Installing Hardwood Floors: Types of Subfloors and Why They Are Important


What lies underneath your hardwood floors is very important when it comes to both hardwood floor installation and maintenance. The installation details for each type of subfloor may become overwhelming fast, so we’ll keep it simple.

Installing Wood Floors Over Concrete

The first question you may have is whether you should build a plywood subfloor over your concrete. This would make the hardwood floor installation easier and faster, even when considering the time you take to put the subfloor together. There are pros and cons for this intermediary step. Adding plywood to the concrete increases the subfloor height, which can make the transition to another room or type of flooring problematic. On the other hand, a plywood subfloor feels softer under your steps and simplifies the hardwood floor installation.

If you decide to install your wood floor directly over the concrete subfloor, the hardwood floor contractor will measure first the moisture content (MC) of the subfloor and try to bring it to an acceptable level. This level is different from area to area, depending on your specific climate. For example, a certain MC percentage may be within the normal range in one climate, but may indicate humidity problems in a drier one. Measuring and recording the MC requires accurate instruments and a bit of expertise in evaluating all areas of the floor, recording the results and properly interpreting them.

If you evaluate that the concrete floor cannot be completely dried, wood floor installation is still possible, but needs extra attention. In this case, use a vapor barrier such as felt paper. Combined with the adhesive and a proper use of expansion gaps, this can give you excellent results.

A crucial step in the installation process is the acclimation of wood boards. Acclimation allows your floorboards to adjust their moisture content to the environment by sitting for a few days in the room in which they will be installed. This minimizes problems down the road when, once installed, insufficiently acclimatized wood boards may cup in the attempt to adapt to the humidity of the room.

It is expected that your concrete subfloor will have slight variations in height. A layer of leveling compound can fix the problem of an uneven subfloor. This may take an extra day, but will make the job easier and will prevent uneven pressure on the boards – and the floors from squeaking later.

Keep in mind that engineered wood is the best option when installing hardwood floors over concrete subfloors. Engineered wood is designed to accommodate environmental changes, which makes it the only type of wood floor that can be installed in basements. In addition, engineered wood can be floated, which allows your floors to move and expand as a whole when adjusting to external conditions.

Installing Wood Floors on Plywood Subfloors

A plywood subfloor is the easiest subfloor to work with. Here, you must take the same preparatory steps as in the case of any other subfloor: the plywood must be dry and the wood boards acclimatized.

A glue-and-nail installation is perfect for such a job. Most likely, every project will come upon a few problems. The room may not be exactly square, so the planks may end up running in an angle against the walls. An uneven subfloor may slightly raise some of the planks above. The wood may have acclimatized unevenly, giving you slight variations in plank width. A hardwood floor professional will know how to measure the room, minimize the irregular appearance of the wood planks, adjust the expansion gaps so that they camouflage the plank width variation or raise boards so that the surface of the hardwood floor is smooth.

Installing Wood Floors Over Radiant Heat

Many homes today have some type of radiant heat installed in their floors, whether the heating tubes lie between the joist and the plywood subfloor or in a bed of gypcrete over the subfloor. Beautiful hardwood boards can be installed on top of such systems, but your hardwood floor contractor has to take a few more precautions.

First, it is very important that the radiant heat is turned on at least a week in advance (we advise one month), before even bringing the wood boards in for acclimation. This gives the heating system enough time to push the moisture out of the subfloor, process during which you do not want your wood planks sitting around and absorbing the moisture.

Second, the wood floor installers have to make sure that they do not damage the heating tubes with nails, an easier mission when the tubes are visible. Third, the boards must be arranged perpendicular to the direction of the tubes, to ensure even heating of the hardwood floor.

Finally, it is critical that you use the radiant heat system properly in order to avoid extreme changes of temperature at the floor level. The rule is simple: turn on your heat system slowly and keep it going. During the cold months, increase the heat gradually every day until you reach the desired heat level. Two degrees a day is a good rule of thumb.