Subfloor thickness is set by joist spacing and the panel's span rating. Under 2021 IRC Table R503.2.1.1(1), joists at 16 inches on center need at least a 32/16 panel (15/32 or 1/2 inch), and joists at 24 inches need a 48/24 or 24 o.c. panel (23/32 or 3/4 inch) unless a footnote condition applies.
The panel thickness is a consequence of the span rating, not the other way round. You pick a panel whose rated floor span is at least your joist spacing, then read the thickness off the same row. This article copies the relevant rows from the 2021 International Residential Code. The code edition your jurisdiction has adopted, plus any local amendments, governs. Proprietary panels and engineered floor systems follow the manufacturer's or APA's published data.
If you have not fixed the joist spacing yet, start with floor joist spacing explained and size the joists from the floor joist span table. The subfloor comes after both.
How the IRC sets subfloor thickness
Section R503.2.1 requires wood structural panels used structurally to conform to DOC PS 1, DOC PS 2, CSA O325 or CSA O437, and to carry a grade mark showing grade, bond classification and Performance Category. The code then says the Performance Category value is used as the "nominal panel thickness" wherever the code refers to thickness. So the number to check on site is on the stamp, not on a tape measure.
Section R503.2.1.1 requires subfloor and combination subfloor underlayment panels to be one of the grades in Table R503.2.1.1(1), and Section R503.2.2 makes that table (or APA E30) the source for maximum span.
What a span rating means
A span rating such as 32/16 has two numbers. The left number is the maximum spacing of roof supports in inches. The right number is the maximum spacing of floor supports in inches. Panels rated 16/0, 20/0 or 24/0 have a floor number of zero, and Table R503.2.1.1(1) gives them a subfloor span of 0, so they are roof sheathing only.
Single floor panels (the table's "Underlayment, C-C plugged, single floor" grades) carry one number, such as 20 o.c., which is the floor span. These are meant to serve as combination subfloor underlayment in one layer.
Subfloor thickness chart by span rating
2021 IRC Table R503.2.1.1(1): wood structural panels, sheathing grades, maximum subfloor span. Panels continuous over two or more spans with the long dimension perpendicular to supports, 24 inches or wider, dead load 10 psf.
| Span rating | Minimum nominal panel thickness (inch) | Maximum subfloor span (inches) |
|---|---|---|
| 24/16 | 7/16 | 16 |
| 32/16 | 15/32, 1/2 | 16 (footnote h) |
| 40/20 | 19/32, 5/8 | 20 (footnotes h, i) |
| 48/24 | 23/32, 3/4 | 24 |
| 60/32 | 7/8 | 32 |
2021 IRC Table R503.2.1.1(1): single floor grades used as combination subfloor underlayment, same panel conditions.
| Span rating | Minimum nominal panel thickness (inch) | Maximum span, combination subfloor underlayment (inches) |
|---|---|---|
| 16 o.c. | 19/32, 5/8 | 16 (footnote i) |
| 20 o.c. | 19/32, 5/8 | 20 (footnotes i, j) |
| 24 o.c. | 23/32, 3/4 | 24 |
| 32 o.c. | 7/8 | 32 |
| 48 o.c. | 1-3/32, 1-1/8 | 48 |
The footnotes matter. Footnote h lets the 32/16 and 40/20 sheathing rows reach 24 inches where 3/4 inch wood finish flooring is installed at right angles to the joists. Footnote i allows 24 inches where 1.5 inches of lightweight concrete or approved cellular concrete is placed over the subfloor. Footnote a notes that the loads assume a 10 psf dead load, and the live load has to be reduced if the dead load is higher.
Subfloor thickness for 16 inch joists
At 16 inches on center, the lightest sheathing rows that work are 24/16 at 7/16 inch and 32/16 at 15/32 or 1/2 inch. For a single layer that also acts as underlayment, the 16 o.c. single floor grade at 19/32 or 5/8 inch is the minimum.
Any panel with a longer rated floor span also satisfies the table at 16 inches. Many builders step up to a thicker panel for a stiffer feel under tile or heavy finishes, but that is a choice, not a code minimum.
Subfloor thickness for 24 inch joists
At 24 inches on center, the table gives a 24 inch subfloor span to two rows without conditions:
- 48/24 sheathing at 23/32 or 3/4 inch.
- 24 o.c. single floor at 23/32 or 3/4 inch.
A 32/16 or 40/20 panel only reaches 24 inches through footnote h (3/4 inch wood finish flooring at right angles to the joists), and the 40/20 row also through footnote i (lightweight concrete topping). If the finish floor is carpet, vinyl or tile, those footnotes do not apply. Wider spacing also puts more load on each joist, so recheck the joist size in the span table before you widen the layout.
