Installing Plywood Flooring Over Concrete

Many homes today are built over a concrete slab, rather than a full basement. While a basement is nice to have, it’s an added expense that many first-time homeowners just can’t afford. Even so, when they want to upgrade their home later on, that concrete slab can be a problem. One way it’s a problem is for installing hardwood flooring. Before hardwood flooring is installed over concrete, a plywood subfloor must be installed.

Not all subfloors are the same. Often, when we talk of subflooring, people think of lauan plywood, which is used as subflooring over thicker plywood, in preparation for installing a vinyl floor. In that case, the only purpose of the subflooring is to provide a smooth surface, without any cracks or height irregularities at the seams.

We’re talking about a different sort of subflooring here. When installing subflooring over concrete, for installation of wood flooring, it is necessary to use thicker subflooring, so that it can be nailed into. A minimum of 5/8” thick plywood is recommended, with 3/4” thick plywood being considered ideal. This should be “exposure 1 sub-flooring,” meaning that it is made to withstand weather (moisture) it may be exposed during construction. This is the sort of plywood used as sheathing when building a house.

Moisture levels in concrete floors

If you’re installing plywood over an existing dry concrete floor, you shouldn’t have any problem. However, a new concrete floor contains a lot of moisture. So, you need to allow it to cure for at least 60 days before even bothering to check the moisture level. The moisture level of the concrete should be down to 5.5% before attaching the plywood subflooring. Floors that are below grade (i.e. basement floors) may never reach this point.

flooring, concrete, wood, working
Finished plywood flooring over concrete, Stephen Perrera

If you live in an area which is known for flooding or your particular home has a history of flooding, you may want to reconsider putting in a hardwood floor. Water damages the wood and will also damage the finish on the wood floor. A hardwood floor in a basement that floods will not last.

Three-quarter inch thick, CDX plywood is normally used for sub-flooring. Some people recommend using pressure-treated plywood, but that is not necessary. Even using pressure-treated plywood in a basement that is prone to flooding won’t help, as the hardwood floor installed to it won’t be pressure treated.

Putting in a moisture barrier

Because of the risk of moisture damaging the flooring, a moisture barrier should always be used. The specific type of moisture barrier depends on how the subflooring will be installed. There are different moisture barriers for the different types of installations.

  • Nail down installation – Either 6 mil poly plastic sheeting or 15 lb. roofing felt should be placed on the concrete, before the sub-floor is installed
  • Glue down installation – Plastic and felt will not work in this application, as they would be weaker than the adhesives used to attach the flooring. Rather a trowled or rolled liquid product is needed
  • Floating floor installation – When doing a floating floor, use a 6 mil poly plastic sheeting

Please note that some plywood, intended for use as an underlayment, may have a moisture barrier already applied to one side. In this case, no further moisture barrier is needed. These underlayments are most often used for floating floor installations.

Leveling the Concrete

Fresh concrete is supposed to be leveled and smooth when it is poured. Unfortunately, not all contractors do this well. It’s possible to have a new floor that is either not level or not plane. In either case, you’ve got high and low spots that you need to deal with. That’s even worse if you’re working on remodeling a home, especially an older home, because chances are pretty high that you’re going to have cracks and uneven places in that floor, due to the home settling, even if the floor was level and plane when new.

If the concrete has any areas which are uneven, those uneven spots need to be corrected before the plywood subfloor is installed. Lumps and bumps of concrete will prevent the plywood from sitting level. Likewise if there is a seam in the concrete, from two separate pours, which is not smooth, even and level, it will cause the plywood subflooring to sit unevenly, ultimately causing problems with the installation of the hardwood floor.

Leveling the concrete is a fairly time-consuming process, but not one that is so complex that you can’t do it yourself. Little in the way of specialized equipment is needed and the materials are readily available at your local home improvement center.

Before starting anything, you need to determine if the floor is plane and level. The easiest way to do this, if you don’t have a laser level available to you, is to simply drop marbles on the floor, several times, and watch what they do.

  • If the marbles all roll to one side, the floor is out of level
  • If the marbles congregate in one area, that is likely a low area
  • If the marbles separate, but go to different areas, where they stop, check those areas to see if they are low spots as well
  • If the marbles avoid any area, rolling away from it or around it, that’s likely a high spot

Grind down the high spots with a surface grinder or planetary floor grinder, which you can rent; and a concrete grinding wheel, which you will probably have to buy. You can use an eight foot contractor’s level or simply a string to see if you have ground it down far enough.

With the high spots eliminated, fill any cracks and holes with epoxy. If the cracks are too narrow to allow you to pour the epoxy in, widen them. You don’t want to leave any gaps. Allow the epoxy to set, then sand or grind it level. You don’t need to use the rental grinder for this, an electric hand sander will do.

