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Installing Hardwood Floors on Concrete Slabs

While I rarely have to install a wood strip floor on concrete here in the North East, a lot of you have houses built with a concrete slab as a subfloor. I must say, you are lucky to lack a basement, it has just been a depository for junk in my case. A slab on grade subfloor presents a lot of concerns when trying to install some types of wood floor. And if it's a fairly new home it may have one of those hydronic-heating systems installed right in the slab. I've decided to write this article siting the best choices, down to the worst choices, in each category of floor types. You may find that some of the worst choices are the most commonly used methods in the construction industry, but I cannot help that.

The first things to consider if you want a wood floor on the concrete are flatness, and moisture incursion. Now don't confuse flatness with levelness, you cannot simply put a 2-foot level on a small patch of bare concrete and say it's flat enough for a wood floor. If you are planning on installing any type of glue down hardwood floor product you must see the entire concrete surface, and make sure that it is flat within 3/16" in a 10 foot radius or 1/8" within a 6 foot radius. The way you do this is to take a 6 or 10-foot straight edge and place it in various areas on the subfloor. It will be very clear where the dips are. If there are just a few dips, you should fill them in with a non-shrinking mortar, available at you local concrete supply store.

The Quikrete ® company make a variety of repair products that have a good track record for this use. Go to their web site at www.quikrete.com or buy their book Build and Repair with Concrete. I highly recommend the book if you want to do any concrete repair work yourself, and Don Knotts, one of my favorite actors is on the cover, to give the book that friendly reassuring look. The Quikrete web site and the book also give detailed instructions on how to apply a floor-leveling compound. These floor levelers made by Quikrete are cement-based. Don't confuse them with the inferior gypsum based products that have a checkered past (lots of cracking and adhesion failures). You can see why a good flooring contractor wants to see the entire concrete, in order to give you an accurate estimate. We don't have X-Ray vision, we can't look through carpet.

So after the floor is filled and patched and these compounds are cured, what choices do you have in hardwood? Not so fast, you still have to do a moisture test if your concrete subfloor sits right on the earth. I wouldn’t consider a solid wood floor unless the slab is at least a foot above the highest outside grade. Moisture can creep into the concrete and by capillary action rise through the minute cracks and get into and swell the wood. A well-sloped and drained outside grade will prevent this. A solid wood floor you are about to lay on that slab is hygroscopic, meaning that it will absorb moisture from surrounding materials. You will need to do a moisture test on the concrete in various areas of the house. You will need to do this during the wettest of your seasons. I’ll bet you won’t hear any of this when you purchase your wood floor from one of those big box stores. But I hear about it later when the floor has failed, and numerous strips are buckled off the subfloor. The easiest way to test for moisture is to duct tape down a 2-foot square piece of clear heavy plastic on all sides. Suspend a 100-watt light 18" over it and wait 3 days. If there is any moisture in the slab it will show up as condensation on the plastic or as a darkening of the concrete. If this is so, you will then need to do a more quantitative test and that is the calcium chloride measure. The easiest way to get the materials and instructions for this is to go to www.vaportest.com. Or try www.sinakcorp.com (this company offers professional and warranted solutions to moisture incursion problems). The results of this test will give you an idea of just how much moisture is coming into the slab. The guidelines for wood floors are no more than 4 lbs. per 1000 sq. ft. over 24 hrs. This will make more sense when you see the vapor test web site. Oh, and by the way if this is new construction, you must wait at least 60 days for the slab to fully cure, and do these tests also.

Of course it should go with out saying that a concrete subfloor that has a basement under it or is on the upper level of a modern building will only experience moisture problems at the edges of the floor. This may be due to wind driven rain and should be only a rare occurrence. On those temporary damp conditions, good glue shouldn’t fail much. And good glue means flexibility for the life of the floor. Dri Tac 6200 (www.dritac.com) has a proven record in this ability, to flex even 5 decades later, if the wood swells a bit out of place during these temporary moist conditions. Later when conditions dry the wood will settle back to it’s original spot, as if it remembered where it was installed.

