How to Prepare a Concrete Sub-Floor

Installing a wood floor over a concrete sub-floor can be tricky. The most important factor for a successful installation is preparation. There are three main areas of concern: structural integrity, a flat and even surface, and moisture content. Here’s what you need to know to make it work and ensure long-lasting beauty and maximum value for your home.

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. Damage to the structural integrity of the sub-floor is common in very old structures, and less of a concern in more modern buildings. If there is a great deal of visual damage, it’s probably best to call in a professional contractor. If the floor is uniform and does not appear damaged, you’re probably good to go on structural integrity.

Flat and Even

The word “level” often comes up when discussing flooring, but it’s not entirely correct, because no level measures an 8 – 10 foot span. Specifications define that sub-floors should be within a tolerance of 3/16 of an inch over a span of 8 to 10 feet. What this means is that within any 8 –10 foot range, there should be no dips or humps that exceed 3/16 of an inch. To find uneven spots, 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 (carefully) sanded down with a rented concrete grinder. Grinding concrete will throw up a lot of grit, so be sure to wear protective goggles and a dust mask.

Fill dips, low spots, and gouges with a cementitious leveling compound, which may also be called floor patch. This is fairly simple; you mix it according to manufacturer instructions and patch the floor. When mixing the patch compound, add the water first and then the dry mix so you don’t get a thick clump in the bottom. Mixed floor patch should be about the consistency of pancake batter. To apply floor patch evenly, take the board you’re using as a straight edge and pour the amount of patch you think it will take to fill the gap, then pull the board back and forth over the area to spread the patch. One person should be on each end of the board to apply equal pressure. It may not look perfect, but as long as it’s even, it should be fine. This is called screeding. Once the gaps are filled, let the screed dry completely; then check that it’s flat and sand if necessary.

Moisture

Concrete is porous material that pulls moisture from the air and ground. This is the reason 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. There is always moisture in the air, but too much will soak into wood planks like water into a sponge, causing them to swell. This is normal, but 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. Eventually, floorboards with too much moisture have nowhere to go but up, resulting in buckling and warping, which may be localized to a small area or ruin the entire surface of the floor.

The first precaution to take is to use a moisture meter to be sure the concrete sub-floor meets the recommended moisture content specified by the manufacturer. Usually this is 4%. Concrete moisture meters are easy to use, you just hold them over the floor and they read the moisture in the air. However, you should know 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. This will give the concrete a chance to compact and allow the moisture to redistribute more evenly for a more reliable moisture content reading.

Unless you are in an area of very low humidity, we recommend an underlayment with a moisture barrier, or 2-in-1 underlayment. Underlayment comes in rolls. Typically, you would roll out strips of underlayment to cover the entire floor and then join the edges of each row with a special tape. Check the manufacturer instructions for specifics. Ends can be left long and trimmed after the flooring is installed.

Once the preparations are done, you should clean the floor thoroughly, removing spatters of compound, dust, nails, or anything else that may cause problems under the floorboards. Remember that a wood floor can last several lifetimes. It’s worth the extra time to ensure that the installation is done properly.