Concrete Sub-Floor Preparation

The installation of the wood floor over a concrete sub-floor can be a bit tricky. We must keep in mind that the key to a successful installation is preparation. There are three main areas of concern: structural integrity, a flat and even surface, and moisture content. We present a list of tasks you need to be concerned about.

Structural Integrity

In a nutshell, this means that the sub-floor is intact: free of cracks, degraded areas, collapsed areas, and soft spots that are in danger of collapsing. In very old structures, damage to the structural integrity of the sub-floor is common. This is less of a concern in modern buildings. We recommend you calling in a professional contractor if there is a great deal of visual damage. If the floor is uniform and does not appear damaged, you’re probably good to go on structural integrity.

Flat and Even

Sub-floors should be within a tolerance of 3/16 of an inch over a span of 8 to 10 feet. Within any 8 –10 foot range, there should be no dips or humps which exceed 3/16 of an inch. Lay out a flat 8 – 10 foot plank and mark trouble spots as you move it across the floor in all directions.

Minor hump problems can be fixed with a hand-held or rented orbital sander. Be sure to wear protective goggles and a dust mask because sanding throws up a lot of dust.

Fill dips, low spots and gouges with a cementitious leveling compound, which may also be called floor patch. The first step is mixing it according to manufacturer instructions. Add the water first and then the dry mix so you won’t get a thick clump in the bottom which should be about the consistency of pancake batter. In order to apply floor patch evenly, take the board you’re using as a straight edge and pour the amount of patch to fill the gap. The next step is pulling the board back and forth over the area in order to spread the patch. in order to apply equal pressure, one person should be on each end of the boarder. As long as it’s even it the process was executed good enough. This is called screeding. Now check if it’s flat, and sand if necessary.

Moisture

Concrete is porous material that pulls moisture from air and ground. This is why thick solid wood installations are not recommended over concrete and why solid wood floors should not be installed at or below grade, or sea level. The truth is that there is always moisture in the air, but too much will soak into wood planks like water into a sponge, causing them to swell. Too much moisture causes the boards to push beyond the small area allocated for movement, and the floor becomes pressurized as the boards crowd the space. In the end, too much moisture resulting in buckling and warping, which may be localized to a small area or ruin the entire surface of the floor.

You should firstly use a moisture meter to be sure the concrete sub-floor meets the recommended moisture content specified by the manufacturer which is usually 4%. For concrete moisture you just have to hold them over the floor and they are able to read the moisture in the air. Keep in mind that they can penetrate only about an inch into the surface of the concrete, and moisture initially settles to the bottom. If this installation is in a new house or building, give the concrete plenty of time, 30 – 60 days, to cure before considering installing a wood floor.

You should use an underlayment with a moisture barrier, or 2-in-1 underlayment, unless you are in an area with very low humidity. Underlayment comes in rolls and generally, you would roll out strips of underlayment to cover the entire floor and then join the edges of each row with a special tape.

After everything is done, you should clean the floor, removing spatters of compound, dust, nails, or anything else that may cause problems. Wood flooring is an investment since it improves the value of your home.

Check the manufacturer instructions for every step of the way.