Installing Subfloors: What You Should Know

Subfloors do more than support the finish flooring: Along with the floor joists that support them, subfloors add strength and rigidity to the entire skeleton, or frame, of the building. The materials and installation method are determined by a number of factors, including the load requirements of the building, the floor-joist spacing and the finished flooring that will be installed.

The words "subfloor" and "underlayment" are often used interchangeably, but they are two separate components. A subfloor is part of the frame of the structure. All houses need subfloors. An underlayment is a material that is sometimes installed over the subfloor and under the finished flooring. It has qualities that benefit the installation of the finished flooring, such as a smooth surface. Underlayments are not always necessary.

Plywood is the traditional subfloor material. It is made of thin veneers of wood that are glued and pressed together. The grain of each veneer runs perpendicular to the ply above and below it. This arrangement gives plywood its strength and stability. The standard plywood sheet is 4 x 8 feet. Plywood comes in a number of thicknesses, with 3/4-inch plywood commonly used in subfloors.

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Oriented strand board (OSB) is fast becoming a popular subfloor material. It is made from strands of ground-up lumber. The individual pieces are aligned in the same direction and are then glued and pressed into thin layers. The typical OSB product can consist of up to 50 individual layers that are then pressed together and cut into 4 x 8-foot sheets.

Concrete slabs make good subfloors for masonry-type flooring, such as tile and stone. The concrete must be smooth and cured properly. For solid wood, vinyl, carpeting and most other materials, a plywood underlayment must be attached to the concrete.

When it was first introduced, OSB was distrusted by professional builders. Of course, a generation earlier, most builders were wary of plywood. Today, both plywood and OSB are used. Most building codes use the term "wood structural panel" to indicate both. Usually, the decision to select one over the other is based on current pricing and availability.

To help speed the acceptance of OSB as a structural panel, the Engineered Wood Association (formerly the American Plywood Association, or APA) developed a rating system for the panels in its program. The system means that a certified panel, whether plywood or OSB, can be specified for its intended use. Panels are certified for use as subfloors as well as roof and wall sheathing. It gets a little confusing because the association still uses the distinctive APA logo on its certification stamps, but it is a sign that they consider certified plywood and OSB as equal.

A house consists of a number of interconnected parts. A decision made on one part affects some of the other parts of the building. This applies to new construction and large home-improvement projects, such as building an addition.

For subfloors, the spacing of the joists that support the subfloor and the finish material determine the selection of the subfloor panel. A designer or architect specify the joists based on the load the floor will hold and the building code.

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Usually, joists are spaced 16, 20 (that is the nominal measurement; it is actually 19.2) or 24 inches on center. On center means from the center of one joist to the center of the next. It is an important distinction because the panels have to fall over the joists correctly in order to be fastened securely. Wider joist spans require stronger panels.

Different finished-flooring materials may require different underlayments — the material that goes between the subfloor and the floor. Vinyl flooring, for example, usually requires a smooth underlayment installed over the subfloor. For ceramic tile and other stone products, manufacturers often require a plywood underlayment.

Subfloor installations are not DIY projects unless you are an experienced builder. Subfloors are part of the framing process. If you are working with a general contractor, he will hire the framer. However, if you are overseeing the project:

• Obtain quotes from multiple contractors
• Make sure the contractor is licensed and insured
• Ask for and contact references
• Ask to see projects that are similar to yours
• Insist on working with a written contract.

Whoever does the installation should be familiar with the installation protocol for subfloors. This is often spelled out in the local building code and always in the manufacturer's literature. Usually, subfloors are exposure-one panels. These panels are made with exterior glue, a necessity for a material that is usually installed before the building is closed in and weatherproofed.

Typically, the contractor applies a bead of construction adhesive along the tops of the floor joists. The panels are set in place with the long dimension of the panel spanning the support joists. They will leave about a 1/8-inch gap between panels to allow for expansion. (Tongue-and-groove panels have the gap built in.) To avoid damaging the edges of the panel, contractors will coax stubborn panels into position using a length of lumber as a nailing block rather than hitting the panel with a hammer.

They are then nailed with either 6d (2-inches long) or 8d (2 1/2-inches long) ring-shank or spiral-shank nails, depending on the thickness of the panel. Nails are usually spaced 6 inches apart along the panel edges and 12 inches apart in the interior of the panel. Secure attachment is necessary to prevent squeaks in the floor.

The contractor will stagger the joints between panels. So, after the first row is installed, the contractor will start the second row with half of a panel. This arrangement prevents a seam running from one wall to the next.

Subfloors are installed early in the construction process. They receive a lot of wear and can be damaged by the construction trades as the house or addition is completed. Damage must be repaired or an underlayment installed before the finished flooring is put in place.