How to Prepare a Concrete Subfloor
Just like with wood subfloors, concrete subfloors have to be dry and flat for a successful wood flooring installation.
It’s important to follow the wood flooring and adhesive manufacturer’s recommendations for subfloor preparation. But following the guidelines of the best industry-wide resource, the National Wood Flooring Association (NWFA),
these are the specifications you need to know.
How to Make a Concrete Subfloor Flat
The flatness tolerance for a concrete floor is 1/8” in a six-foot radius or 3/16” within 10 feet. If the concrete slab does not meet these specifications, there are solutions.
High spots often can be removed by grinding. It’s necessary to minimize the amount of silica dust produced by grinding. The Occupational Safety and Health Administration recommends using dust collection devices or applying water to the concrete before sanding. For your safety, wear a respirator to minimize exposure to inhaling the dust.
Low spots can be brought to level by using a patching compound or self-leveling product.
Testing and Preventing Moisture in a Concrete Subfloor
Before a concrete slab can be moisture tested, it must be a minimum of 30 days old. Use a moisture meter designed for concrete moisture testing. Test at the surface of the slab (electrical impedance test) and within the body of the slab (electrical resistance).
The NWFA specifies three moisture content testing methods: relative humidity, calcium chloride or calcium carbide.
If a slab tests too high in vapor emission to glue down a wood floor, consider installing a vapor retarder and a plywood subfloor.
A note specific to calcium chloride testing: A reading of more than three requires a vapor retarder with a perm rating of one or less to be used, but it is recommended that an impermeable vapor retarder with a perm rating of 0.13 or less, such as a six mil polyethylene film, be used.
Concrete Subfloor Surface Prep
Make sure the concrete slab is free from non-compatible sealers, waxes, oil, paint, etc. You can check for sealers by applying drops of water to the surface. If the water beads up, there might be sealers or oils. Burnished or slick slabs might require screening or sanding with a 30-grit abrasive.
Do not attempt to glue a wood floor over a chalky or soft concrete slab.
A clean, dry, flat subfloor will give you the start you need for a successful installation.