Over Concrete Slab

There are three methods used to attach subfloors to concrete slabs—direct attachment, sleeper systems, and floating floors. All methods require the preparation of the slab. For best results, the slab should be covered with a minimum 6 mil preferred polyethylene sheeting. The polyethylene sheet should be spot glued to the slab with an adhesive that will bond to both concrete and plastic. All joints in the polyethylene sheeting should be lapped a minimum of 4 inches. These laps should also be glued to ensure an adequate moisture seal. The plastic should extend up the sides of the walls a few inches, with the excess trimmed after the subflooring is installed. In this method, the panels can be placed directly on the polyethylene sheeting. Panels should be properly spaced at edges and ends according to the manufacturer's directions and fastened with masonry nails, concrete fasteners, or powder-actuated fasteners. Again, manufacturer's directions should be heeded for instructions on how far to place fasteners from panel edges.

Alternatively, the subfloor panels can be applied directly to a sleeper system. If installing the subfloor panels over a sleeper system, all edges should be fully supported. This can be achieved using blocking or a tongue and groove profile.

After the slab is correctly prepared, the sleeper system should be installed with 2x4s laid flat. Manufacturers generally recommend joist spacing of 16 inches o.c. Subfloor panels should then be laid on the joist system with the long dimension across the joists. All panels should cover at least three joists. Manufacturers' directions on spacing panels and fastener types should be followed closely.

Whether by code or by the building's designer, multifamily residential structures frequently have strict sound requirements for floors in order to cut down on noise from the units above. Many times, these requirements do not allow a subfloor to be fastened mechanically to the concrete substrate. One solution in a solid plank floor installation is a “floating floor,” an isolated floor system that sits on top of the concrete without glues or nails or screws and is held in place entirely by its own weight. The typical profile involves a concrete substrate, on top of which is a sound isolation mat, and then two layers of subflooring panels that are both glued and nailed to each other. Plywood is susceptible to delamination in this type of application because if it is not fastened directly to the substrate, it can undulate and lift, compromising the viability of the entire floor system.

High-performance panels have the resistance to water absorption and lower water induced edge swell to enable a gravity-loaded system to maintain flatness during construction and after installation. This translates to a high-quality wood floor installation.

Over Light Gauge Metal Framing

Light gauge metal framing is used in many different construction applications. It is more prevalent in light commercial buildings and accounts for only a small percentage of residential work.

Screws and pins are the two types of fasteners recommended in attaching subflooring to the metal framing. Self tapping or self drilling bugle head type screws are recommended so they can be installed flush or counter sunk. Hex screws or other fasteners that cannot be installed flush may interfere with the finish floor or wall covering installations. Typically, a minimum of three threads should be exposed through the framing beneath the subfloor. Alternatively, steel pins can be used. These fasteners contain specially designed grooves called knurls that spiral down the shaft. The knurls help to penetrate and grip the metal framing. They are installed with pneumatic air guns similar to those used for typical framing nails. As always, the local building codes or light gauge framing manufacturer's recommendations should be followed for fastening requirements.

To achieve optimum performance, a continuous bead of construction adhesive should be applied to the light gauge metal joists prior to installing the subfloor. Because metal can expand and contract more than wood, polyurethane adhesives are recommended when installing engineered wood subfloor over metal framing. Panels should be installed with the long dimension perpendicular to supports, with the light gauge manufacturer's installation instructions followed for size of fasteners.

In certain situations for accelerated installation time, powder-actuated fasteners can be used to attach engineered wood subfloors to light gauge metal framing. These specialty fasteners provide the ability to fasten into concrete, masonry and structural steel. When considering using a powder-actuated fastening system, it is advisable to request an Evaluation Service Report or an approved fastener design.

Prepping a Subfloor

A subfloor must be prepared to certain standards in order to adequately form the base for a high-performance finish floor.