Though it's not evident from simply looking at a floor from above, every flooring system is actually a complex system of multiple layers: structural joists, subfloor, underlayment, and surface flooring. Some form of subfloor is found beneath every type of floor, whether it is hardwood, carpet, cermic tile, natural stone, vinyl, or laminate, and choosing the right subfloor material and installing it correctly is the key to a great floor that performs well and lasts for decades.
What Are Structural Joists?
Structural joists are the horizontal counterpart of beams, which runs perpendicular to the roof and helps support its weight. Structural joists run between beams and help frame a space.
The Four Flooring Layers
The smooth surface flooring that you see is really a layered "sandwich" composed of several different layers, each of which serves essential functions in making a floor that looks and performs adequately.
- Joists. The joist system is the series of parallel structural members that run between beams or load-bearing walls and which holds up the entire floor. Joist can be made from 2 x dimension lumber in sizes ranging from 2 x 8 to 2 x 12s, depending on the load the floor is expected to carry. In some modern construction, very long joists may be engineered joists, which use an I-beam construction of plywood or OSB (oriented strand board) panels framed at the top and bottom by dimension lumber. Note: The joist layer is not present in basement floors or in slab home construction, where the floors rest directly on concrete slabs.
- Subfloor. This is a sturdy structural layer resting directly on the joists. Usually made of plywood or OSB, the subfloor provides a stable surface, not just for floor coverings but for all of the heavy "live-load" elements in the living space, such as furniture, people, kitchen and bath cabinets, showers, and tubs. Even though joists have side bracing, the subfloor also acts as additional bracing to control lateral movement in joists. The term subfloor is sometimes incorrectly used to refer to the thin layer of plywood, cement board, or even foam padding that lies immediately under the surface floor covering. These materials actually comprise the underlayment.
- Underlayment. Over the subfloor, there is an underlayment that smooths out the subfloor and provides a flat, level surface for the finished flooring surface. Unlike the subfloor, this is not a structural element, and it provides no strength to the floor. Several materials can be used for the underlayment, chosen based on the type of surface flooring being installed. Common materials include plywood, cement fiberboard, cement board, troweled mortar, and foam or cork padding.
- Floor covering. The floor covering is the visible, decorative surface that you walk on—the hardwood planks, stone or ceramic tiles, vinyl tiles or sheets, carpeting, laminate planks, etc.
Anatomy of the Subfloor
Subflooring is the bottom-most horizontal layer of a flooring system—the layer that rests directly on the structural joists that span the space between support beams or load-bearing walls. Normally, the only time you will repair, replace, or make changes to the subfloor is during major remodeling or construction jobs.
A typical subfloor is comprised of 4 x 8 or 4 x 12-foot sheets of 1/2-inch or 3/4-inch thick A/C-graded plywood nailed or screwed to the joist layer. The A/C grade means that one side is finished smoothly while the other side is relatively rough (this is usually the bottom side).
OSB is also frequently used for the subfloor layer. Opinions vary depending on the merits of plywood vs. OSB, but both are very common, and both are entirely adequate when installed correctly. Like plywood, OSB sheets are nailed or screwed to the joists.
In older homes, it was not uncommon for the subfloor to consist of simple 1 x 6, 1 x 8, or 1 x 10 planks nailed diagonally across the joists. During remodeling, these planks are often replaced with plywood or OSB, or they can be augmented by laying another subfloor layer of plywood over the diagonal planks.
How to Install a Subfloor
Whether the subfloor consists of plywood or OSB sheets, installation is similar. First, make sure the underlying joists are relatively flat and level. Where there are structural issues with the joists, these problems should be remedied before laying the subfloor. Where the joists are slightly uneven, some carpenters lay down a bead of construction adhesive over the tops of the joists before installing the subfloor panels. This offers some rigidity to the installation and prevents flexing and squeaking.
