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Flooring Installation Basics The below gives the proper steps and sequence for installing a tongue-and-groove wood-strip floor. Four issues always seem to generate the most interest and discussion: acclimation of the flooring, use of 15-lb. asphalt-saturated felt paper, frequency of nailing, and whether to leave an expansion space. To acclimate or not? Ideally, the proper moisture content for the installation of any wood floor is midway between the seasonal high humidity, which occurs during the summer, and the seasonal low humidity, which occurs near the end of the first winter heating season after construction is completed. In my 20 years of consulting in the wood flooring industry, I have often seen damage done to wood flooring from "acclimating" the flooring to the job before installation. Because the quality of kiln-drying within the wood flooring industry is high, acclimation is only appropriate in extreme climates where interior relative humidity levels are substantially above or below the 7.5% MC/40% RH manufacturing specifications. This includes the arid regions of the western U.S. and the humid southeastern U.S. Acclimating wood flooring in other regions actually risks exposing it to the high levels of relative humidity and moisture present during summer or on a new construction site. This will cause the wood flooring to swell before and during installation. Then, during the first heating season, it will shrink and permanent spaces will be left between the flooring strips. Instead, dry out the building before bringing the flooring on site.

Felt paper.

I’ve often heard doubters ask, "Why should I use 15-lb. felt, and what does it really do?" It performs four functions:

1. Felt slows the flow of water vapor.

It gradually absorbs moisture in the subflooring, allowing it to pass through the flooring slowly, instead of flooding the underside of the flooring and causing it to swell and possibly cup.

2. Felt increases the friction between the flooring and the subfloor,

which in turn helps to resist lateral movement in the event that swelling occurs.

3. Felt adheres to the subfloor

and wood flooring and will help to eliminate any vertical looseness or movement that might occur.

4. Finally, felt will help safeguard a manufacturer’s warranty

in the event that you encounter a problem like cupping or large spaces between the boards. The majority of the wood flooring manufacturers consider the use of 15-lb. felt to be mandatory and will deem your installation to be negligent if you don’t use it. The cost for materials and labor to include 15-lb. asphalt-saturated felt underlayment is approximately 5¢ per square foot. That’s cheap insurance even if its only function is to avoid warranty issues in the event of a problem.


I go nuts when I hear a person say that you should not use too many nails because then the flooring can’t move — the argument of many hackers and trunk slammers in the construction business. There is no such thing as too many nails unless they start splitting the flooring strips into little pieces. Nails are what hold the flooring in place and keep it from moving. I prefer to think in terms of nails per square foot to determine the proper fastening for flooring of different widths. If the optimal nailing interval for 21/4-inch-wide strip flooring is 6 to 8 inches, this is equal to 9 or more nails per square foot. The schedule below gives the average nailing interval necessary for 9 nails per square foot for other flooring widths.

Recommended Nailing for Strip Flooring

Flooring Width





10 to 11


6 to 8

5 to 6


3 to 1

2 to 3


Expansion gap.

Since wood shrinks and swells very little in length (with the grain direction), there is no need to allow any space at the ends of the flooring. In fact, I generally don’t worry about leaving expansion spaces anywhere — sides or ends — when installing a traditional nail-down T&G floor (Figure 8).

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Figure 8. Because he uses plenty of nails to keep the flooring from moving, the author doesn’t leave perimeter expansion spaces on typical residential strip flooring installations. Countersunk face nails hold the perimeter strips snug; the last strip is scribed tight to the wall. To prevent splitting when nailing these last courses, the author uses a nail spinner from Vermont American (right), which chucks into a drill, to drive the finish nails. This is because, as mentioned above, I use enough nails to firmly hold the individual pieces of flooring in place. As long as the moisture cycle stays within the normal range for the region where the floor is installed, movement isn’t usually a problem when the flooring shrinks and swells. Wood is somewhat elastic and can be slightly compressed without becoming permanently deformed. The exception is when there’s an unexpected source of excessive moisture, such as a plumbing leak or an abnormally long period of extremely high humidity. In that case, the swelling of the floor can exert such pressure that the wood actually compresses permanently. Then, when the moisture subsides and the flooring shrinks, large gaps will result and the floor will creak and pop when it is walked on. In the case of excessive moisture, leaving a perimeter expansion gap will do nothing to prevent damage. Expansion is typically less of a problem than shrinkage. In the Boston area, for example, most nail-down solid tongue-and-groove flooring is fit tight against pre-installed baseboards and other moldings. Later, during the first heating season, the flooring shrinks, leaving gaps between the perimeter flooring and the baseboards. To remedy at least part of this problem, it’s wise to install baseboards over the already installed wood floor. Remember, this discussion applies only to nail-down floors over attached subfloors. When installing floating or glue-down wood floors, however, you must be certain to leave adequate expansion spaces. Those are entirely different installations, and a subject for a different article.

Prep Work

Make sure the moisture content (MC) of the subfloor is within a normal range (no more than 2% beyond the maximum MC for the region). Be sure to eliminate all excessive moisture before you even deliver the flooring to the site. Check the wood subfloor for looseness and proper nailing. Remove squeaks now! Repair any damaged areas in the subfloor. Point out any major uneven spots to the customer and find out if they want you to repair them. Lay down 15-lb. asphalt-saturated felt underlayment. This is the cheapest insurance policy you can buy. Mark floor joist locations for the first and last courses of flooring.


Pick out straight pieces of flooring to start and finish the floor. Nothing is quite as frustrating as trying to rip a warped and twisted piece of flooring to fit against the wall. Use a string, chalk line, or straightedge to start the floor. Start straight to stay straight (Figure A).

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Figure A. Snapping a chalk line to start the layout in a new room ensures the flooring runs parallel to the walls.

This, in conjunction with the straight boards, makes nailing a walk in the park. Stagger the end joints at least 6 to 8 inches as you lay out the floor. Nail every 6 to 8 inches for 21/4-inch strip flooring and even closer as width increases (see Figure 7).

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Figure 7. You need to drive lots of nails into wood flooring to hold it firmly in place during regular seasonal climate swings. The wider the flooring, the more nails that need to be driven along the nailing edge of each board.

Use splines whenever you have to reverse direction — in other words, run the flooring groove to groove (Figure B).

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Figure B. The author makes splines to use whenever he has to run the flooring groove side to groove side.

Be sure to nail down the groove edge so that it doesn’t rise up from the pressure applied during installation. Howard Brickman is a flooring contractor and consultant based in Norwell, Mass.