As an Amazon Associate I earn from qualifying purchases.
Plywood and OSB roof sheathing are stamped with a span rating such as 24/16, where the first number is the widest roof support spacing the panel may span, in inches. The 2021 International Residential Code lists the minimum thickness of each rating and its maximum roof span with and without edge support in Table R503.2.1.1(1): 16/0, 20/0 and 24/0 panels are 3/8 in; 24/16 is 7/16 in and spans 24 in either way; 32/16 is 15/32 in (32 in with edge support, 28 in without); 40/20 is 19/32 in and 48/24 is 23/32 in. Edge support means panel clips, tongue-and-groove edges or blocking. So trusses at 24 in need 7/16 in sheathing without clips, or 3/8 in with them, and rafters at 16 in need at least 3/8 in. Table R602.3(1) fastens roof sheathing with 8d common nails (2 1/2 × 0.131 in) at 6 in on panel edges and 12 in in the field. Enter the spacing, the edge support and, if you like, the roof area for the sheets and nails.
Plywood and OSB roof sheathing carry a span rating such as 24/16: the first number is the maximum roof support spacing in inches. The 2021 IRC gives each rating a minimum thickness and a maximum roof span with and without edge support (panel clips, tongue-and-groove edges or blocking) in Table R503.2.1.1(1). Table R602.3(1) fastens 3/8 to 1 in roof sheathing with 8d common nails (2 1/2 × 0.131 in) at 6 in on edges and 12 in in the field.
| Span rating | Minimum thickness | Max span with edge support | Max span without edge support |
|---|---|---|---|
| 16/0 | 3/8 in | 16 in | 16 in |
| 20/0 | 3/8 in | 20 in | 20 in |
| 24/0 | 3/8 in | 24 in | 20 in |
| 24/16 | 7/16 in | 24 in | 24 in |
| 32/16 | 15/32 in | 32 in | 28 in |
| 40/20 | 19/32 in | 40 in | 32 in |
| 48/24 | 23/32 in | 48 in | 36 in |
| Spacing | No edge support | With edge support |
|---|---|---|
| 12 in | 3/8 in (16/0) | 3/8 in (16/0) |
| 16 in | 3/8 in (16/0) | 3/8 in (16/0) |
| 19.2 in | 3/8 in (20/0) | 3/8 in (20/0) |
| 24 in | 7/16 in (24/16) | 3/8 in (24/0) |
| 32 in | 19/32 in (40/20) | 15/32 in (32/16) |
| 48 in | none listed | 23/32 in (48/24) |
It depends on the rafter or truss spacing and whether the panel edges are supported. Under 2021 IRC Table R503.2.1.1(1), 24 in spacing needs a 24/16 panel (7/16 in) without edge clips, or a 24/0 panel (3/8 in) with clips or blocking; 16 in spacing allows a 16/0 panel (3/8 in). Many builders still use 7/16 in OSB or 1/2 in plywood as a stiffer minimum.
Yes, on supports up to 24 in apart. 7/16 in OSB is normally rated 24/16, which the IRC allows at a 24 in roof span with or without edge support. At wider spacing you need 15/32 in (32/16) or thicker.
The first number is the maximum spacing of roof supports in inches, the second the maximum for floors. A panel stamped 32/16 can span 32 in on a roof (28 in without edge support) and 16 in as subfloor.
2021 IRC Table R602.3(1) calls for 8d common nails (2 1/2 in × 0.131 in) at 6 in on centre along supported panel edges and 12 in in the field for 3/8 to 1 in roof sheathing. High-wind areas need closer spacing at edges and corners — check your local amendments.
Only where the table needs edge support for your panel and spacing — for example 3/8 in 24/0 panels on 24 in trusses. Panel clips, tongue-and-groove edges or blocking all count as edge support. With 7/16 in 24/16 panels on 24 in centres the IRC does not require them, though many builders use them anyway.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.