The very best choice of wood floor on concrete is solid wood parquet, which comes in a variety of patterns and styles, so don’t dismiss this elegant flooring just because you think it will give your floor a cheap look. In fact some of these 3/4" block floors in the Montecello pattern are more expensive than most strip floors, and are better suited to a glue on slab method. The plain mosaic pattern you see in most stores is a poor representative of this durable solid hardwood. These 6" by 6" tongue and groove prefinished squares don’t install very well except on the flattest of subfloors. Where as the smaller unfinished 4" slats of a square edged parquet easily follow slight undulations of the concrete. All parquet has opposing grain patterns that are much less likely to fail, if some excess humidity gets to them. You can read more about the special considerations of this type of floor in the Hardwood Authority section.

The next best choice for this application is the laminated or engineered plank floor. These look and sound just like a solid wood floor, but unfortunately don’t wear as well. They cannot be repeatedly sanded and refinished like a solid wood square edged parquet. But these laminated wood floors sure look nice. Read more about these pre-finished floors in the Pre-finished vs. Sand-on -site section. I’m sure that you can find an engineered floor that will also give you up to 50 years of light duty service, in say a living room or bedroom. This would be a poor choice in a kitchen or even a dining room.

You could also try one of the floating floors, these come in a thicker top lamination (less moisture stable), and like the laminated plank are not too much affected by temperature and humidity swings. Unfortunately these don’t sound like a solid floor when you walk on them, it’s like walking across a drum. And no amount of padding will quiet them, and these pads degrade in time anyway. The unnatural looking joints between the panels are getting some complaints.

But some people still insist that they want a real wood floor. What they really mean is a more traditional hardwood strip floor, either prefinished or sanded and finished on site. You will then need a wood surface to nail it to. The sports floor industry has solved this problem, albeit expensively. They have proven systems, that consist of a kiln dried wooden sleepers with a conventional plywood subfloor nailed to it. The sleepers are padded off the slab and bracketed to each other so not to damage to the concrete. With a well installed system like this you will be adding as much a 3" or more to the surface of the floor. But these are warranted and time proven methods, that will give you the solid feel of a wood floor with just a bit of bounce, depending on the system you choose. There are various web sites out there and I’ll give you 2 to get you started. The two commercial sites have good illustrations, which will give you a good idea how these sports floors are laid up. Try www.maplefloor.org OR www.maplefloor.com.

For the die hard D.I.Y.’er you can instead install kiln dried 2 by 4 (laid flat) screeds either in a floated framework, attached to themselves or in random lengths fastened to the concrete. If you choose a floated framework you can lay them on 16" centers and then install a 3/4" plywood subfloor to that. Some companies suggest installing thinner 1" by 4" screeds, but this doesn’t give the subfloor much of anything to nail to. You can also glue the screeds down with the new urethane construction glues in a random lengths, about foot from center to center. These boards should cut in lengths of18" to 4 feet. This will allow you to install a 3/4" strip floor directly on the screeds themselves. You don’t have to have all the hardwood floor joints on the screed surface, but don’t lay more than two joints in between the screed less than 4-5 rows apart.

You should also have a 6 mil poly film barrier on the slab and on top of the screeds. Even though you have proven that there is no moisture in the concrete, humid summer time air will condense on the cool cement, and form water droplets. This should be minimized with this double layer of poly. And for extra insurance you should be using pressure treated 2 by 4 ‘s and kiln dried (to 6%-10% MC depending on your area) so they don’t warp out of place. Don’t install any screed boards over 4 feet, also make sure to overlap and randomize the rows, so you have a more random nailing surface. To assist the glue you can use powder actuated nails, Tapcon screws, or expanding concrete anchored screws. This will be just to hold down the ends of the wood until the glue dries under the screeds. In this type of floor the joints that fall between the screeds will be weak points in the floor. Years later, you may get the occasional broken board between the screeds as the floor gets sanded thinner, but this is fairly simple to repair.