The large sheets of plywood or OSB are laid across the joists so the ends are centered on joists. Use as many full sheets as possible for maximum strength. Where cut sheets are necessary, make sure the ends are centered on joists. Lay the sheets with gaps of 1/8 inch between them to allow for expansion. The sheets are then attached to the joists with nails or screws driven every 8 inches or so. Where rows of subfloor sheets are laid side-by-side, try to stagger the end joints so they do not line up from one to the next. Avoid layouts in which four sheets all meet at one point.
If there are minor low spots in the subfloor, you can fill them with a liquid floor-leveling compound, such as Durock's Multi-Use Self-Leveling Underlayment, which can be applied in layers up to 3 inches thick.
Recommendation for Specific Flooring
Where you are laying a subfloor from scratch and know what the surface floor covering will be, this may affect how you install the subfloor.
- Hardwood flooring: Plywood is the best subfloor for hardwood flooring installation. CDX plywood ranging from 1/2 to 3/4-inch-thick and rated A/C will serve well for any hardwood flooring installation. Tongue-and-groove plywood is available to reduce squeaks and help the subfloor fit together better.
- Ceramic or porcelain tile: The subfloor for tile can be tricky since it is important to avoid any flexing that would later cause cracking in the tile's grout and in the tile itself. Make sure the joists are sturdy and not subject to flexing; doubling them up ("sistering") may help make the floor more rigid. Thicker, 3/4-inch A/C plywood is the best choice for underlayment here, and make sure it is solidly anchored to the joists. Above this subfloor, a good underlayment of cement board will further guard against flexing.
- Natural stone: Tiles of marble, slate, travertine, and other natural stone are very heavy flooring materials that are often quite brittle as well. Surprisingly, they are not nearly as strong as ceramic tiles. For this reason, a sturdy subfloor that doesn't flex is very important. The Tile Council of North America (TCNA) calls for a subfloor of 19/32-inch tongue-and-groove plywood to be installed over wood joists spaced 16 inches on center, with a 1/8-inch gap between sheets. This should be followed by a second layer of 15/32-inch-thick plywood installed with a 1/4-inch gap between sheets. Both layers should be installed so the face grain runs perpendicular to the joists. Only now is the subfloor ready for cement board underlayment.
- Laminate flooring: As with hardwood, laminate flooring is best installed on a 3/4-inch-thick plywood subfloor. If the existing subfloor is in poor condition, you will need to add a secondary underlayment of thin plywood over the subfloor. Laminate is a fairly thin material and is not forgiving when it comes to grooves, dents, and ridges. Over the subfloor, a foam underlayment is applied. Laminate underlayment is thin (6 to 8 millimeters) foam padding that comes in rolls about 3 feet wide; it can help smooth out very minor dents and bumps in the subfloor.
- Carpeting: Either plywood or OSB is sufficient as a subfloor for carpeting. In most instances, simple carpet padding serves as a sufficient underlayment over a plywood or OSB subfloor, but in instances where the subfloor is in poor shape or for older subfloors with diagonal 1 x boards, a 1/4-inch thick A/C plywood underlayment is advised before laying padding and carpeting. When laying carpeting over concrete, a layer of plywood resting on sleepers is advised, or use interlocking subfloor panels, such as DriCore.
Considerations for Concrete Slabs
Subflooring in a basement or for other concrete slabs is an entirely different matter since you have no joists and there may be moisture issues. Even if the basement's concrete floor looks and feels dry to the touch, residual moisture may wick up over time and damage your finish flooring.
Unless you are installing tile—which can be installed directly on the concrete—a moisture-impermeable subfloor or underlayment is required. You can lay down plywood on top of sleeper strips (serving as mini-joists resting directly on the slab), or you can lay the plywood directly on the concrete. In both cases, place a vapor barrier or a foam underlayment under the wood.
As an alternative, there are special basement subfloor panels, such as DriCore or BARRICADE, that lock together in a tongue-and-groove fashion. These panels consist of a top layer of OSB attached to a rigid moisture-blocking underlayment that raises the panels up off the floor. This offers an advantage over sleepers and plywood since the overall height is much thinner. DriCore subfloor panels are suitable for any flooring material but are especially good for carpeting or laminate, both of which can be susceptible to moisture from concrete slabs.