Now you’ll need to prepare the floor for using self-leveling compound. Clean the floor thoroughly, removing any grease or oil, as the leveling compound won’t stick to it. Sweep or vacuum up any dust and particles. Then, once the floor is dry, apply the latex bonding compound. This can be applied with a paint brush and roller, just as if you were painting the floor. Allow it to dry.

Finally, you’re ready to use the self-leveling compound. Mix the compound with water, according to the manufacturer’s directions, creating a slurry. It helps to use a drywall mixer in a drill and a five-gallon bucket, so that you can get the compound smooth.

Pour the compound immediately, as it dries quickly. Be sure to start in the corner farthest from the door and pour as many buckets of the compound as needed to cover the entire floor. There is a special tool for smoothing it out, but you can use a garden rake to do so, just as well. Allow the compound a few days to dry, before installing the plywood subfloor.

3 Ways of Attaching the Plywood

The plywood itself should be laid down with the long direction perpendicular to the grain direction of the hardwood floor. This will help prevent cracks forming when the seam in the floor is directly over the seam in the plywood.

Do not butt the edges of the plywood together; rather, leave a 1/8” expansion gap between the sheets to avoid buckling. In addition, leave a 3/4” gap around the edges of the floor or wherever the floor comes into contact with vertical structures, such as columns. Most of this gap will end up being covered by the baseboard.

If you are working on a remodel, you’ll probably already have drywall on the walls. As long as the drywall is mounted high enough above the existing floor, to allow the plywood you are using to slip under it, you can measure that 3/4” gap from the studs. However, if it is not that high, you will need to measure the 3/4” from the face of the drywall.

Please note that this expansion gap is extremely important to prevent your floor from heaving and buckling. Putting the flooring in tight, without leaving an expansion gap, doesn’t leave anywhere for the individual sheets of plywood to expand, as they absorb moisture. The only place they have to go, is to go up. As they do, they will pull fasteners through the veneers of the plywood, cause delamitation and could cause the flooring to come loose from the subflooring.

If this gap is not provided, the flooring will ultimately become so wavy, that it needs to be repaired. Unfortunately, there is no way of repairing the floor, without removing it entirely and starting over. That’s a rather expensive mistake to make.

There are three ways of attaching the plywood to the concrete floor. They are:

  • Screws – Concrete screws, such as Tapcons provide for very secure fastening. This requires the use of a hammer drill to create a pilot hole for the Tapcons to go into and an impact driver to seat the screws. When installed, the heads of the screws must be at or below flush, as compared to the surface of the plywood. Screws should be installed every 16 inches.
  • Powder Actuated Fasteners – Concrete nails, shot into the floor work well, if you can get them to install with the heads flush or slightly below flush. They cannot be reset or driven deeper once they have been driven in by the powder actuated tool, so run a test before using them. Like the screws, these should be 16” apart.
  • Adhesive Mastic – Newer adhesives are providing an excellent alternative to more traditional methods of installation. When using an adhesive mastic, the vapor barrier is not used; but rather, a painted-on sealant is. These adhesives are expensive, making for a higher overall cost. The mastic itself is troweled in place, providing an additional moisture barrier between the plywood and the concrete.

Once the plywood subflooring is installed, check the edges for any ridges caused by the sheets of plywood not laying flat or being of different thicknesses. Any ridges found require filling with a flooring grade wood putty and then sanding to provide a smooth, continuous surface for the wood floor to be mounted to.

Floating Floors

Floating floors are becoming more common, both in response to HOA rules on how much noise residents can make and also to provide cushioning in the floor, for physical activities. In a floating floor application, two layers of plywood are stacked, with offset seams; the top layer laid at a 45 degree angle to the bottom one.

In these cases, a pad is attached underneath the bottom layer of plywood. These pads vary, with some coming already attached to the underlayment. Cork is a common material, although rubber pads are used as well. Pads can also be separate of the floor and laid on the concrete floor, before laying the plywood down. In either case, the pads offer some flexibility to the floor, making it more comfortable to walk on, as well as reducing the amount of noise produced when walking on the floor.

Plywood Over Battens

While not as common anymore, plywood can be installed over battens. This provides the same benefits of a floating floor and can be installed over a floor that isn’t as smooth. It is even possible to install a plywood and batten subfloor over a concrete floor that isn’t level and plane, although you need to have a means of verifying that you are shimming the battens correctly, such as a laser level. This method should not be used if ceramic tile is to be applied over the subfloor.

The battens themselves are usually 1”x 4” dimensional lumber. These are attached to the floor, every 16’, running the length of the room. They can be attached with powder actuated fasteners or with tapcon type screws. In either case, the battens should be checked, to verify that they are level with each other and that there are no high or low points. Low points can be shimmed, and high points planed down, if necessary.

