Plywood Subfloor Types: Thickness by Joist Spacing (APA Ratings)

For most residential floors, pick APA-rated subfloor panels whose span rating matches your joist spacing: 19/32″ panels (Sturd-I-Floor 20 oc or Rated Sheathing 40/20) reach 20″ on center, and 23/32″ panels (24 oc or 48/24) reach 24″. Get the rating wrong and the floor bounces, squeaks, or fails inspection. This guide covers panel types, thickness by joist spacing, plywood versus OSB, and fastening. Always confirm with local code.

Types Of Subfloor Materials

Subfloor plywood is structural plywood fastened across floor joists to create a flat, rigid base for finished flooring. APA, the engineered wood trade association, labels it with a span rating (for example 24/16 or 24 oc) that sets the maximum joist spacing, and with a bond classification, either Exposure 1 or Exterior, that describes how the glue handles moisture.

Plywood is one of two panel types APA rates for subfloors. The other is oriented strand board (OSB), and both carry the same span ratings. For homes built on wood joists, tongue-and-groove (T&G) panels are the usual choice because the interlocking edges support each other between joists.

Sturd-I-Floor vs Rated Sheathing: The Difference Most Guides Skip

Most guides call every subfloor panel “CDX,” but APA sells two different products for floors. APA Rated Sturd-I-Floor is a combined subfloor and underlayment for single-layer floors, and its span rating is one number: 20, 24, or 48 oc. APA Rated Sheathing is a general-purpose panel for roofs, walls, and floors, so it carries two numbers such as 24/16, 40/20, or 48/24, where the second number is the maximum spacing for subflooring. Used on a floor, Rated Sheathing typically needs a separate layer of underlayment before the finish flooring goes down.

Panel thickness maps to span rating: 19/32″ panels are rated 20 oc (Sturd-I-Floor) or 40/20 (Rated Sheathing), and 23/32″ panels are rated 24 oc or 48/24. Source: APA, Underlayment and Subfloor

Choosing The Best Plywood For Subfloor

When choosing the best plywood for subfloors, match the span rating to your joist spacing and the exposure rating to how wet the job will get. Tongue and groove is the best plywood subfloor type for preventing floor deformation and squeaks. If you are pricing panels at a big-box store, see our guide to Lowe’s 3/4 plywood subfloor options. Here are the common APA panel ratings for floors:

Panel type	Span rating	Thickness	Joist spacing it covers	Notes
Rated Sturd-I-Floor	20 oc	19/32″	Up to 20″ on center	Single-layer floor under carpet and pad
Rated Sturd-I-Floor	24 oc	23/32″ (3/4″)	Up to 24″ on center	Most common residential pick; T&G available
Rated Sturd-I-Floor	48 oc	1-1/8″	Up to 48″ on center	Wide-spaced framing such as post-and-beam
Rated Sheathing	32/16	15/32″	Up to 16″ on center	Needs separate underlayment for finish flooring
Rated Sheathing	40/20	19/32″	Up to 20″ on center	Needs separate underlayment for finish flooring
Rated Sheathing	48/24	23/32″ (3/4″)	Up to 24″ on center	Needs separate underlayment for finish flooring

Exposure rating matters as much as thickness. Panels sold as CDX are APA Exposure 1, meaning the glue is designed to survive moisture during construction, such as a rained-on floor before the roof is dry. They are not Exterior-bond panels and should not be left in long-term weather. If a floor sits open to rain for weeks, expect trouble regardless of the grade stamp, and cover it.

Marine-grade plywood is rarely necessary for a subfloor. Bathrooms and basements usually get a standard Exposure 1 panel, with moisture handled by the layers above it and by keeping the subfloor dry, so spend the extra money only if a manufacturer or local code calls for it.

Plywood Subfloor Thickness by Joist Spacing

Thickness follows joist spacing, because the span rating stamped on the panel is the maximum center-to-center distance it is rated for. APA rates these panels with the long dimension, or strength axis, running across three or more supports. This table turns the ratings into a quick lookup:

Joist spacing	Sturd-I-Floor (single layer)	Rated Sheathing (needs underlayment)
12″ on center	20 oc (19/32″) or 24 oc (23/32″)	32/16 (15/32″), 40/20 (19/32″), or 48/24 (23/32″)
16″ on center	20 oc (19/32″) or 24 oc (23/32″)	32/16 (15/32″), 40/20 (19/32″), or 48/24 (23/32″)
19.2″ on center	20 oc (19/32″) or 24 oc (23/32″)	40/20 (19/32″) or 48/24 (23/32″)
24″ on center	24 oc (23/32″)	48/24 (23/32″)

