At this time, Lumber Liquidators does not recommend gluing down ¾”Solid wood directly to a concrete slab, typically this has been a nail down product.
In recent times, some of the big home center stores have been advocating this procedure. Adhesive manufacturers recommend precision milled boards and the use of shorts or board length sizes under 40”, such as our HAND SCRAPED SOLID collection.
Experience with site related moisture, temperature and humidity fluctuations has shown the longer ¾” solids to be unstable when glued to cement, in fact, without raising subfloor base too high, engineered floors are actually the floor of choice for direct glue down on concrete slabs. These are constructed or engineered with a plywood core, designed to withstand temperature and moisture changes.
However, solid flooring sizes 5/16”, 3/8”, ½” and 5/8” can be glued to cement. Solid wood glue down installations can be more difficult when compared to engineered styles. The longer solid boards can be twisted or bowed right out of the box. While this is not an issue when nailing, these can present problems when gluing down. When working with solid wood it is advisable not to remove the plastic binding straps keeping the boards secure until ready to install, thus ensuring straighter boards. Do not lay out more boards that can be installed the day of installation. Culling or selecting out bowed or twisted boards can produce greater than normal waste and should be accounted for when ordering. Boards with slight bows can be cut in half then used. However, for these reasons we recommend the use of professional flooring installers for solid wood glue-down installations.
In addition, due to the rigidity of solid wood, a flat surface is required (deflection not to exceed 3/16” in 10’). Floor flatness issues can be corrected by filling low areas with cement patches or levelers and grinding down high spots. Blue painters tape (#2080) should be used to help keep surrounding boards grouped together while the adhesive cures. Some installers use straps in an effort to tighten up their glue-down installations. Caution is needed. Be aware that over-strapping can produce board lifting, peaking, board-edge crushing, or glue-bond failure. Many installers use a 75-100lbroller to ensure adhesive transfer and evenness of the board edges.
A successful ¾” solid installation on concrete can be accomplished using several methods as outlined below. First test for excess moisture and remedy
You will need a moisture barrier and a nailing base.
(1) A platform sleeper system- Install a 2”x4” sleeper framing system (2x4s laid down) all around the perimeter and ½” from the walls (to prevent side wicking of moisture). Followed by 2x4 sleepers laid every 12” on center. This framing is both urethane glued and nailed to the concrete. Insulating material can be used to fill in the space between the sleepers. Next, completely cover this framing system with heavy Plastic 6mil sheeting, overlap the seams by 6” and securely tape. Then install a wood sub-floor platform (minimum ¾ inch plywood) over the 2x4 sleepers. Install the new flooring over 15lb roofing felt. NOFMA
When addressing height restrictions;
(2) A plywood nailing base “wet set” using urethane glues- First, trowel on Bostik’s or Mapei’s moisture barrierallow to cure 16-20hrs. To make the 4’x8’ plywood nailing base flexible, score plywood sheets on the backside every 8” to 10” by using a circular saw, cutting one half the thickness of the sheets. Scoring or “kerfing” relives the tension from plywood, allows for floor contour and helps prevent future warping or curling. Or one can cut the 4x8 plywood down into 12”x 8’ flexible boards. Using a ¼” by ¼” square notched trowel, apply Bostik’s Best (BBA) or Mapei (990) urethane adhesive. Set the 12”x 8’ plywood boards into the wet adhesive. Use weights to hold in place. Allow to fully cure before nailing or gluing any new installation. NOFMA
(3) A Floating plywood sub-floor system- After laying 8mil plastic sheeting as a moisture barrier over the cement, overlap seams 4” and duct taped. Lay one layer of 5/8” plywood over the plastic sheeting, lay another layer of 5/8” plywood on a diagonal over the first for stability, screw both together using a 6” screw pattern do not puncture plastic. Keep an expansion space of ½” from all walls. If plywood sheets are square-edge, provide a 1/8” gap between all sides of adjoining plywood panels. Install new flooring over 15 Lb roofing felt. NOFMA
(4) PLYWOOD FASTEN-TO-SLAB SYSTEM- This system uses 3⁄4” or thicker sheathing grade exterior plywood as the subfloor over an appropriate moisture barrier, 8mil plastic sheeting. 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 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. Start at the center of the panel and work toward the edges, fasten securely. NOFMA
*Note; Although many installers have had real success nailing or fasten plywood to the cement for use as a nailing base, keep in mind that fastening plywood with nails or screws will breach any applied moisture barrier. Therefore, there is a propensity for future moisture contamination as moisture can slowly wick up into the subflooring by way of the fasteners. Thus promoting mold growth or board edge cupping.
a. 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”. 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.
b. 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.
*Note; Do not use salt or pressure treated lumber with wood installations as these can have high moisture and chemical content. Moisture content of any wood components for the wood sub-floor or new floor should not exceed 12%.
As can be seen, there are many methods to successfully install solid hardwood over cement. Therefore, the methods outlined here are intended to be guides only. Whichever nailing base method you choose must also provide enduring protection against moisture.
*Always consult your professional installer for his guidance and established procedures for your region of the country.
