What thickness of plywood is best for a subfloor?
Match thickness to joist spacing. On 16-inch on-center joists, 23/32" (3/4") tongue-and-groove is the residential default, with 19/32" (5/8") as the minimum. On 19.2" OC use 23/32" T&G, and on 24" OC use 3/4" minimum, stepping up to 7/8" for tile or heavier live loads. These follow IRC R503.2.1.1 and the APA E30 span tables; non-standard loads should defer to an engineer.
Is OSB or plywood better for a subfloor?
It depends on build conditions. OSB is cheaper and meets the same span ratings for a dry, well-detailed floor, which is why it holds most of the US subfloor market. Plywood resists moisture cycling better, recovers from edge swell, and holds edge screws more securely, so it is the safer choice when the framing will be exposed to weather before dry-in or in a wet climate.
Can you use regular plywood (CDX) for a subfloor?
CDX is acceptable as a subfloor when a separate underlayment will follow over it, or on a budget build where the panel edges are blocked. Because CDX has square edges, unsupported edges between joists need blocking. For a single-layer subfloor that the finish floor fastens straight into, tongue-and-groove plywood is the better answer because the interlocking edges support each other.
What plywood should I use for a bathroom or wet-area subfloor?
Use an exterior-rated panel such as ACX or exterior-rated CDX, but do not rely on the plywood for waterproofing. The waterproofing is a continuous membrane installed on top of the subfloor, run up the walls at the transition. Most wet-area floor failures come from unsealed cut edges, missing slope to the drain, or a discontinuous membrane.
Does the subfloor need a span-rating stamp?
Yes. IRC R503.2 requires wood structural panel subflooring to conform to the code span tables or APA standards, and a building inspector reads the APA stamp on the panel. A sheet with no span-rating stamp, even if it looks identical, is not code-compliant subflooring and may not carry the design load.
