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Sub Floors

Substrates fall under one of three different categories’:

   A) Concrete, gypsum or lightweight concrete
   B) Plywood or chipboard
   C) Sleepers, floor boards (1”x 6”) or floor joists

Regardless of which of these subfloors you currently have in place, it is important to note that finished floor will only be as good or as flat as the substrate beneath. Therefore we believe it is important to take the time and effort to get your sub-floor where it needs to be for a successful hardwood flooring installation.


According to National Wood Flooring Association Standards, the concrete or screed must be allowed to dry to a maximum of 75% relative humidity before installation takes place. To properly test for this, the hardwood-flooring contractor will drill into the concrete and place a probe to determine the amount of moisture the floor is giving off over a 72-hour period. Simple surface tests will only tell you what is going on the surface of the concrete and not if the concrete has dried out to any depth.

The concrete must also be dry before attempting to install the floor. You often hear of allowing one month of drying time per inch of concrete, or a minimum of 4 weeks drying time. The truth is that these are at best ‘guesstimates’ and at worst potentially reckless. If the sub floor is not dry enough you risk the floor failing in the future.

For those who cannot wait for your concrete to dry out to 75% relative humidity, there are materials on the market that will allow you install a hardwood floor while the concrete is still green. These products can be very expensive.

Concrete floors must be flat to within plus or minus 1/8″ over a 6 span before any hardwood floor can be installed. This applies whether you are nailing the floor or gluing the floor directly to the sub floor. At Gäte Hardwood Floors we have perfected our methods of leveling sub straights through the years. We understand that this process can be large investments for our clients, and provide the highest quality services when preparing our sub straights. Cost of which can be as high as a third of the floor itself at times.


Wood sub floors of any kind must be flat to within plus or minus 1/8″ over a 6′ span must be securely fixed, and any panels that have rot must be replaced prior to installation of a hardwood floor. We thoroughly test and document the moisture content of the wood sub floor using a hygrometer. The sub floor moisture content should be within 2% of the hardwood flooring to avoid excessive contraction or expansion once installed.


As with any other sub floor, joists must be level to within plus or minus 1/8″ over a 6′ span. The distances between the joists will determine the amount of fixings that can be used (you can only nail where there is a joist). Solid floors must be fastened every 10 inches. Most houses in Southern California have joists that are 16 inches apart. If you are fastening a hardwood floor to two by fours on top of existing concrete or screed sub floor, the concrete/screed must comply with the conditions laid above.