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​Gluing Down Solid Wood Flooring to Concrete

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Gluing Down Solid Wood Flooring to Concrete

This is a question that I have started to receive more and more. My answer to that is yes and it is still warranted. But, the catch is that the solid wood flooring we have that can be glued over concrete and warranted is 5/16″ and 7/16″ solid hardwood, not 3/4″ solid hardwood and can be only on on-grade and suspended concrete. Most 3/4″ solid hardwood is made as a nail/staple down installation. It is not because the adhesives will not adhere to the solid hardwood. It is the fighting of the natural tendencies of the solid hardwood to bow, twist, and curl. I have looked through many suppliers web sites such as Anderson, Armstrong, Bruce, Carolina Mountain, Cikel, Columbia, Harris Wood, Indusparquet, Lauzon, Lumber Liquidators Bellawood, Mirage, Mercier, Mohawk, Shaw, Somerset and all were a Nail/Staple Down Installation. All wood flooring is a natural product and will move with varying moisture and humidity levels, and

solid wood flooring will move more than engineered because it is not stabilized by alternating directional layers. Due to this movement, solid wood should not be installed in basements since they are below-grade and highly prone to moisture and relative humidity issues. If someone wants a 3/4″ hardwood product, we have several 3/4″ Engineered Hardwood products that can go below grade.

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The direction of the hardwood movement is determined by the annual growth rings. Plain-sawn boards will expand and contract through the width of the board. The wider the plank, the more potential for movement. Typically, solid flooring that is quarter-sawn produces the most stable board possible. “Quarter-sawing” is considered the best hardwood flooring, and generally refers to wood that ranges in grain angle from 60 to 90 degrees. By having such a high grain angle, the usual expansion and
contraction that hardwood floors experience with changes in humidity are kept to an absolute minimum because the expansion of the flooring is up and down rather than side to side. A quarter-sawn board expands and shrinks mostly across its thickness. Therefore, a floor made of quarter-sawn boards shrinks and expands significantly less across its width, making a much more stable floor.

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There is always that customer out there who insists they want 3/4″ solid hardwood installed in their basement. This is a decision you will have to make, as this will void any and all warranties. Below is what I found under the warranty section of most suppliers;

NO WARRANTY for special applications such as gluing solid hardwood flooring to concrete subfloors. Contact the sealer/adhesive system manufacturer for their recommendations and warranty.

Now, there are ways to install 3/4″ Solid Hardwood to an on-grade or suspended concrete slab. Per the NWFA and , you may install a subfloor over concrete and there are different ways to do this;
Floated Subfloor
A. In on-grade and below-grade applications, always add vapor retarder before applying underlayment.
B. In above-grade applications, follow the flooring manufacturer’s recommendations.
C. A vapor retarder is recommended anytime solid 3/4″ wood flooring is installed over concrete. A vapor retarder is required for installation over concrete with a calcium chloride reading greater than 3 pounds, a relative humidity reading of greater than 75%, or a calcium carbide (CM) reading of greater than 2.5%.

D. Floated Subfloor System
1. Materials a. 2 layers nominal 3/8” minimum CD Exposure 1 Plywood subfloor panels (CDX) 4′ X 8′ sheets.
2. Installation method:
a. Place the first plywood layer with edges parallel to wall, without fastening. Leave 3/4″ space between wall and plywood.
b. Plywood panels should be placed with 1/8″ gaps between sheets.
c. Lay the second layer perpendicular or at 45 degree angle to the first.
d. Plywood panels should be placed with 1/8″ gaps between sheets and a 3/4″ minimum expansion space at all vertical obstructions and wall lines.
e. Staple or staple and glue (with urethane or construction adhesive) the second layer to first layer on 12″ interior grid pattern (6″ on the perimeter). Be careful not to penetrate the vapor retarder.

Glue-Down Subfloor
A. Always follow the adhesive manufacturer’s recommendation for proper subfloor, spread rate and trowel notch.
B. If necessary, add vapor retarder before applying underlayment. A vapor retarder is recommended anytime solid 3/4″ wood flooring is installed over concrete.
C. Glue-Down Subfloor System:
1. Materials a. Use nominal 5/8″ (19/32) CD Exposure 1 Plywood subfloor panels, (Exposure 1), 4’x8’ sheets.

2. Installation method:
a. Cut the plywood panels to 2′ X 8′ or 4′ X4′ sections.
b. Score the back of the panels ½ the thickness on a 12″x12″ grid.
c. Apply an adhesive approved for the installation of plywood, per the plywood manufacturer’s recommendations.

d. Lay sections in a staggered joint pattern in the adhesive, with 1/8″ spacing between sheets, and 3/4″ minimum expansion space at walls
and all vertical obstructions.

Nail-Down Subfloor
A. Always follow the manufacturer’s recommendation for proper subfloor.
B. In on-grade and below-grade applications, always add vapor retarder before applying underlayment. In above-grade applications, follow the flooring manufacturer’s recommendations.
C. A vapor retarder is recommended anytime solid 3/4″ wood flooring is installed over concrete.
D. Nail-Down Subfloor System Over Concrete
1. Materials a. Minimum: use nominal 5/8″ (19/32) CD Exposure 1 Plywood subfloor panels (CDX), 4′ x 8′ sheets
2. Installation method
a. Stagger panel joints allowing approximately 1/8″ expansion space around all panels to prevent edge peaking due to compression caused by panel swell.
b. Allow 3/4″ minimum expansion space at all vertical obstructions.
c. Panels should be mechanically fastened. For powder load or pneumatic pressure information, contact your local supplier.
d. Fasten 2″ from the edge every 6-8″ along the perimeter of the sheet and one fastener or more spaced every 12” in the interior of the panel. Fasten the center first to prevent the subfloor from bowing. (See diagram.)
e. Areas with higher humidity may require additional fasteners.

NOTE: Fasteners may be powder-driven pins, pneumatic driven nails, screws, deformed pins, or other fasteners suitable for concrete application. Check with fastener manufacturer for specification such as length, drill size, and/or shot load where applicable.

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For the Nail Down Subfloor, one subfloor manufacturer that has written installation instructions for going over concrete is AdvanTech. http://www.huberwood.com/technical-library/1/16?
alias=advantech-installation-tips

I have seen some glue down 3/4″ solid hardwood floors directly to concrete and many installers use straps or clamps in an effort to force board rows tighter together during installation. Be advised that over-strapping can adversely affect the floor and may result in glue-bond failure, seam peaking, twisted boards or out-of-square floor board alignment.
The Sika AcouBond System (below) used for sound control, allows solid hardwood flooring to be glued over concrete. Again, this requires straps and clamps for installation.

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Special Thanks to JJ Haines for the info!