Standard 3/4-inch-thick solid strip flooring must be nailed
into a wood subfloor. This includes plywood, OSB, or plank
subflooring — as long as certain conditions are met. My
preference is 3/4-inch plywood; OSB can swell if it gets wet.
Old plank subfloors are usually okay as long as there’s
not a moisture problem in the building. (As an aside, I do not
recommend the use of fire-retardant-treated plywood as
subflooring. The water used in the process is not always
properly removed before delivery, and the salts used in the
treatment process may cause fasteners to corrode and work loose
Beyond acceptable dryness, a few other qualities are
important for a nailing substrate to function properly. First,
the subfloor needs to be thick enough to allow the use of
standard length nails. There must be enough substrate area for
plenty of nails throughout the floor, which is why two-by
sleepers by themselves do not make an adequate nail base. Last,
to prevent movement during seasonal changes in RH, a subfloor
needs adequate strength and stiffness.
The key to prepping conventional subfloors is taking the
time to get rid of any potential squeaks and uneven areas
before the wood flooring goes down. In new construction,
properly glue and nail T&G plywood to floor joists to
provide a uniform, squeak-free surface. On an existing
subfloor, check for and nail off any loose spots and repair any
unevenness at joints.
Wood Floors Over Concrete
Concrete under a wood floor must be dry. If it’s not dry,
use heat and ventilation to dry it. Also, concrete should be
level to 1/4 inch over 10 feet and should be clean and free of
surface contamination before a flooring installation begins.
Concrete in contact with the ground should be placed over at
least 6 inches of crushed stone or gravel with a 6-mil
polyethylene vapor barrier under the concrete (for a more
complete discussion, see "Laying Wood Floors Over Concrete
A vapor barrier is vital because concrete is not waterproof,
but somewhat porous. Looked at under a microscope, concrete has
tiny spaces throughout. If you doubt this, pour a cup of water
on any unsealed concrete surface and watch the concrete absorb
the water. The tiny voids in concrete allow water and
especially water vapor to move through the concrete in a manner
similar to capillary action.
Before delivering flooring to a wood-over-concrete job,
check moisture levels. If there is excessive moisture present,
it will continue to evaporate up through the surface of the
concrete. There are basically three ways to test for moisture
in concrete: electronic moisture meters, calcium chloride
crystals, and the rubber mat/polyethylene test, which I use
because it’s easy and dependable. I simply place several
solid, smooth-backed rubber or vinyl mats on the surface of the
concrete. A variation of this test is to tape 2 x 2-foot
squares of clear polyethylene to the surface of the concrete
with duct tape. After 24 hours, any dampness or even slight
darkness beneath the mat or polyethylene indicates excessive
moisture. Many times you can observe the presence of excessive
moisture in a slab by looking under boxes, plywood, or other
items that may have been stacked on the surface overnight.
It is vital that you document the results of this test
somewhere in your field notes or job file. Many times the
customer wants work completed as quickly as possible and will
try to push you to "Just do it!" If you fail to point out
potential problems to the customer and make them understand the
risks, you may assume the liability.
When my company encounters this problem, I write a letter
explaining the situation and include an unconditional release
of liability with signature blocks for the customers. I also
request a meeting where I present the facts calmly and
professionally. I then present the letter for their signature
and explain that I am unwilling to assume the increased risk of
proceeding with the delivery of materials until the excessive
moisture conditions are eliminated. If a customer won’t
accept the risk, I believe it’s more cost-effective to
find some other work rather than to do a job twice but get paid
Plywood Substrate Over
I don’t install wood flooring over
2x4 sleepers and I don’t recommend it under any
circumstance. Instead, I provide a nailing substrate using
fastened or floating plywood systems laid over a poly vapor
barrier right on top of the concrete. The reason often stated
to justify sleeper systems is to provide space for the floor to
breathe. If we agree that breathing is unnecessary because no
moisture is present and that the use of sleepers without a
plywood subfloor doesn’t provide an adequate nailing
substrate, then the extra cost and hassle of installing
sleepers under plywood is unjustified.
Leveling the slab.
Check to see that the slab is flat and does not vary more than
a gradual 1/4 inch over 10 feet when checked with a
straightedge. Fill any low spots with a leveling product
— I usually just use clean dry sanitary mason’s
sand (Figure 4).
4. The author screeds dry mason’s sand to level a
low spot on a slab before installing a plywood nailing
substrate. The trick with this technique is to avoid stepping
in the sand after you’ve screeded it level and covered it
with a poly vapor barrier.
Just be careful not to walk in the sand before you cover it
with rigid foam insulation or plywood.
Plywood fastened to
After placing a poly vapor barrier over the
slab, lay down sheets of 3/4-inch-thick plywood, leaving
1/4-inch spaces between panels and at least a 1/2-inch space
around the room perimeter. Then fasten the plywood directly to
the concrete with power-driven concrete nails. The poly vapor
barrier is still 99% intact, with small punctures every square
foot or so. Under normal conditions these punctures should not
dramatically decrease moisture resistance.
Next, proceed with the normal installation by using 15-lb.
asphalt-saturated building paper or felt between the plywood
and the finish flooring. It will be necessary either to use
shorter 13/4-inch flooring fasteners or to slightly tilt the
nailing machine so that nails don’t strike the concrete
and damage the tool (Figure 5).
5. When installing flooring over a single sheet
of plywood on a slab, a shim beneath the nailer shoe
will angle the nail to prevent its striking the
Floating plywood on
slab. Whenever I’m worried about future
moisture intrusion through the slab, I use the floating plywood
method. First, place 6-mil poly over the concrete. To protect
against anticipated moisture problems, take the extra steps of
taping the overlaps with duct tape and extending the poly up
the walls several inches. You may want to put down a layer of
compression-rated rigid foam insulation (no gluing necessary)
to limit heat loss through the floor (Figure 6).
6. To insulate a floor installed over concrete,
the author installs compression-rated rigid foam
insulation under the floating plywood nailing
Then, place a layer of 1/2-inch plywood oriented with the
long direction of the room. Space the panels 1/4 to 1/2 inch
apart and leave at least 1/2 inch along the perimeter. Do not
fasten this layer to the concrete.
Then lay a second layer of 1/2-inch plywood over the first,
orienting these panels 45 degrees to the first layer with the
same spacing at the edges and perimeter. The 45-degree
orientation of the second layer keeps the joints of the two
layers from lining up and helps to form a rigid monolithic
system when the installation of the flooring is complete.
Staple the two layers together with a pneumatic stapler, making
sure that the staples don’t go completely through the
bottom layer. You may substitute screws for staples, but be
careful not to puncture the polyethylene vapor barrier. Then
proceed with normal installation, using 15-lb.
asphalt-saturated felt between the plywood and finish