Correcting a common rule of thumb: many articles say 3/4″ is required at 16″ on center and 5/8″ at 12″. APA span ratings do not work that way: a 19/32″ panel rated 20 oc already covers 16″ and 19.2″ spacing, and a 23/32″ panel covers up to 24″. Plenty of builders still choose 23/32″ at 16″ for a stiffer, quieter floor, and finish-flooring makers often set their own minimums, so follow the stricter of the panel rating, the flooring instructions, and your local code. Our 19/32 plywood guide explains why a “5/8″ sheet is really 19/32” thick, and the joist span calculator helps check the framing itself.

“designed for single-layer floor construction beneath carpet and pad”

Why Plywood Is Better Than Other Subfloor Materials

Plywood earns its place because it is a code-recognized, widely stocked structural panel with predictable span ratings, and its cross-laminated veneers hold fasteners and keep edges stable. The alternatives each have a catch worth knowing.

Plywood vs. OSB: APA rates both, and a panel of either type with the same span rating is rated for the same joist spacing. The real differences show up on the job site. Both are Exposure 1 and both dislike prolonged wetting, but OSB edges are generally considered more prone to swelling after repeated soaking, while plywood tends to dry back flatter. OSB is usually cheaper, and plenty of new homes are built on it.

Plywood vs. Particle Board: Particle board is not an APA-rated structural subfloor panel. It soaks up water and crumbles, so use it, if at all, only as underlayment in dry rooms and never as the structural layer over joists.

Plywood vs. Solid Wood Planks: Older homes often have 1x board or diagonal plank subfloors. Boards can cup, shrink, and gap as they age, and they do not spread loads across a panel. Plywood laid over them or in their place gives a flatter, more uniform base.

Bottom line: for residential subfloors, tongue-and-groove plywood rated for your joist spacing is the safest default. OSB is acceptable where the manufacturer, code, and your climate allow it, and the AdvanTech vs plywood comparison covers the premium OSB option.

Comparison Of Plywood And Osb

Plywood is a strong and durable option for subfloors, and APA rates both plywood and OSB panels for the same floor spans. Tongue and groove flooring plywood is recommended for the best performance. Here is a closer look at how the two compare in practice.

Moisture Resistance

Plywood is made of cross-laminated wood veneers glued together, and APA Exposure 1 glue is designed to handle the moisture of normal construction delays. OSB uses the same Exposure 1 classification. The difference shows up at the edges: OSB edges are generally considered to swell more after repeated wetting, and the swelling often does not fully reverse. Either panel should be kept dry and protected before the roof is on.

Strength

Strength is set by the span rating, not the panel type. A 23/32″ panel rated 24 oc is rated for 24″ joist spacing whether it is plywood or OSB, so compare ratings on the grade stamp rather than assuming plywood is always stiffer. Gluing the panels to the joists adds stiffness to either one.

Thickness Requirements

Thickness follows the span rating, as shown in the table above: 19/32″ for rated spans up to 20″, and 23/32″ for spans up to 24″. For a deeper OSB comparison, see our CDX plywood vs OSB guide.

Fasteners, Glue and Gaps for Subfloor Panels

APA recommends a glued floor system, which uses construction adhesive in addition to nails to help eliminate squeaks and nail popping and to increase floor stiffness. Apply a bead of adhesive that conforms to APA specification AFG-01 (ASTM D3498) along the top of each joist, then fasten the panel before the adhesive skins over.

• Orientation: run the panel’s long dimension across the joists, over at least three supports.
• Fastener spacing: APA’s minimum recommendation is nails 6″ on center at panel ends and edges and 12″ on center at intermediate joists. Ring-shank nails or screws are common upgrades, and the panel stamp or local code can require more.
• Gaps: leave about 1/8″ between panel edges and ends so the panels have room to expand without buckling.
• Stagger the joints: offset end seams between rows so they do not line up on one joist.

Common Subfloor Materials

Plywood is a strong and durable subfloor material commonly used in construction projects because it installs quickly in large sheets. Here are the materials you will meet most often in residential construction, and how plywood compares.

Water and Mold Resistance

Exposure 1 plywood is designed to resist moisture during construction, not long-term weather exposure. Any wood panel left wet can grow mold, so the practical protection is keeping the subfloor dry: fix leaks, ventilate crawl spaces, and cover the floor if rain gets in before the roof is on. In humid climates or over vented crawl spaces, follow local code for vapor retarders and ground cover.

