Subfloor Guide: Materials, Installation, and Common Problems

What Is a Subfloor?

The subfloor is the structural layer fastened to the floor joists, on top of which flooring materials are installed. It provides the flat, stiff surface that flooring needs to perform correctly. Most subfloors are 3/4" plywood or OSB (oriented strand board), though older homes may have 1" boards run diagonally. Concrete slabs in basements and slab-on-grade homes serve as the subfloor.

Plywood vs. OSB Subfloor

Plywood: More moisture-resistant, stiffer, holds fasteners better. Preferred for areas prone to moisture and for tile installation. Costs 10–15% more than OSB.
OSB (oriented strand board): Cheaper, structurally adequate for most applications. More susceptible to moisture damage — swells and delaminates when wet. Standard in most new residential construction due to cost.

Subfloor Thickness Requirements

Hardwood/laminate over plywood: 3/4" minimum (19/32" actual)
Tile: Combined subfloor + backer board thickness of 1-1/8" minimum; deflection must be limited (L/360 rule)
LVP: 3/4" minimum; very flat (3/16" variance per 10 feet maximum)

When to Repair vs. Replace Subfloor

Repair small damaged areas (less than 4 sq ft): cut out damaged section, sister new joists if needed, screw in new plywood patch. Replace larger areas or when there's structural damage from moisture, rot, or termites. Signs of subfloor problems: squeaking floors (loose fasteners), springy/soft spots (wet damage, rot), visible low areas or high spots that cause flooring to buckle.

Frequently Asked Questions

How do I fix a squeaky subfloor?

Squeaks are almost always caused by subfloor movement against joists or fasteners. From below: drive screws from below through the joist into the subfloor (don't penetrate the finish floor). From above: drive screws through the subfloor into the joists — use a countersinking bit and fill screw holes. Construction adhesive between subfloor and joists during original installation prevents most squeaks.

Can I install tile directly on a plywood subfloor?

Usually not on plywood alone — tile requires a rigid, non-flexing substrate. Install cement board or uncoupling membrane (like Schluter DITRA) over the plywood first. The combined thickness must be at least 1-1/8". The subfloor must also have adequate stiffness (L/360 deflection or better). Tile installed on flexible substrates cracks at grout lines within years.