Tongue and groove subfloor and edge support
Panel edges that run perpendicular to the joists fall between supports. Table R503.2.1.1(1) handles them in footnotes j and k: unsupported edges shall have tongue-and-groove joints or be supported with blocking, unless nominal 1/4 inch wood panel-type underlayment, fiber-cement underlayment with joints offset not less than 2 inches, or 3/4 inch wood finish flooring at right angles to the supports goes on top. Footnote d defines edge support as lumber blocking, panel edge clips, tongue-and-groove edges, or another approved type.
In practice, tongue and groove subfloor panels are what make an unblocked floor work. Square-edge panels need blocking under every edge that does not land on a joist. The blocking used for joist restraint is a separate requirement, covered in joist blocking and bridging.
OSB vs plywood subfloor
The IRC does not treat OSB and plywood as different subfloor products. Both are wood structural panels under Section R503.2, both are graded by span rating and Performance Category, and both are read from the same rows of Table R503.2.1.1(1). A 23/32 inch 48/24 OSB panel and a 23/32 inch 48/24 plywood panel are equivalent as far as the prescriptive table goes.
The differences that show up in the field, such as edge swelling when wet, weight, and cost, are product and manufacturer questions. If a proprietary panel claims more than the table, its own evaluation report or APA data governs, and the building official decides whether to accept it.
Sanded plywood as combination subfloor underlayment
Sanded plywood is the one case where the code gives plywood its own table, based on species group.
2021 IRC Table R503.2.1.1(2): sanded plywood combination subfloor underlayment, minimum thickness (inch). Plywood continuous over two or more spans, face grain perpendicular to supports, all grades of sanded exterior-type plywood.
| Species group | 16 inch joists | 20 inch joists | 24 inch joists |
|---|---|---|---|
| 1 | 1/2 | 5/8 | 3/4 |
| 2, 3 | 5/8 | 3/4 | 7/8 |
| 4 | 3/4 | 7/8 | 1 |
Lumber subfloor thickness
Board subflooring is still in the code. Table R503.1 sets the minimum net thickness.
2021 IRC Table R503.1: minimum thickness of lumber floor sheathing.
| Joist or beam spacing (inches) | Perpendicular to joist | Diagonal to joist |
|---|---|---|
| 16 | 5/8 | 5/8 |
| 24 | 11/16 | 3/4 |
Section R503.1.1 requires end joints to fall over supports unless end-matched lumber is used, and lets you omit the subfloor entirely where joists are no more than 16 inches apart and 1-inch nominal tongue-and-groove wood strip flooring runs perpendicular to the joists. Table R503.1 also has rows for 1-1/2 inch tongue-and-groove decking at wider beam spacings, with minimum stress grades in its footnotes; read the table for those.
Subfloor nailing
Section R503.2.3 says wood structural panel subfloor on wood framing is fastened per Table R602.3(1), and on cold-formed steel per Table R505.3.1(2). These rows of Table R602.3(1) are for combination subfloor underlayment:
2021 IRC Table R602.3(1), items 38 to 40: wood structural panels, combination subfloor underlayment to framing.
| Panel thickness | Fastener | Edges (inches) | Intermediate supports (inches) |
|---|---|---|---|
| 3/4 inch and less | Deformed (2 x 0.113 inch) or deformed (2 x 0.120 inch) nail, or 8d common (2-1/2 x 0.131 inch) nail | 6 | 12 |
| 7/8 to 1 inch | 8d common (2-1/2 x 0.131 inch) nail, or deformed (2-1/2 x 0.131 inch) or (2-1/2 x 0.120 inch) nail | 6 | 12 |
| 1-1/8 to 1-1/4 inch | 10d common (3 x 0.148 inch) nail, or deformed (2-1/2 x 0.131 inch) or (2-1/2 x 0.120 inch) nail | 6 | 12 |
For sheathing-grade subfloor panels, use items 31 to 33 of the same table. Footnote h of Table R602.3(1) says the edge spacing applies at panel edges on framing members and required blocking and at floor perimeters, and that the floor perimeter shall be supported by framing members or solid blocking, which in a platform floor usually means the rim joist. Footnote c requires nails at no more than 6 inches on center at all supports where spans are 48 inches or greater.
Showing the subfloor on a framing plan
A floor framing plan normally carries the subfloor as a general note rather than a drawn element, for example "23/32 inch 48/24 T&G wood structural panel, long dimension perpendicular to joists, fastened per IRC Table R602.3(1)." Put it near the joist callout so a reviewer can check the panel against the spacing. The floor framing plan guide shows where notes sit, and how to read a floor framing plan covers the callouts a reviewer looks for. Exterior decking follows different rules, set out in the deck joist span and spacing guide.
Framing Plan draws joist layouts, beams and openings and exports PNG, PDF and DXF as a preliminary drawing aid. It does not check code or size members: its sample profile uses demo span figures and metric member sizes, and the editor works in millimeters. Choose the panel from the tables above, then open the framing studio to lay out the joist field your subfloor note will sit beside.