Knowing what type of plywood for subfloor use gives you a floor that feels solid and stays quiet, while the wrong choice leads to bounce, squeaks, and swelling that telegraphs through your finished flooring. The answer to what type of plywood for subfloor installation comes down to three things: thickness, grade, and how it is rated for moisture and spanning. This guide breaks down each factor so you can buy the right panels the first time.
Subfloor vs. Underlayment
People often blur these two layers, but they do different jobs. The subfloor is the structural panel fastened directly to the floor joists, carrying the load and tying the framing together. Underlayment is a thinner, smoother layer sometimes added on top to create a flat surface for vinyl or tile.
This distinction matters because the plywood you choose depends on the role. Structural subfloor panels must be thick and rated to span between joists, while underlayment panels prioritize a smooth, void-free face. Confusing the two, such as using thin underlayment as a structural subfloor, produces a floor that flexes and fails.
Some products blur the line intentionally. Combination “single-layer” subfloor panels, often labeled Sturd-I-Floor, are engineered to serve as both structural subfloor and a surface ready for carpet or resilient flooring in one 3/4-inch or thicker layer. These save a step in new construction, but if you later plan to install tile or hardwood, you may still add a dedicated underlayment or cement board on top for a flatter, more stable surface.
Recommended Thickness
Thickness is dictated by joist spacing. The wider the gap between joists, the thicker the panel must be to avoid deflection and the bouncy feel that develops over an unsupported span.
- 16-inch on-center joists: 5/8-inch tongue-and-groove plywood is the practical minimum; 3/4-inch is better and standard.
- 19.2 to 24-inch on-center joists: use 3/4-inch or thicker rated sheathing to prevent sag.
- Tile installations: add stiffness with a thicker or doubled subfloor, since tile and grout crack over any flex.
For most homes, 3/4-inch tongue-and-groove is the reliable default, and the interlocking edges resist the up-and-down movement at panel joints that causes squeaks.
CDX Plywood vs. OSB
The two most common structural subfloor materials are CDX plywood and oriented strand board (OSB). Both are code-approved and widely used, and the choice often comes down to cost, moisture behavior, and personal preference.
OSB is typically a few dollars cheaper per sheet and comes in larger panels, which is why most new construction uses it. Plywood tends to be stiffer, dries out faster after getting wet, and holds fasteners slightly better. OSB, by contrast, swells at cut edges when it absorbs water and does not always return to its original thickness, which can leave raised ridges under thin flooring. For a bathroom or any moisture-prone room, many builders favor plywood or a premium moisture-resistant subfloor panel.
Both materials perform well when kept dry and installed to spec, so the decision often reflects the finished flooring above. Under carpet or a floating laminate floor, OSB is perfectly adequate and saves money. Under tile, sheet vinyl, or nailed hardwood, where any edge swelling or minor unevenness shows, the extra stiffness and dimensional stability of plywood earns its higher price. Whichever you pick, always store the panels flat and off the ground before installation so they do not warp or absorb ground moisture.
Understanding Plywood Grades and Ratings
Plywood is graded by the quality of its two faces using letters A through D, where A is smooth and defect-free and D allows knots and voids. CDX means a C-grade face, D-grade back, with X standing for exterior-grade glue, not full exterior use.
- Face grades (A-D): for hidden structural subfloor, a rough C or D face is fine; for underlayment under vinyl, choose a smoother A or B face.
- Exposure rating: “Exposure 1” panels use waterproof glue and tolerate construction moisture; fine for interior subfloors.
- Span rating: stamped as two numbers, this tells you the maximum joist spacing the panel can bridge.
Always check the span-rating stamp rather than guessing from thickness alone.
Moisture and Exposure Ratings
Water is the enemy of any subfloor, so match the panel’s moisture rating to the room. Standard “Exposure 1” plywood handles the incidental dampness of normal construction and everyday interior humidity. In wet areas like bathrooms, laundry rooms, and kitchens, step up to a moisture-resistant subfloor product or a treated plywood designed to resist swelling.
Never use interior particleboard as a subfloor; it disintegrates when wet and has no place in a floor assembly. If the subfloor will ever see standing water, such as in a mudroom or near a shower, prioritize a panel engineered to shrug off moisture and dry without permanent damage.
Below-grade and over-crawlspace floors deserve special attention. A subfloor above a damp crawlspace can slowly absorb humidity from below and cup or delaminate over years, so pair a quality panel with a properly sealed vapor barrier over the ground and adequate ventilation. When wood flooring is planned, measure the subfloor’s moisture content with a meter before installation; it should generally read within a few percentage points of the flooring itself to avoid gaps and cupping after the finished floor acclimates.
Tongue-and-Groove vs. Square Edge
Panel edges affect how well the floor performs at the seams. Tongue-and-groove (T&G) panels interlock along their long edges, transferring load across the joint even where there is no joist beneath. This locking action all but eliminates the differential movement that causes squeaks and uneven spots at panel edges.
Square-edge panels are cheaper but require solid blocking beneath every unsupported edge to prevent the panels from moving independently. For subfloors, T&G is worth the small upcharge; for underlayment installed over a solid subfloor, square-edge panels are standard because the layer below already provides support.
Installation Best Practices
Even the right plywood squeaks if installed poorly, so the details matter. Run panels perpendicular to the joists with the long edge crossing multiple joists, and stagger the end joints from row to row like brickwork so no continuous seam runs across the floor.
- Leave expansion gaps: a 1/8-inch gap at panel ends and 1/8 inch along edges lets the wood expand without buckling.
- Glue and screw: apply construction adhesive to the joists and use ring-shank or subfloor screws, not smooth nails, to lock panels down and stop squeaks.
- Fastener spacing: place fasteners every 6 inches along panel edges and every 8 to 10 inches in the field.
- Acclimate the panels: let plywood adjust to the home’s humidity for a day or two before installing.
Making the Final Choice
For the majority of residential floors, 3/4-inch tongue-and-groove plywood rated Exposure 1 with a span rating that matches your 16-inch joist spacing is the correct, worry-free answer. Upgrade to a moisture-resistant panel in bathrooms and laundry rooms, add thickness or a second layer under tile, and always glue and screw rather than nail. Get these fundamentals right and your subfloor will stay flat, silent, and solid under whatever finished flooring you install on top of it for decades.
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