hen preparing the concrete slab, the
slab must be sound and flat. To prepare the slab
grind off any high spots, fill low spots, clean up grease,
oil and other contaminants, and sweep clean. If the
slab is "mealy" and excessively dusty, it may not be of
proper strength.
Vapor Retarder
To be certain normal slab moisture does not reach the
finished floor, a proper vapor retarder must be used
on top of the slab. Where this is placed will depend on
the type of system used. The vapor retarder should have
a U.S. perm rating of less than 1 perm. 6 mil polyethylene
film has a 0.04 perm rating and is considered a good
choice.
With 3/4in plywood used as a nailing base, the recommended
vapor retarders are affixed to the slab. These
systems may be either 2 membrane asphalt felt/building
paper and mastic or a 4-6 mil polyethylene film or an
equivalent system as described below.
Two membrane asphalt felt or building paper system
Prime and apply cold cut-back asphalt mastic
with a notched trowel at the rate of 50 sq. ft per gallon.
Let set 2 hours. Roll out 15 lb. asphalt felt/building
paper, lapping edges 4in. Butt ends. Over this apply a
second similar coating of mastic and roll out a second
layer of asphalt felt/building paper. Lay both layers
of felt in the same direction, but stagger the overlaps
to achieve a more even thickness.
Polyethylene method. When slabs are well above
grade and the expected annual rainfall is light to moderate,
cover the entire slab with 4- to 6-mil polyethylene
film, overlapping edges 4-6in and allowing enough
to extend under the baseboard on all sides.
Where moisture conditions are considered more severe,
prime and apply cold-type cut-back asphalt mastic with
a straight-edge or fine tooth trowel over the entire slab
surface (100 sq. ft. per gal.). Allow to dry about 1 hour.
Lay the 4-6 mil polyethylene film over the slab, covering
the entire area and lapping edges 4-6in. "Walk in" or roll
in the film, stepping on every square inch of the floor to
insure proper adhesion. Small bubbles are of no concern,
and may be punctured to allow captive air to escape.
Plywood-On-Slab System
This system uses 3/4in or thicker sheathing grade exterior
plywood as the subfloor over the appropriate vapor retarder.
Loose lay 3/4in plywood panels over entire floor. Laying plywood
on a diagonal to the direction of the finished floor will
help prevent cracks associated with panel edges.
Stagger plywood and joints every 4ft by cutting the first
sheet of every other run in half. Leave 3/4in space at all wall
lines and 1/4in to 1/2in between panels. Cut plywood to fit within
1/8in near and around door jambs and other obstructions
where finish trim will not be used.
Fasten the plywood with a powder-actuated concrete
nailer or hammer-driven concrete nails. To be sure to
flatten out the plywood, start at the center of the panel
and work toward the edges. Use at least nine nails per
panel or more to fasten securely.
An alternate method is to glue the 3/4in plywood over the
vapor retarder systems which include the cut-back mastic.
Cut the 3/4in plywood into 4ft x 4ft squares or 16in x 8ft
planks, score the back 3/8in deep on a 12in x 12in grid, and
lay panels in the cut-back mastic applied with a 1/4in x 1/4in
notched trowel (35 sq. ft. per gal.). Remember to stagger
panel joints by 2 ft.
Screeds System
This system uses as a nailing base flat, dry 2in x 4in screeds
of Group 1 density wood (sometimes called sleepers) of
random lengths from 18in to 48in, as a nailing base. They
must be preservative treated with a product suitable for
interior installation. After treatment screeds must be
dried to a Moisture Content of 12% or less, if saturation
with water is involved.
Screeds are laid on their flat face in rivers of mastic
with screed runs 12in on center at right angles to the direction
of the finished floor.
Sweep the slab clean, prime with an asphalt primer
and allow to dry. Apply hot (poured) or cold (cut-back)
asphalt mastic and imbed the screeds. Stagger joints and
lap ends at least 4in and leave 1/2in space between lapped
edges. Be sure there is enough mastic for 100% contact
between screeds and slab. Leave 3/4in space between ends
of screeds and walls with a continuous run of screeds at
end walls.
Over the screeds lay a 4- to 6-mil polyethylene vapor
retarder with edges lapped over rows of screeds. Avoid
bunching or puncturing it, especially between screeds.
The finish flooring will be nailed to the screeds through
the film.
The system with screeds spaced 12in on center and a
moisture retarder without a subfloor is satisfactory for
all 3/4in Strip Flooring and Plank Flooring less than 4in wide.
Plank Flooring 4in and wider requires either the Plywood-
On-Slab subfloor, or screeds plus a wood subfloor, to provide
an adequate nailing surface. The subfloor over
screeds may be 5/8in or thicker plywood, 3/4in OSB (performance
rated), or 3/4in Group 1 dense softwood boards or
equivalent no wider than 6in. If subfloor boards are used
over sleepers or screeds, allow 1/2in spacing between boards.
When area moisture conditions are
considered high (Gulf coastal area) use the
vapor retarder glued directly to the slab system
in addition to or in substitution for the
film draped over screeds.
Installation Over Wood Joists
Outside cross ventilation in the foundation walls must
be provided through vents or other openings with no dead
air areas. A surface cover throughout the crawl space
(100%) of 6 mil polyethylene film is essential as a moisture
retarder.
Subflooring. With 3/4in thick strip flooring use either kilndried
boards of NO. 1 or NO. 2 Common Pine or other
dense, Group 1 softwoods suitable for subfloors over wood
joists, or exterior sheathing grade plywood. If plywood,
5/8in (19/32in) or 3/4in (23/32in) performance rated products are preferred.
Also, 3/4in (23/32in) OSB is a comparable substrate. With
1/2in thick strip flooring use a 3/4in (23/32in) subfloor. Thinner materials cannot be recommended as a preferred
subfloor material.
A summary of subfloor test results is available. Install
subfloor panels as recommended by the panel manufacturer.
They should be installed with grain of faces at right
angles to joists, nailed every 6in along each joist with
appropiate nails and with appropiate spacing at panel
ends and edges unless otherwise recommended by the
panel manufacturer.
For a board subfloor, use only flat, dry 3/4in dressed
square edge boards no wider than 6in. Lay diagonally
across the joists; allow 1/4in to 3/8in expansion space between
boards. Don't use tongue and groove boards. Nail to every
bearing point (includes blocking) with two 8d common
nails. All mitered joints must rest on joists.
Mark location of joists so flooring can be nailed into them.
Good nailing is important. It keeps the boards rigid,
preventing creeping sometimes caused by shrinkage in
subfloor lumber. Without adequate nailing it is impossible
to obtain solid, non-squeaking floors.
Flooring Installation Guide :: Index
1) Installation Introduction
2) Handling & Storage
3) Job Site Conditions
3.1) Installing over Concrete
3.2) Testing Concrete for Excessive Moisture
3.3) Concrete Slab Preparation Prior to Installation
4) Laying & Fastening
5) Plank Flooring
6) Laying a New Strip Floor Over an Old Floor
7) Parquet, Block Herringbone & Similar Flooring
8) Special Construction Situations
9) Tips for Easier & Better Flooring Installations
10) Strip Flooring on Walls & Ceilings
11) Installation of Gymnasium Floors over a Concrete Slab
Installation content courtesy of NOFMA. FloorMall.com is a Proud Member of NWFA.
