Job Site Conditions

Check the job site before delivery. Be sure the flooring will not be exposed to excessive periods of high humidity or moisture. The surface grade or slope should direct water away from the building.

Basements and crawl spaces must be dry and well ventilated. In joist construction with no basement, outside cross ventilation through vents or other openings in the foundation walls must be provided with no dead air areas. A surface cover of 6 mil polyethylene film is essential as a vapor retarder in crawl space construction.

The building should be closed in with outside windows and doors in place. All concrete, masonry, sheetrock and framing members, etc. should be thoroughly dry before flooring is delivered to the job site. In warm months the building must be well ventilated; during winter months heating should be maintained near occupancy levels at least five days before the flooring is delivered and until sanding and finishing are complete.

Because materials used to provide energy efficient structures trap moisture in a residence, it may be necessary to delay delivery and installation of flooring to allow the excessive moisture trapped during construction to evaporate. The average moisture content of framing members and subflooring should be below 12%-14% before delivery of the flooring. Moisture contents above 12%-14% can cause moisture related problems.

When job site conditions are satisfactory, have the flooring delivered and broken up into small lots and stored in the rooms where it will be installed. Allow 4 to 5 days or more, for the flooring to become acclimated to job site conditions. If flooring is packaged, open or remove packaging for acclimation.

From the time flooring is delivered and until occupancy, temperature and humidity should be maintained at or near occupancy levels. After occupancy, continue to control the environment. Extended times (more than 1 month) without HVAC controls can promote elevated moisture conditions which can adversely affect flooring.

Protect flooring from excessive heat. Flooring installed over a heating plant or un-insulated heating ducts may develop cracks unless protection from the heat is provided. Use a double layer of 15 lb., or a single layer of 30 lb. asphalt felt/building paper, or 1/2" standard insulation board between joists under the flooring in these areas. Over a heating plant the insulation used should be non-flammable.


Hardwood flooring can be installed successfully over a slab which is on-grade or above grade. Below-grade installations are not recommended. The slab must be constructed properly (dry and flat with a trowel finish).

Watch out for water. New concrete is heavy with moisture, an inherent enemy of wood. Proper on-grade slab construction requires a vapor retarder such as 6 mil polyethylene film between the gravel fill and the slab. While this prevents moisture entry through the slab, this membrane also retards curing of the slab. So test for dryness, even if the slab has been in place over two years. Slabs younger than 60-days are generally too wet for flooring installation.


NOTE: Make tests in several areas of each room on both old and new slabs. When tests indicate too much moisture in the slab, do not install hardwood floors. For a moist slab, wait until it dries naturally, or accelerate drying with heat and ventilation then test again.

1. The Rubber Mat Test. Lay a smooth, non-corrugated rubber mat on the slab, place a weight on top to prevent moisture from escaping, and allow the mat to remain 24 hours. If the covered area shows water marks when the mat is removed too much moisture is present. This test is worthless if the slab surface is other than light in color originally.

2. The Polyethylene Film Test. Tape a one-foot square of 6 mil clear polyethylene film to the slab, sealing all edges with plastic moisture resistant tape. If, after 24 hours, there is no "clouding" or drops of moisture on the underside of the film, the slab can be considered dry enough to install wood floors.

3. The Calcium Chloride Test. Place a quarter teaspoonful of dry (anhydrous) Calcium Chloride crystals inside a 3-inch diameter putty ring on the slab. Cover with a glass so the crystals are totally sealed off from the air. If the crystals dissolve within 12 hours the slab is too wet.

4. The Phenolphthalein Test. Put several drops of a 3% Phenolphthalein solution in grain alcohol at various spots on the slab. If a red color develops in a few minutes, too much moisture is present.


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.



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 3/4" 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 4". 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 4-6" 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 4-6". "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.



This system uses 3/4" or thicker sheathing grade exterior plywood as the subfloor over the appropriate vapor retarder. Loose lay 3/4" 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 4' by cutting the first sheet of every other run in half. Leave 3/4" space at all wall lines and 1/4" to 1/2" between panels. Cut plywood to fit within 1/8" 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 3/4" plywood over the vapor retarder systems which include the cut-back mastic. Cut the 3/4" plywood into 4' x 4' squares or 16" x 8' planks, score the back 3/8" deep on a 12" x 12" grid, and lay panels in the cut-back mastic applied with a 1/4" x 1/4" notched trowel (35 sq. ft. per gal.). Remember to stagger panel joints by 2 ft.


This system uses as a nailing base flat, dry 2" x 4" screeds of Group 1 density wood (sometimes called sleepers) of random lengths from 18" to 48", 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 12" 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 4" and leave 1/2" space between lapped edges. Be sure there is enough mastic for 100% contact between screeds and slab. Leave 3/4" 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 12" on center and a moisture retarder without a subfloor is satisfactory for all 3/4" Strip Flooring and Plank Flooring less than 4" wide. Plank Flooring 4" 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 5/8" or thicker plywood, 3/4" OSB (performance rated), or 3/4" Group 1 dense softwood boards or equivalent no wider than 6". If subfloor boards are used over sleepers or screeds, allow 1/2" spacing between boards.

NOTE: 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.


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 3/4" thick strip flooring use either kiln-dried 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, 5/8" (19/32") or 3/4" (23/32") performance rated products are preferred Also, 3/4" (23/32") OSB is a comparable substrate. With 1/2" thick strip flooring use a 3/4" (23/32") 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 6" along each joist with appropriate nails and with appropriate spacing at panel ends and edges unless otherwise recommended by the panel manufacturer.

For a board subfloor, use only flat, dry 3/4" dressed square edge boards no wider than 6". Lay diagonally across the joists; allow 1/4" to 3/8" 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.