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| Southern Pine Flooring: Subfloor Installation |
A suitable moisture barrier and subfloor nailing base (over concrete slab or
wood joists) must be installed to provide adequate protection and support for
the flooring.
Remember: the nailing base should be in place prior to flooring delivery and
the acclimation period.
Recommended Moisture Barriers
Two types of moisture barrier material are recommended beneath Southern
Pine flooring:
Polyethylene: A 6-mil thickness is preferred; 4-mil is acceptable. The 6-mil
material has a perm rating of 0.08. A perm rating below 1.0 for polyethylene is
required to retard moisture.
Roofing felt paper: Two layers of 15 lb. paper, or one layer of 30 lb. paper.
Subfloors Over Concrete
Concrete is a porous material that allows the migration of water vapor from
the ground into the structure. Concrete hardens over time, but it can still contain
water up to 60 days after installation, and sometimes longer.
Proper construction practices require that a vapor barrier be installed prior to
the concrete installation to reduce the moisture absorption process. Southern
Pine flooring can be installed on concrete subfloors, either on or above grade.
Once the concrete is determined to be dry, a moisture barrier and nailing
base can be installed. Refer to Figures 5 and 6.
To create a nailing base over concrete, two methods can be used: either a
layer of dimension lumber strips (called "screeds") or a layer of plywood. When
using the screed method, it is recommended that 2x4 material be used, cut into
lengths varying from 18" to 48".
Screeds must be dry. If pressure-treated screeds are used, they
MUST be kiln dried after treatment (KDAT).
Clean the concrete, removing any oils or other foreign material. For a screed
nailing base, apply a cold cutback asphalt adhesive mastic to the surface at a
spread rate of 35 square feet per gallon and embed screeds on the wide face,
12" on center, perpendicular to the direction the flooring will lay. End walls
should have a continuous screed run. A 3/4" gap should be left between walls
and screed ends. Apply the moisture barrier, overlapping sides 4" and touching
end walls.
For a plywood nailing base over concrete, spread cold cutback mastic at a 50
sq.ft. per gallon rate. Allow two hours setting time; lay the moisture barrier with
a 4" side overlap and touching the end walls. When using a 15 lb. felt moisture
barrier, an additional layer of a cold cutback asphalt adhesive mastic and felt is
applied. Stagger felt seams to prevent direct alignment with the first layer. Laying
plywood diagonally to the direction of the flooring will offset plywood joints
with floor nailing.
Start with a 4'x4' section every other course, staggering plywood joints. Allow
a gap of 1/4" to 1/2" between panels, and a 3/4" gap at walls. Use at least 9
nails per panel, nailing outward from the center, flattening the panels. Refer to
Figures 5 and 6.
Concrete Condition Check
If a moisture meter is unavailable to check the condition of the concrete,
a sheet of polyethylene cut to 24"x24" will reveal the slab’s moisture
condition. Secure the polyethylene to the concrete with duct tape (A), forming an air-sealing
bond. Wait 24 hours. If condensation develops between the concrete and the polyethylene
sheet (B), do not install flooring; the concrete is too wet for wood flooring and must be permitted
more time to cure. If the structure is several months old and the concrete has cured, yet
the poly test still reveals condensation formation, inspect the site for moisture problems caused
by improper drainage. Slower percolating soils do not drain as efficiently, causing hydrostatic
water pressure. Correct a drainage problem by upgrading or adding drain tiles at the foundation
footings.
A similar moisture test for concrete is the "rubber mat" test. Lay a rubber mat flat on
the concrete floor. Place a concrete block, tool box or other heavy object on top of the mat,
(C) trapping any air from escaping. After 24 hours, lift the mat. If the concrete is darker
beneath the mat on a lighter background concrete, then moisture has formed, indicating the
concrete is too wet for flooring. Dark concrete surfaces tend to camouflage wetness; therefore,
use the rubber mat test on light-colored concrete surfaces for best results.
Until the concrete is noticeably dry on the underside of the polyethylene or the rubber
mat, do not install the subfloor. Should wet concrete still be evident, fans and dehumidifiers
will improve air circulation and lower the humidity.
When the concrete is determined to be dry, a moisture barrier and nailing base can be
installed.
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Subfloors Over Wood Joists
For installation of flooring over wood floor joists, a subfloor nailing base
consisting of a minimum 5/8" plywood or 3/4" oriented-strand-board (OSB)
is required. Lay the subfloor panels perpendicular
to the joists.
Use of a caulk-type
construction adhesive
between plywood subfloor
and joists provides
additional anchoring
support. Starting with a
single 4'x4' panel in
one corner, alternating
courses of 4'x8' panels
are to be staggered, offsetting the end seams. An 1/8" space between panels
allows for panel expansion and prevents panel squeaks. Position all seams
between panels directly over the joists. A 6d screw or ring-shank nail is recommended
every 6" for fastening panels to floor joists.
Lay the moisture barrier atop the plywood, overlapping edges 4" and touching
all walls. If 15 lb. felt is chosen, stagger the edges of the second layer in relation
to the first.
Check local building codes to verify subfloor requirements for your area.
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