Roof sheathing thickness

Roof sheathing thickness by rafter spacing

The IRC sets roof sheathing by the panel's span rating, the stamp on every sheet of OSB or plywood: the first number is the maximum roof support spacing in inches, the second the maximum floor joist spacing^[1][2]. The roof column of IRC Table R503.2.1.1(1), the same in the 2021 and 2024 editions, reads:

So 7/16 in OSB is the code minimum for rafters or trusses at 16 or 24 in on centre. Thicker panels are a choice for stiffness, nail holding or a heavy roof covering, not a code requirement at those spacings. The 40/20 and 48/24 sheathing rows were not in the copy of the table we read; check them in your adopted code.

7/16, 1/2 or 5/8 inch roof sheathing?

Pick the panel by rating first and thickness second, because two panels of the same thickness can carry different ratings:

• 7/16 in (24/16): the common minimum for 24 in truss spacing with edge support.
• 15/32 or 1/2 in (32/16): allowed up to 32 in with edge support or 28 in without; a little stiffer underfoot at 24 in.
• 19/32 or 5/8 in: sturdier under heavy tile or slate and in high-snow areas when the design calls for it; also usable at 24 in without edge clips.
• 23/32 or 3/4 in: mostly a floor panel; on roofs it reaches 48 in supports with edge support.

Count how many sheets you need with the plywood calculator; it multiplies the footprint by the pitch factor the roof pitch calculator gives.

Edge support: panel clips, blocking or tongue and groove

Edge support means the long panel edges between rafters are held level, by metal panel clips (H-clips), solid blocking or tongue-and-groove edges. The table allows wider spacing for 32/16 panels only when the edges are supported^[1]:

• Use clips, blocking or tongue-and-groove edges wherever the panel's rating needs edge support at your spacing.
• Leave the edge and end gaps the panel maker prints on the sheet so panels can swell.
• Run panels with the long dimension across the rafters and stagger end joints.

Roof sheathing for rafters and trusses

Rafter and truss spacing drives the sheathing, so check both together. Rafters at 24 in on centre save lumber but need a panel rated for 24 in; the rafter span chart shows which rafter sizes span your building at 16 or 24 in, and the roof truss calculator counts trusses at either spacing. Roof sheathing also braces the roof, and IRC Chapter 8 covers its attachment to the rafters^[3].

Roof sheathing thickness mistakes

• Reading thickness instead of the span rating. The stamp, not the caliper, sets the allowed spacing.
• Skipping clips on 32/16 panels at 32 in. Without edge support the limit drops to 28 in.
• Using floor-only panels on a roof. Use panels rated for roof use, and follow the exposure rating on the stamp.
• Ignoring local amendments. High-snow and high-wind jurisdictions often amend sheathing and nailing; confirm with your building department.