Question
I have purchased solid Indonesian hardwood flooring (18mm kiln dried T&G pre-finished planks designed for false floors) to install over an existing concrete slab. The instructions I have are for secret nailing to 38 mm joists, which would end up too high. I wish to glue it as a floating floor. Ideally, I'd glue the T&G, as is the practice with the laminate floating flooring. I have been told, however, that this can be joined in such a manner because it is more stable and that I need to cover the entire concrete slab with a flexible adhesive and glue each plank to the concrete, similar to tiling. I am in Australia, if that causes different conditions.
Forum Responses
You need a membrane between the concrete subfloor and wood floor. I suggest laying out felt paper over the concrete, sealing the joints with an asphalt adhesive and then laying the wood floor over as though you were installing a floating floor.
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.
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.
Laminated floors:
These floating floors have two important design differences to solid flooring:
1) They are extremely straight out of the box - this means they will form a good, tight glue bond strip to strip, without minimal side pressure and without nailing to the floor below.
2) They are relatively stable with seasonal moisture changes - this means the laminated floor should stay quite flat over a period of years.
Solid wood floors:
1) Will always have some minor crook (American "bowed" type crook, not the Aussie version), which you will need to force straight as you glue - this could prove very difficult and time consuming.
2) With normal seasonal moisture changes (unless you live out in the Simpson desert), the thicker solid wood flooring may try to move - this could force splits if the glue holds, or open joints if it doesn't, and ugly bulges during the rainy season.
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.
If you insist to live dangerously:
a) Leave wood in room (unwrapped) for two weeks before starting.
b) Include moisture barrier.
c) Include some type of sub-floor.
d) Toe-nail it down using angular floor nailer (can be rented).
Comment from contributor A:
I wanted a hardwood floor over a concrete slab also. This is how I did it. First I sealed the floor with a good commercial grade sealer. I then laid two layers of heavy gauge plastic sheeting, overlapping all joints by 24 inches and duct taping them. On top of the plastic I laid 1/2" X 4 X 8 sheets of dense styrofoam insulation. Taped it together at the seams. On that layer I laid 1/2" X 4 X 8 sheets of OSB (oriented strand board) using roofing clips spaced every 18". I then duct taped those seams. Finally I laid my pre-finished oak hardwood floor on the OSB using the standard method of a pneumatic floor stapler setting a staple every 6 inches. For good measure I glued every tongue and groove joint with a heavy duty carpenters glue. I left adequate spacing all around the walls for expansion. This floor is rock solid. The OSB is much more stable than plywood, the styrofoam is vapor resistant, also. It's gone through 2 dry Michigan winters and humid summers with no problems.
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.
