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IRC Table R503.2.1.1(1) and the APA span rating: two numbers, one sheet
Every structural panel is stamped with two numbers separated by a slash, and they are the roof span and the subfloor span for that one sheet. A 32/16 panel spans 32 in over rafters and 16 in over joists. Most people buy on the thickness, which is the least informative number on the stamp.
Sheathing takeoff
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Spans are from IRC Table R503.2.1.1(1). The rating holds only across three or more supports with the strength axis across them.
Specification summary
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One sheet, two spans, and the stamp that says both
Worked example
A 28 ft by 40 ft roof sheathed in 32/16 panels over rafters at 24 in, with panel clips, 10% waste.
The rating. 32/16 means 32 in on a roof and 16 in on a floor. Your 24 in framing passes comfortably.
Remove the clips. The roof figure drops from 32 in to 28 in, a 13% reduction, and 24 in still passes.
Try 40 in framing with clips and it fails: 40 exceeds the 32 in this panel is rated for.
Sheets. 1,120 sq ft at 32 sq ft per sheet with 10% waste is 39 sheets.
Fasteners. About 74 per sheet at 6 in edges and 12 in field, so 2,886.
The edge support step is the cheap one. Panel clips cost pennies and buy 4 in of span on a 32/16 panel, or 12 in on a 48/24. That is the least expensive span you will ever buy.
The formula
Sheets are area over 32, and the rest is a lookup that can refuse:
waste. 10% on a plain rectangle, more with hips, valleys and openings
rating
the two numbers on the stamp: roof span, then subfloor span
edge support
blocking, panel clips or tongue and groove. Changes the roof figure only
Waste on a roof is genuinely higher than on a wall because hips and valleys cut every sheet on an angle and the offcuts rarely fit anywhere. On a complex roof 15% is closer than 10%.
Buy on the stamp, not on the thickness
The single most useful thing to know about structural panels is that the thickness is not the specification. The span rating is, and it is stamped on every sheet.
It is two numbers with a slash: roof span on the left, subfloor span on the right. A 32/16 panel spans 32 in over rafters and 16 in over joists, and it is the same sheet in both cases. That difference exists because a floor carries far more load and is held to a much tighter deflection limit.
The thickness is a consequence rather than a specification, and it is not even quoted honestly: a panel sold as 1/2 in is a 15/32 in performance category, and 3/4 in is 23/32. Two sheets described the same way at the counter can carry different ratings.
The two conditions the rating depends on are easy to break. It applies across three or more supports, so a sheet bridging only two is a single span without the rating. And it applies with the strength axis across the supports, so a sheet turned the other way to save cutting has no rating at all.
The two numbers stamped on the sheet
Every structural panel carries a span rating in its trademark, and it is two numbers with a slash. APA defines them precisely:
The left-hand number denotes the maximum recommended spacing of supports when the panel is used for roof sheathing with the long dimension or strength axis of the panel across three or more supports. The right-hand number indicates the maximum recommended spacing of supports when the panel is used for subflooring with the long dimension or strength axis of the panel across three or more supports. A panel marked 32/16, for example, may be used for roof decking over supports up to 32 inches on center or for subflooring over supports up to 16 inches on center.
APA, The Engineered Wood Association, Form No. TT-037A, February 2007
Roof on the left, subfloor on the right. A 32/16 sheet spans 32 in over rafters and 16 in over joists, and it is the same sheet. People read the first number, or the thickness, and buy on that.
IRC Table R503.2.1.1(1), allowable spans for wood structural panels
Span rating
Nominal thickness
Roof, with edge support
Roof, without
Subfloor
16/0
3/8 in
16 in
16 in
not rated
20/0
3/8 in
20 in
20 in
not rated
24/0
3/8 in
24 in
20 in
not rated
24/16
7/16 in
24 in
24 in
16 in
32/16
15/32 or 1/2 in
32 in
28 in
16 in
40/20
19/32 or 5/8 in
40 in
32 in
20 in
48/24
23/32 or 3/4 in
48 in
36 in
24 in
60/32
7/8 in
60 in
48 in
32 in
IRC Table R503.2.1.1(1), allowable spans and loads for wood structural panels, City of Republic, Missouri, revision date January 2017
Two conditions the rating depends on, and one that changes it
Three or more supports. Read the APA definition again: it says across three or more supports. A sheet bridging only two supports is a single span and does not get the rating printed on it. That is a real case at the end of a run and around openings.
Except for APA RATED SIDING panels, the Span Rating applies when the long panel dimension or strength axis is across supports, unless the strength axis is otherwise identified.
