hence, the bottom faces of the boards swelled more than the top faces and the cupping resulted. It could have been somewhat, if not completely, prevented by pre-finishing the bottoms of the boards.
I agree with all the above, particularly the need of a moisture barrier over the concrete, but I think we are still missing a couple of important points:
Solid wood floors:
Since a lot will depend on the species you are using and your local climate, I suggest you pay a reputable company to lay it for you.
The movement can not be ignored. I would never suggest a floating solid hardwood floor. The better laminate products (Kahrs, Award, etc) are the better choice for floating. You still have to vapor barrier over the slab and equally, you have to leave room around the perimeter (5/8 to 3/4), depending on the size of the space. This space will allow for the movement, and don't short it. You then cover this space with a trim not attached to the floor, so as not to restrict the movement (thus floating). These laminates usually carry about 5/16" solid wood top cross banded onto a lumber core and are so engineered as to allow you to glue them together. Never get solid hardwood to lay straight enough!
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
Comment from contributor B:
I recently installed a hickory 5 ply 1/2" wood floor using a float down method over a concrete subfloor. I live in North Carolina and the concrete slab is above grade. I installed a foam/plastic barrier down as is recommended. A floating floor is not as solid feeling as a glue down floor. If your flooring is flat with no bow and your floor is level/flat, a float down can feel very solid. Glue down has an installation time disadvantage and also makes it difficult to replace the floor if needed. I floated my floor but I occasionally cut through the foam underlayment and glued a board down with Liquid Nails, especially in high traffic areas, and the floor turned out great with no cracking or popping, and it feels very solid.
Comment from contributor C:
Ensure you select planked flooring when floating a floor. Attempting to float a strip floor (2 1/4") will create too many seams in the floor and may cause the floor to be unstable if floated.
Comment from contributor D:
Lay 6mil poly over your concrete slab and fasten three quarter inch plywood to the slab. Lay your sheets at a 45 degree angle opposite of what your flooring will run. Fasten your strip flooring with a cleat nailer using 1 1/2 inch cleat nails. Try to keep your rh constant.
Comment from contributor V:
I've been using a product called t2, made by Sika, to directly stick solid hardwood floors to concrete for the last seven or so years with no problems. There is an accoustic reduction mat laid first onto the slab. Tthis is made from recycled rubber and there are holes about 3 inches long by half inch every inch or so that adhesive is extruded into from a 600cc cartridge gun. Fantastic system, but not cheap. It's a polyurethane moisture-cured rubbery glue that stays flexible to allow for expansion and contraction when fully set. One guy said he used no nails. Don't do that, because it sets brittle and can shear when floor move and in Ireland, we all know they move.
Comment from contributor E:
Our living room floor (prefinished oak hardwood) was installed without a moisture barrier. It was not able to withstand the humidity of Boston summers. It buckled and within a month the planks popped out. There was mold and wetness under the floor, and the entire thing had to be removed and replaced.
Comment from contributor J:
I live in Italy where cement is the basis of all home construction. Here hardwood floors are in fashion and are regularly and successfully glued directly to a cement base. I have teak in the living room, bedroom, kitchen and bathrooms. After 5 years I don't have any cupping or bulging or loose boards.
One precaution is to let the cement dry thoroughly before you lay the floor. This could take several months and the humidity should be checked by an installer before proceeding. They tell me however that long planks of wood with considerable movement need to be nailed and to do this they sink wooden dove tail shaped strips into the cement to which the floor is subsequently nailed. As far as floating floors go, a simple moisture barrier such as plastic plus soft poly is sufficient as long as the cement has dried sufficiently.
Comment from contributor F:
I have been a flooring installer for nearly a quarter of a century. What I am surprised not to have seen mentioned here are products like RedGard by Custom Building Products or Hydroban by Laticrete. These type polyethylene products permanently seal the concrete slab from the surface material - waterproof for life. They also are antifracture membranes which is an additional plus. Properly applied, they create a permanent moisture barrier as well as allowing a glued/mortared down surface material to expand and contract independent of the slab.
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