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
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
3. Felt adheres to the
subfloor and wood flooring and will help to
eliminate any vertical looseness or movement that might
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
Nailing. 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
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.
Nailing for Strip Flooring
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. 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
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.
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
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).
A. Snapping a chalk line to start the layout in
a new room ensures the flooring runs parallel to the
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
Nail every 6 to 8 inches for 21/4-inch strip flooring and
even closer as width increases (see Figure 7).
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. The author makes splines to use whenever he
has to run the flooring groove side to groove
Be sure to nail down the groove edge so that it
doesn’t rise up from the pressure applied during
Howard Brickman is a flooring contractor and consultant
based in Norwell, Mass.