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Installation: Subfloor Over Concrete

How to Install a Subfloor Over Concrete.

The National Wood Flooring Association issues guidelines for installing wood subfloor over concrete. There are several subfloor installation methods, each with its own guidelines, including:

  • Floated subfloors
  • Glue-down subfloors
  • Nail-down subfloors
  • Screen systems

Follow these installation guidelines when installing a wood subfloor.

How to Install a Floated Subfloor Over Concrete

For above-grade applications, your flooring manufacturer should provide recommendations for installing a floated subfloor. For on-grade and below-grade installations, however, NWFA guidelines recommend a vapor retarder anytime you’re installing solid ¾” wood flooring over concrete. A vapor retarder is required under the following conditions:

  • Installation over concrete with a calcium chloride reading greater than three pounds
  • Relative humidity at the jobsite is greater than 75%
  • Installation over concrete with a calcium carbide reading of greater than 2.5 percent.

Once you’ve installed a vapor retarder, install your subfloor. To install a floated subfloor system, first place a layer of plywood subfloor panels with the edges parallel to the wall. Use a ¾” spacer between the plywood and the wall and a 1/8” spacer between panels.

Next, lay a second layer of plywood at a 45-degree angle to the first layer. Again, use spacers to leave gaps between panels and at the walls.

Finally, using a 12” interior grid pattern, staple or glue the two plywood layers to one another, being careful not to puncture the vapor retarder. Use a 6” staple pattern at the perimeter of the subfloor.

How to Install a Glued Subfloor Over Concrete

Check your flooring manufacturer’s recommendations to ensure you’re using a proper subfloor. As with a floated subfloor, NWFA recommends a vapor retarder for glue-down subfloors when installing ¾” wood flooring over concrete.

After installing a vapor retarder (if necessary), cut your plywood panels into 2’x8’ or 4’x4’ sections. The NWFA recommends plywood with a minimal thickness of ¾”. Score the panels to a depth of half their thickness using a 12” x 12” grid. Apply an approved adhesive and lay the plywood sections in a staggered joint pattern, with 1/8” spacers between sheets and ¾” spacers at walls.

How to Install a Nail-Down Subfloor Over Concrete

As with other installation methods, a moisture-control vapor retarder is recommended for on-grade and below-grade nail-down installations. Plywood subfloor panels of 5/8” thickness or higher are recommended.

To begin, stagger panel joints and use 1/8” spacers between panels. This will prevent panels from peaking due to swelling caused by heat and humidity. Use ¾” spacers against walls and other vertical obstructions.

To nail down the subfloor, always use a mechanical system such as a powder load or pneumatic pressure nail gun. Beginning at the center to prevent bowing, fasten the subfloor using a 12” x 12” grid on the interior and every 6-8” along the perimeter (2” from the panel edge).

How to Install a Wood Subfloor Over Concrete with Screeds

Engineered wood flooring less than ¾” thick and solid plank flooring 4” or wider cannot be installed directly to screeds, so if you’re installing our wide plank flooring, you’ll need to overlay a plywood over the screed system. More details for installing over screeds is available here.