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Dry out the job site, prep the subfloor, and don’t skimp on nails

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Even after 20 years in the wood flooring industry, I still get immense satisfaction when my tools are packed up in the van and I can admire the natural beauty of a newly completed job. For that wood floor to perform flawlessly throughout its projected 50- to 80-year life span, however, there are many crucial steps I have to carefully perform.

Moisture Control Is Critical

Accounting for moisture is the single most important factor when laying a hardwood floor. Understanding how moisture affects wood is not a mystical art taught only in the inner sanctum of the Temple of Wood Flooring, it’s common sense. Simply put, wood reaches a state of equilibrium with the relative humidity (RH) of the surrounding materials and environment. When RH is low, wood loses water molecules and shrinks. When RH is high, wood gains water molecules and swells. Wood scientists and lumber manufacturers long ago figured out the relationship between the amount of water in wood and the surrounding RH levels (see Figure 1).

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Figure 1. As relative humidity (RH) in the air rises, so does wood’s moisture content (MC). Wood swells in size until it reaches its fiber saturation point, at an MC level of 28%.

We call the amount of water contained in a wood sample its moisture content (MC), which is calculated for each type of wood as a percentage of its weight when oven dry (OD). A sample is deemed oven dry when it has been baked in a 200°F oven for 24 to 48 hours and all of the water molecules have been removed. A wood sample shrinks to its minimum dimensions at 0% RH and 0% MC. Maximum swelling occurs at 100% RH and 28% MC, at which point wood reaches its fiber saturation point (FSP). The FSP is the total amount of water molecules that can be absorbed within the microstructure of wood.

At the Mill

Because of the shrinking and swelling that accompanies changes in RH, it makes no sense to manufacture any wood product to its final dimensions and shape until it has been dried. The first step in lumber processing is to remove the excess moisture from the green wood. After the lumber is rough-cut, it’s dried to an MC falling in the range of RH levels to which the wood will be exposed as a finished product. During the 1930s, a nationwide survey of interior RH and MC by the USDA found regional variations based on local climatic conditions (Figure 2).

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Figure 2. Seasonal ranges in RH vary across the U.S. Optimally, both the subfloor and the strip flooring should be at mid-range MC levels shown on this map during installation. This means drying out new construction sites before delivering materials, and acclimating materials only if you are in extremely dry or moist areas of the country. Today most manufacturers kiln-dry wood building materials to an MC of 7.5% at a corresponding RH of 40% — roughly the national average.

Job-Site Moisture

Excessive moisture at the job site is the leading cause of problems with wood floors. All of the expense and effort to properly kiln-dry and precisely manufacture wood strip flooring are for naught if it is later exposed to excessive moisture and swells before, during, or after it is installed. Much of the wood flooring industry literature is misleading because it emphasizes acclimating wood flooring to job sites. In reality, the reverse is true: A job site needs to be dried out before any wood flooring arrives. There is nearly always excessive moisture on new construction sites and major remodeling job sites. Wood flooring should never be delivered to the job until all excessive moisture has been eliminated. The quickest and most effective method for removing job-site moisture is to run the heating system and increase fresh air ventilation. Wood flooring should be installed only after the interior MC level of a structure has been reduced to within the range that will prevail during the life of that structure after it is occupied. This prevents the excessive moisture present during any major renovation or new construction project from being absorbed into a kiln-dried wood floor. If the wood subfloor over which the wood floor is to be installed contains excessive moisture, then the wood flooring will absorb the moisture and swell. Three of the biggest lies I’ve been told in my 20 years as a flooring contractor are, "We’re from the government and we’re here to help you;" "The check is in the mail;" and "This job is as dry as a bone — you can start laying the floor next Monday." Even if there is no intent to deceive, you need to confirm that a job site is dry by measuring the moisture content of the subfloor. If you’re acting as the flooring sub, don’t rely on the contractor to control site moisture. In the end, it’s the installer who needs to take responsibility for checking MC levels. Water vapor is colorless and odorless, so it can only be reliably measured with moisture meters (Figure 3).

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Figure 3. Using a moisture meter is the only accurate way to check for excessive moisture in subfloors and finish flooring strips. A pocket-size meter costs less than $200 and is an essential tool for good wood flooring installations.

Electrical resistance meters are the simplest type to use and provide the only practical non-destructive way to determine moisture content in wood frame construction. Two pins are driven into a wood surface parallel to the grain and the meter gives an MC reading. Small pocket versions are available from several manufacturers for less than $200.