At the base of nearly every floor--tile, hardwood, laminate, vinyl--is a subfloor. Understanding and installing the correct subfloor is key to a great floor that performs well and lasts for decades.
What Is Flooring Subfloor?
Subflooring is the bottom-most layer of floor that rests directly on the joists. It is below the floor covering and is never seen in daily use.
A typical subfloor is 1/2" or 3/4" A/C-graded plywood. A/C means that one side is finished relatively smoothly and the other side is relatively rough (usually the bottom side). Subflooring is nailed or screwed directly to the joists.
Flooring Layers Explained
When you look at a completely installed floor, it is really a "sandwich" composed of many different layers. Flooring layers, starting from the bottom and working upward, will likely include:
- Joist. The bottom-most layer that holds up the entire floor. A joist might be a 2" x 10" timber such as hem-fir or it might be an I-beam made of engineered wood. Not all floors will have joists. Concrete floors do not require joists.
- Subfloor. A structurally-sound layer, usually plywood. Subfloor provides a stable surface not just for floor coverings but for all of the heavy elements that go in homes: furniture, people, kitchen and bath cabinets and showers and tubs. Even though joists have side bracing, subfloor also acts as additional bracing to control lateral movement in joists.
- Underlayment. Thin layer of plywood, cement fiberboard, or cement board to smooth out the subfloor and provide a flat, level surface for the finish floor.
- Finish Floor or Floor Covering. The decorative floor that you walk on, such as tile, hardwood, or laminate.
Subfloor is sometimes used in the wrong context, such as referring to thin plywood underlayment or even foam padding "floating" underneath laminate flooring (another kind of underlayment, not a subfloor).
Types of Subfloor Materials
- Plywood: True subfloor, 1/2" or 3/4" A/C-graded plywood is a mainstay of flooring construction.
- OSB: Oriented-strand board (OSB) ranks up there with plywood as "real subfloor." Which is better--plywood or OSB? It tends to be a matter of opinion.
- Cement Board: Used for tile, cement board is more like an underlayment because it must rest atop a solid surface such as plywood or concrete.
- Foam and Cork Padding: Underlayment. Foam and cord padding helps cushion your step when installing laminate flooring.
- Radiant Heating Subfloor: Another "true" subfloor, Warmboard is 1 1/8" thick subfloor--thicker than typical plywood subfloor--with grooves cut into the surface to accommodate PEX tubing through which heated water is run. Aluminum sheathing on top, conforming to the grooves, provides a surface to reflect heat up into the room.
- Dricore: More like an underlayment, Dricore is a brand name for a panel system of engineered wood and padding. Good for basement flooring.
Plywood is the best subfloor for hardwood flooring installation. CDX plywood ranging from 1/2" to 3/4" and rated A/C will serve any hardwood covering well. Tongue and groove plywood is available to reduce squeaks and help the subfloor fit together better.
Ceramic or Porcelain Tile
Subfloor for tile can be a tricky issue, since you want to avoid flexing which would later cause cracking in the tile's grout and in the tile itself. Joist inflection is one cause for cracking, but the plywood atop the joists should be solid enough to prevent additional flex. On top of the plywood, you can install an underlayment of cement board.
Laminate flooring can be installed on the same subfloor as noted for hardwood flooring. If the subfloor is in poor condition, you will need to add a secondary underlayment of thin plywood. Laminate is a fairly thin material and is not forgiving when it comes to grooves, dents, and ridges. Laminate underlayment is thin (6-8 mil.) foam padding that comes in rolls about 3 feet wide and can help smooth out very minor dents and bumps.
Subfloor for basements is an entirely different matter: you have no joists and there likely is a moisture problem. 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 (same thing in this case) is required. You can lay down plywood on top of sleepers, to create mini-joists, or you can lay the plywood directly on the concrete. In both cases, place a vapor barrier under the wood. Or you can lay down a foam underlayment.
Finally, special basement subfloor panels, such as DriCore or BARRICADE lock together in a tongue-and-groove fashion and have foam underlayment attached to the bottom. This is a good basement subfloor option if cost isn't an issue but height is. These subfloor panels keep the subfloor low, as opposed to the highest option--plywood atop sleepers, which can add several inches.
When you install subfloor over joists, keep several points in mind:
- Keep the plywood sheets as large as possible; avoid cutting wherever possible. Larger sheets provide surer strength.
- Edges of the plywood should rest on top of joists. Never leave an edge of plywood hanging in mid-space between joists.
- Never have four corners of the plywood subfloor meet. Stagger the plywood to avoid this.
- Keep 1/8" gap between sheets and walls to allow for expansion and contraction.
- Applying construction adhesive between joists and plywood is not necessary but will help prevent squeaks in the future.
For minor low spots, you can pour liquid floor leveling compound. DUROCKâ„¢ Multi-Use Self-Leveling Underlayment is one example of a self-leveler, and will create underlayment as thick as 3 inches.