Custom Subfloor Systems

Custom subfloor systems add layers to a plywood subfloor for specific conditions, such as a concrete slab or a cold basement. Plywood still supplies the structural layer.

Foam-Backed and Dimpled Panel Systems

Commercial panels with a foam or dimpled plastic underside are sold for basements and slab-on-grade floors to add a thermal and moisture break under a floor. If you are pairing a subfloor with in-floor warmth, our guide to heated flooring costs outlines the systems that sit above the subfloor.

DIY Plywood Over A Concrete Slab

Over a slab there are no joists, so plywood is typically laid as a floating layer or fastened to sleepers on top of a vapor barrier. Test the slab for moisture first, and follow the flooring maker’s instructions for the barrier. Our guide to installing wood flooring on a concrete slab walks through the full method.

Benefits Of Different Subfloor Materials For Different Floors

Different finished floors ask different things of the subfloor. Here is what each needs from a plywood subfloor:

Tile

Tile needs a stiff, flat floor that does not flex, because movement cracks grout and tile. Many installers add a second layer such as cement board or an uncoupling membrane over the plywood rather than tiling on it directly, and the tile setter’s instructions set the exact requirement. Our guide on plywood subfloor thickness for tile goes deeper.

Finished Wood Floors

Solid hardwood needs a firm subfloor with no bounce. Check the flooring maker’s installation guide for its minimum panel thickness and flatness tolerance, and use a long straightedge to find high and low spots before you install. Engineered wood and LVP are typically less demanding but still need a flat, dry base.

Frequently Asked Questions For Plywood Subfloor Types

What Type Of Plywood Should I Use For Subfloor?

Use tongue-and-groove, APA-rated panels whose span rating matches your joist spacing. For 16″ on center, Sturd-I-Floor 20 oc (19/32″) or 24 oc (23/32″) both qualify, and 23/32″ is the popular pick for a stiffer floor. CDX panels are Exposure 1, so keep them dry during construction and confirm the rating with local code.

Should I Use 5/8 Or 3/4 Plywood For Subfloor?

A “5/8″ panel is really 19/32″ thick and rated 20 oc, which covers 16″ and 19.2” joists. A “3/4″ panel is 23/32″, rated 24 oc, and covers up to 24”. Many builders choose 3/4″ at 16″ for a stiffer feel, and flooring makers may require it, so check their instructions and your local code.

What Is The Most Common Subfloor?

Tongue-and-groove plywood and OSB panels are the most common subfloors in new wood-framed homes, usually 23/32″ thick over joists at 16″ or 24″ on center. Older homes may have 1x board sheathing laid straight or diagonally, which should be checked for gaps and rot before any new floor goes over it.

Should I Use Tongue And Groove Plywood For Subfloor?

Yes. Tongue-and-groove edges interlock, which supports panel edges between joists, reduces squeaks, and removes the need for blocking along the long edges, according to APA. Gluing the panels to the joists with subfloor adhesive adds stiffness on top of that.

Is CDX Or OSB Better For Subfloor?

Neither wins on span: APA rates plywood and OSB panels with the same span rating for the same joist spacing. Plywood is generally considered more forgiving of repeated wetting at the edges, while OSB usually costs less. Pick a panel with the right span rating, keep it dry, and follow local code.

How Thick Should Subfloor Plywood Be?

Match thickness to joist spacing: 19/32″ (rated 20 oc or 40/20) covers up to 20″ on center, and 23/32″ (rated 24 oc or 48/24) covers up to 24″. Many builders still use 23/32″ at 16″ for stiffness, and tile or hardwood instructions may call for more, so follow the strictest requirement.

What Is The Difference Between Sturd-I-Floor And Rated Sheathing?

Sturd-I-Floor is a combined subfloor and underlayment for single-layer floors, with one span number like 24 oc. Rated Sheathing is a general-purpose panel with two numbers like 48/24, where the second is the floor span. Rated Sheathing typically needs a separate underlayment before finish flooring, according to APA.

Conclusion

Choosing a plywood subfloor comes down to three checks: a span rating that covers your joist spacing, an exposure rating that matches how wet the job will get, and fastening that follows the glued floor system. Whether you use 19/32″ panels at 16″ on center or 23/32″ tongue-and-groove at 24″, read the grade stamp, follow the flooring maker’s instructions, and confirm local code before you buy.