What Thickness Plywood for a Subfloor?

The short answer

For joists at 16 in on center, 23/32 in tongue-and-groove plywood or OSB, sold as ¾ in, is the most common single-layer subfloor in North American homes. Thinner panels rated for 16 in spacing are allowed in many cases, but ¾ in gives a stiffer floor. At 24 in joist spacing, use a panel rated "24 oc", which in practice means 23/32 in or thicker. Always check the stamp and your local code.

Nominal and actual thickness

Like lumber, panels are sold by a rounded name. The actual thickness is slightly less.

Reading the span rating

Structural panels made in North America carry a grade stamp. For single-layer floors, look for a rating such as "16 oc", "20 oc" or "24 oc". That number is the maximum joist spacing, in inches, the panel is rated for. Sheathing panels show two numbers, such as 32/16: the first is the maximum roof rafter spacing, the second the maximum floor joist spacing when used under a separate underlayment.

The rating covers code-level strength. A stiffer floor, which feels more solid and protects finishes, often means stepping up a size.

What goes on top matters

• Carpet and vinyl plank: a rated ¾ in subfloor is usually fine. Sheet vinyl and thin resilient floors may also need a smooth underlayment layer so seams and fastener heads do not show through.
• Hardwood: nail-down flooring makers commonly ask for a ¾ in (23/32 in) subfloor so the nails hold well. Read the flooring instructions.
• Ceramic and stone tile: tile needs a stiffer floor than other finishes, and natural stone needs more than ceramic. That often means a second layer of plywood or a tile backer over the subfloor. Follow the tile and backer makers' instructions.

Walls and roofs

Wall and roof sheathing use the two-number rating. A panel stamped 24/16 can span rafters 24 in apart on a roof, and is a common choice in 7/16 in OSB for walls with studs at 16 or 24 in. Roof decks under heavy snow, tile roofs or wider rafter spacing call for thicker panels such as 15/32 or 19/32 in. Unsupported panel edges between rafters often get metal edge clips. Your local code and roofing maker set the minimum.

Signs a subfloor is too thin

• The floor bounces or the dishes rattle when someone walks past.
• Tile or grout cracks along a line between joists.
• Squeaks keep coming back after fasteners are added.
• Soft spots near the edges of panels, where they meet between joists.

Bounce can also come from undersized joists, which a thicker subfloor will not fix. If in doubt, have the floor framing checked.

Metric markets

In Canada, panels carry the same North American span stamps alongside metric sizes. In Australia, structural plywood and particleboard flooring are sold in metric thicknesses with their own span tables that depend on joist centers. Follow the manufacturer's tables and the local building rules there, not the US ratings above.

Worked example

A 12 × 16 ft room addition with joists at 16 in on center. You choose 23/32 in tongue-and-groove panels rated 24 oc for a stiff floor. The area is 192 sq ft. With 10 percent extra for cuts it is 211.2 sq ft, and 4 × 8 ft sheets cover 32 sq ft each, so you need 7 sheets. The plywood calculator does the same sum for any room and for 1200 × 2400 mm sheets.

Installation basics

• Run the long edge of each panel across the joists and stagger the end joints.
• Leave about 1/8 in between panel ends and edges, as panel makers recommend, unless the tongue-and-groove edge sets the gap for you.
• Glue to the joists with subfloor adhesive and fasten with ring-shank nails or screws at the spacing your code or the panel maker gives.
• Keep panels dry. If a subfloor gets soaked, let it dry out and sand any swollen edges flat before the finish floor goes down.

Frequently asked questions