APA, The Engineered Wood Association, Form No. TT-037A, February 2007
Strength axis across the supports. The long dimension of the sheet must run perpendicular to the framing. Laying sheets the other way, which people do to reduce cutting, voids the rating. It is one of the easiest inspection failures to create and one of the hardest to see afterwards.
Edge support changes the roof number. A 32/16 panel spans 32 in on a roof with blocking, panel clips or tongue and groove edges, and only 28 in without. A 48/24 drops from 48 to 36. That is a 25% reduction for a handful of H-clips, and the subfloor column has no such distinction because tongue and groove is assumed.
Fastening is where sheathing actually earns its keep
Sheathing is not just a surface. Nailed correctly it is the shear diaphragm that resists wind and seismic load, and it is what provides the lateral support the stud height table assumes.
The common schedule is 6 in on panel edges and 12 in in the field, and the edge spacing is the one that matters: that is where the shear transfers. Braced wall panels under R602.10 have their own tighter schedules, sometimes 4 in or 3 in on edges, and getting that wrong turns a rated wall into a decorative one.
Overdriving is the other common failure. A nail driven through the face veneer has lost most of its shear capacity, and a pneumatic nailer set too high does that invisibly across an entire wall.
OSB and plywood are rated the same way
Both carry APA span ratings and the table above applies to either. Structurally they are interchangeable at the same rating, which is the whole point of a performance rating rather than a material specification.
Where they differ is moisture. OSB swells at cut edges and is slower to dry than plywood, which matters on a job that gets rained on before it is dried in, and it is why OSB subfloor edges sometimes telegraph through flooring.
Plywood holds fasteners slightly better near edges and is more forgiving of repeated wetting. Neither of those is a code question and both are rated the same.
Subfloor is a system, not a panel
The table gives the panel span, but a squeak-free floor is glue plus fasteners plus the panel. Construction adhesive on every joist and screws rather than nails is the difference between a floor that stays quiet and one that does not.
Tongue and groove edges are assumed in the subfloor column of the table, which is why there is no edge support distinction there. A square edge panel used as subfloor needs blocking under the unsupported edges.
Single-floor panels, sold as Sturd-I-Floor, carry one number rather than two because they are only rated for that use. Those come as 16, 20, 24, 32 and 48 oc.
Frequently asked questions
What do the two numbers on plywood mean?
Roof span and subfloor span, in inches on centre. APA defines the left-hand number as the maximum spacing for roof sheathing and the right-hand as the maximum for subflooring, both with the strength axis across three or more supports. A 32/16 panel spans 32 in on a roof and 16 in on a floor.
What thickness plywood for a roof at 24 inch centres?
A 24/0 panel at 3/8 in is rated for it with edge support, and 24/16 at 7/16 in is rated for 24 in with or without. Buy on the span rating rather than the thickness, since the fractions quoted at the counter do not match the performance category.
Do I need H-clips on a roof?
Only if you need the higher span figure. A 32/16 panel spans 32 in with edge support and 28 in without; a 48/24 drops from 48 in to 36 in. Clips, blocking or tongue and groove edges all count as edge support.
Does the span rating apply if the sheet spans only two supports?
No. APA states the rating applies with the strength axis across three or more supports. A sheet bridging two supports is a single span and does not carry the rating printed on it, which is a real case at the end of a run.
Can I lay sheathing the other way to save cutting?
Not without losing the rating. The span rating applies when the long dimension, the strength axis, runs across the supports. Turning sheets to run along the framing voids it, and it is one of the easiest inspection failures to create and hardest to see afterwards.
Is OSB as strong as plywood?
At the same span rating, yes, structurally. That is the point of a performance rating. They differ in how they handle moisture: OSB swells at cut edges and dries more slowly, which matters on a job that gets wet before it is dried in.
How many sheets of plywood for a 28x40 roof?
1,120 sq ft at 32 sq ft per sheet is 35 sheets, or 39 with a 10% waste allowance. On a hipped or valleyed roof 15% is a more realistic allowance because every cut is on an angle.
Why is 1/2 inch plywood not half an inch?
Because the nominal fraction is a trade name and the real specification is the performance category. A panel sold as 1/2 in is 15/32 in, and 3/4 in is 23/32 in. Specify by span rating and performance category.
APA Span Ratings, Form No. TT-037AAPA, The Engineered Wood Association, February 2007. The definition of the two numbers and the three-support and strength-axis conditions.
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Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.
The rating has conditions. It applies with the strength axis across three or more supports. A sheet turned the wrong way, or spanning only two supports, does not carry it. Fastening schedules for braced wall panels under R602.10 are tighter than the general schedule used here.