Ask three contractors what size plywood for flooring and you might get three different answers. The right thickness depends on whether the plywood is the structural subfloor or just the underlayment beneath the finish, what spans the joists, and what finished floor goes on top. Get it wrong and the floor squeaks, bounces, or telegraphs every joint through the finished surface. Get it right and the floor stays silent and flat for 50 plus years. APA (American Plywood Association) grade stamps and IRC R503.1 code requirements both spell out exactly what works where.
Subfloor vs Underlayment: A Critical Distinction
A subfloor is the structural panel that spans the joists and carries the live and dead loads of the room. An underlayment is the smooth top layer that bridges minor irregularities and provides a clean surface for the finish flooring. Many people use the terms interchangeably, but the IRC treats them as separate components with separate thickness requirements.
OSB and plywood both work for subfloors, but only specific underlayment-grade plywood (typically 1/4 inch lauan or sanded ply) belongs on top of the subfloor as a smoothing layer.
Subfloor Thickness by Joist Spacing
IRC R503.2.1 references the APA span ratings stamped on every panel. The first number is the maximum unsupported span on a roof and the second is the maximum on a floor. A panel marked 32/16 spans 16 inches on a floor.
- 16 inch on-center joists: 5/8 inch tongue-and-groove minimum, 3/4 inch recommended
- 19.2 inch on-center joists: 3/4 inch tongue-and-groove minimum
- 24 inch on-center joists: 3/4 inch tongue-and-groove minimum, 7/8 inch for stiffness
For tile floors, the Tile Council of North America (TCNA) recommends adding a second layer for total assembly thickness of 1-1/4 inches. This prevents the deflection that cracks grout and breaks tile.
Tongue and Groove vs Square Edge
Subfloor panels with tongue and groove long edges (T&G) lock together at unsupported edges, eliminating the need for blocking under the seam. Square edge panels require blocking, which adds labor and material. Every floor subfloor sold at lumberyards is T&G for this reason.
Apply a 1/4 inch bead of subfloor adhesive (Liquid Nails Subfloor LN-902 or PL Premium) along every joist top and inside each tongue groove before setting the panel. Fasten with 8d ring-shank nails or 1-5/8 inch deck screws every 6 inches at edges and 12 inches in the field.
Plywood Grades That Matter
Subfloor plywood typically carries a CDX (C face, D back, exposure 1) or APA-rated Sturd-I-Floor grade stamp. Sturd-I-Floor is engineered specifically for combination subfloor and underlayment service, with smoother top veneers and tighter face grades. Look for the green APA stamp showing span rating, exposure category, and panel grade.
For underlayment beneath vinyl, LVT, or carpet, switch to AC or BC grade sanded plywood. The smoother face eliminates telegraphing of grain or knots through soft finished flooring. Lauan underlayment (1/4 inch tropical hardwood) was once the standard but has fallen out of favor due to sustainability concerns and inconsistent quality.
Plywood Under Different Finish Flooring
The finished flooring on top affects subfloor and underlayment choices.
- Solid hardwood (nailed down): 3/4 inch T&G subfloor, no underlayment needed
- Engineered hardwood (floating): 3/4 inch subfloor plus 6 mm cork or foam underlayment
- LVP (rigid core SPC): 3/4 inch subfloor, attached pad usually included
- Laminate: 3/4 inch subfloor plus 3 mm cushioned foam underlayment
- Ceramic and porcelain tile: 3/4 inch subfloor plus 1/2 inch cement board or uncoupling membrane
- Carpet: 3/4 inch subfloor, 8 lb rebond pad on top
For ceramic tile, Schluter Ditra ($2 per sq ft) or Wedi systems replace cement board with a lighter and faster install. Both meet TCNA ANSI A108 standards for tile installation.
Fastening and Gluing the Subfloor
Glue plus screws is the modern standard for new construction. The combination eliminates squeaks for decades. Use construction adhesive rated for floor framing, apply continuously along joists, and screw the panel down within 30 minutes before the adhesive skins over. Ring shank nails work but back out over time as wood dries, causing the squeaks most older houses develop.
- Lay out joists with the tongue facing the next panel
- Apply a continuous 3/8 inch adhesive bead along each joist top
- Set the panel, tap the tongue and groove home with a sledge and 2×4
- Drive 1-5/8 inch deck screws or 2-1/2 inch screws every 6 inches at edges
- Field screws at 12 inches on center across each joist
Leave a 1/8 inch gap between panels to allow for seasonal expansion. Skipping this gap causes the panels to buckle slightly during humid weather.
When to Add a Second Layer
For tile installations on 16 inch on-center joists, double the subfloor to 1-1/4 inch total (3/4 inch plus 1/2 inch). For wood floors on 19.2 inch joists, the same double layer prevents the bounce that creates squeaks at fastener locations. Stagger the second layer joints so they do not line up with the first layer.
Gluing the second layer to the first with PL Premium plus 1-1/4 inch screws every 8 inches turns two thinner panels into a single thick assembly. This is how high-end builders eliminate any subfloor deflection in luxury kitchens and bathrooms.
Moisture Considerations
Plywood subfloors over crawl spaces or basements need vapor barrier protection underneath. A 6 mil polyethylene sheet over the ground in the crawl space reduces moisture migration up into the subfloor by 80 percent. Without it, plywood absorbs moisture, cups, and eventually fails.
For bathrooms, use AC2 pressure-treated plywood as the subfloor if the room sits over an unconditioned space, or apply a Wedi Subliner waterproofing layer on top of standard plywood before any tile work.
Common Mistakes
Three errors cause most subfloor failures. First, using 5/8 inch on 19.2 or 24 inch joists, which causes bounce and squeaks. Second, butting panels tight without expansion gaps, which produces summer buckling. Third, nailing instead of gluing, which leaves room for movement and inevitable squeaks within 5 to 10 years.
Plan ahead, follow the APA stamp, and pair the right subfloor thickness with the finish floor system. The answer to what size plywood for flooring is almost always 3/4 inch T&G Sturd-I-Floor on 16 inch joists, with appropriate underlayment added based on the finish material above.
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