Floating Subfloors For Hardwood Flooring Over Concrete

Floating sub floors are gaining more recognition as an alternate method to fastened sub floors on concrete. Chief reasons are two fold and depends on the application when it comes to installing a solid ¾” hardwood floor.

For apartment or high rise residences with concrete sub floors, reducing sound transmission is getting more attention and older systems are not being accepted. For those with plywood sub floor bases on grade, direct fastening is no longer becoming acceptable due to penetrating the moisture barriers in place. More reasons for the switch and benefits of floating sub floors are contained below.

Plywood Fastened To Concrete Slabs On Grade

Although the National Wood Flooring Association advocated using the older asphalt tar method prior to fastening plywood to concrete, this method was shunned by many as being troublesome, messy, and unhealthy for those with allergy problems. In effect the tar was mopped, covered with a plastic sheathing (visqueen) and finally plywood was laid into place and attached into the concrete with fasteners.

There have always been issues of fastening through a moisture barrier into concrete. The older method allowed the puncturing to be sealed while the tar was still wet. Tar would encapsulate the hole made by the fastener keeping the moisture barrier intact and functioning. Since those days, tar has all but been eliminated with only the sheathing working as a moisture barrier. Unfortunately the barrier was compromised through a large number of penetrations with the concrete fasteners.

Plywood Fastened To Concrete Slabs Above Grade

In the case of plywood being attached in apartments or condominium high rises, the fasteners once again became a problem. By attaching into the concrete, even with an approved acoustical underlayment, sound transmission still existed. The need for an uninterrupted solution to reduce sound was needed.

Floating Sub Floors. The Solution

Floating sub floors for solid hardwood installations generally consist of two layers of plywood, or allowing enough wood for a 1 ½ inch flooring cleat or staple to hold effectively. Considering the fasteners are installed at a 45 degree angle or blind nailed, overall thickness required runs about 5/8 of one inch. The minimum being used for these applications consists of two layers of 3/8 inch thickness CDX plywood.¹

Installation

The preferred method of installation permits laying the first layer in full 4x8 foot sheets over an approved moisture barrier such as 6 mil plastic sheathing. For added protection #15 asphalt felt paper or Aquabar B underlayment can be used. The second layer of plywood should overlap all seams of the first layer and placed either perpendicular or diagonal while leaving ¾ inch expansion at wall areas and at least 1/8 inch between sheets.

At this time both layers are attached to one another either by stapling only or gluing and stapling.

Ideal For Lightweight Concrete

Lightweight concrete, gypcrete, or other variations are ideal candidates for floating floor systems if one desires a traditional ¾ inch solid hardwood floor.

In addition, concrete slabs of questionable appearance such as chalky and sandy characteristics are unlikely to have the proper strength for other sub floor systems.

Photo Courtesy: Deerfield Construction

¹ Actual thickness of 3/8” plywood is 11/32” Two layers would equal 22/32” or 11/16”