And that brings me to the next method, as we go down the choices, fastening a plywood subfloor directly to the concrete. The methods suggested so far keep the wood and concrete more or less separate, and allow for the different rates of expansion of these dissimilar materials. When you try nailing wood, even plywood directly to the slab, you’ve got to take care. Don’t try to nail down one thick layer of plywood, this rarely works in the long run. And the 3/4" plywood most commonly available is not thick enough for the 2" flooring nails (never use staples) to penetrate the required 7/8". It is too stiff to follow any minor undulations in the concrete and will put undue stress on the fasteners. Instead install 2 layers of 1/2" plywood in opposing directions. You should lay a heavy poly film down first, with the seams taped. Then install cork or foam pad on top of that to protect this moisture barrier from getting punctured by the plywood. Float the first layer of plywood in one direction. Then glue and screw (with short 3/4" screws) the next plywood layer (in the opposite direction) so that no concrete is damaged. This is the best system for installing on a heated slab system. Can’t use nails in the concrete here.

But if you insist, you can nail the first layer of plywood with powder actuated nails. These powder driven nails are tricky to use and you must test the concrete for suitability.

Hand hammer the powder driven nail and see if it splits the concrete. It must be hammered at least 1" into the concrete. Go to the web site of this rather specialized trade before you start at www.patmi.org. I see far too many poorly installed subfloors on concrete, avoid this with a little knowledge from the powder activated manufacturers association. You can also try Tapcon screws, but I have not had much good luck with these. And the concrete anchor sleeves are a rather slow and clumsy method for this operation. Find a method you are most confident and comfortable with. As an added measure of security you can add glue (try the new urethane construction adhesives, with their tremendous holding power) to the plywood. Scoring the plywood (with 1/8" deep circular saw kerfs) helps it lay down flatter. You can also try installing 1/2 sheets of 4’ by 4’ plywood. But really, your concrete slab will need to be flat like I described earlier, or you will have no end of frustration with this method. It all sounds like a bit much doesn’t it? But if you don’t take the time now to get the plywood down firmly, you’ll regret it later.

The last and the most experimental method of installing a solid wood strip floor to your concrete subfloor, is to glue hardwood shorts directly to it. These have to be specially milled for this purpose. They will need to be no more than 2 feet long, and have a flat underside. And they should not be more than 1/2" thick and 2" wide. If you look at the standard strip floor you will notice it has a grooved bottom which will not contact the glue as well as the flat milled bottom. This will become apparent as you start installing with this rather clumsy and slow method. In this case, use a good urethane adhesive, nothing less will do, and Dri Tac 7500 is one of the best. But even Dri Tac’s technicians admit this solid wood glue down application is terribly difficult, and should be left up to the professional. But for you die even harder D.I.Y.’ers I have a few hints. Don’t spread more glue than you can install floor on, in a half-hour. These may be the most precious few rows indeed, when you see how these solid wood shorts move out of place, as soon as you look away. Use a 3M releasable tape product to keep them in place, or use the ratchet straps that the Pergo floor installers use. Let the first few rows set up for an hour or two, so the whole start of the job doesn’t shift.

The trouble with this last method is that if the finished floor encounters excess moisture or simply gets a little wet from a spill, the floor will buckle off the glue. Unlike parquet (with expansion in all directions) strip floor directly on the concrete is really susceptible to even minor water damage. The floor is basically trapped in it’s own glue, and the stress of even minor expansion just wrecks this type of floor installation.

So there you have it all the good and not so good methods of installing wood floor on concrete. If it were me I would choose an elegant parquet pattern (saxony is my fav.) and glue it directly to the slab. Of course I would pick the unfinished square edged parquet so that I could sand it silky smooth, apply dark walnut colored stain, and finish it with at least 3 coats of satin polyurethane. This makes for an elegant, simple and durable floor, and puts those clumsy multi-layer nail down operations to shame. But make up your own mind, I hope I’ve a least opened your mind to the various possibilities.