The plywood floor is then screwed to these battens, much like nailing plywood subfloors to floor joists in a multi-story home. Screws are used, rather than nails, because they are only going into a 3/4” thick board and you don’t want them loosening up and causing a squeaky floor.

Hardwood Floors Directly Over Concrete

Although it is not common, there are some applications, where it is possible to install hardwood floors directly over concrete, without bothering to install a subfloor. This can only be done in arid places, where there is little possibility of moisture coming up through the concrete and attacking the wood.

In such cases, special engineered hardwood flooring products are used. These consist of a void-free hardwood plywood, with about 1/8” of hardwood laminated on top, as the top veneer of the product. These products are also tongue-in-groove for easier and more secure installation. A variety of different styles and finishes are available.

For this to be possible, the floor still must be leveled, if it was not made perfectly level and plane when it was poured. Check the floor carefully for this and if necessary, level the floor, using the same kind of floor leveling compound mentioned above. Then seal the floor, providing a moisture barrier.

The wood flooring itself is installed in this case with a polyurethane flooring adhesive, troweled onto the floor, rather than installed with fasteners, as the nails normally used for installing hardwood floors aren’t strong enough to nail into concrete. To tighten up the boards and eliminate any gaps, holes are drilled, and finish nails are installed every 10 boards or so.

Nailing these boards is done in conjunction with a clamping device, which is bolted to the floor for stability. Once bolted, a scrap piece of the tongue-and-groove flooring is used as a pad and the hydraulic jack in the clamp is pumped up, pressing the boards together and eliminating gaps. Then the holes are drilled and the nails installed. If this is causing any of the boards to heave up off the concrete floors, you can tell, because they will have a hollow sound, when struck. To correct that, simply drill through the flooring at those locations and nail there as well.

Vinyl Flooring Over Concrete – an Alternative to Wood Floors

If you are needing a cost-effective alternative to wood flooring, you might want to consider installing imitation wood, vinyl planks. These come in two basic styles and a variety of finishes, simulating a variety of different kinds of wood. Typically, cushioned vinyl only needs to be bonded to the floor around the perimeter, while non-cushioned flooring needs to be bonded across the whole floor. Always check the manufacturer’s instructions, before installing.

When installing cushioned vinyl floor planks that are only bonded around the perimeter, the planks themselves are designed to lock together, with a modified tongue-in-groove connection. The difference in the joint causes the planks to actually snap together, providing a positive lock. This is important, as the vinyl flooring is thinner than wood flooring typically is. With this sort of flooring, a special double-sided tape is applied to the concrete floor or wood subflooring, all around the perimeter of the room.

In the case of non-cushioned flooring, which requires a mastic across the entire flooring surface, a toothed trowel is used for applying the adhesive. Be careful to ensure that no debris gets into this adhesive, as it will cause bumps in the flooring, which could ultimately cause it to crack. In all cases, it is always critical that the subfloor or concrete be clean, before installing the vinyl flooring.

The Reverse – Concrete Over Plywood

Pouring concrete over plywood can be even more challenging than installing plywood over concrete. Nevertheless, this is not all that uncommon. It might be done to create a subfloor for ceramic tile to be installed over an existing wood subfloor or for making a concrete countertop. In either case, there are two issues to content with: rigidity and the porosity of the plywood. That porosity can make the concrete settle unevenly as the water soaks into the highly porous wood unevenly.

The first thing that needs to be done in this process is to ensure that the plywood floor is rigid. An additional layer of 3/4” plywood should be installed, with screws, over the existing subfloor. Install it at right angles to the existing, with the seams staggered to stabilize the floor.

Galvanized metal lath should then be installed over the plywood as a reinforcing material and to help the concrete to bond to the plywood. Screw the lath down on six inch centers. If desired, roofing felt can be placed under the lath, between it and the plywood, as a slipsheet. This will act as a moisture barrier, helping keep the water from the concrete from soaking into the plywood, thereby helping it to settle more evenly.

Not all products will work as a reinforcing and attaching material between the concrete and the plywood. However, there are a few overlay materials which have been specially designed for this. These materials have a smooth and rough side, so that the smooth side can be glued to the plywood and the rough side can give the plywood something to “bite” and adhere to. Some of these materials also act to provide waterproofing or muffle sound, providing something to absorb impact and vibration, much like a floating floor.

These materials are primarily used to provide structural stability for the concrete, so that it will not fracture. If you are using a waterproof version of these materials, it is recommended to attach galvanized lath over it. In this case, since the screws would pierce the moisture barrier, place a small amount of vinyl caulking on the screws, before installing them, to seal the hole that the screw is creating.

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