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One H-clip per rafter bay on each joint between panel courses: 4 on an 8 ft sheet edge at 24 in centers, 6 at 16 in, two per bay at 48 in. Over a whole roof that averages about 3 per sheet at 24 in. They are required only when the spacing exceeds the panel's span without edge support — e.g. 3/8 in 24/0 panels at 24 in.
Plywood clips, or H-clips, are galvanized steel panel edge clips that slide onto the long edges of two roof panels between rafters, tying the unsupported edges together. The 2021 IRC (footnote to Table R503.2.1.1(1)) counts them as edge support with one clip midway between each pair of supports, or two equally spaced clips at 48 in spacing. So the count is one clip per rafter bay along every horizontal panel joint: a 4 × 8 sheet on 24 in centers crosses four bays and takes 4 clips on its edge, 6 at 16 in, 3 at 32 in. The eave and ridge edges take none, so a 40 ft gable roof with 16 ft slopes on 24 in trusses needs 120 clips for 40 sheets, 3 per sheet on average. Clips are only required when your spacing is longer than the panel's span without edge support: 3/8 in 24/0 panels need them at 24 in, 7/16 in 24/16 OSB does not.
Short answer: one H-clip in every rafter bay along each horizontal joint between two courses of roof panels — two per bay when the rafters are 48 in apart. A 4 × 8 sheet on 24 in centers takes 4 clips on its 8 ft edge; because the eave and ridge edges take none, a whole roof averages about 3 per sheet. Clips are required only when the rafter spacing is longer than the panel can span without edge support.
| Spacing | Bays under an 8 ft sheet | Clips per 8 ft edge |
|---|---|---|
| 16 in o.c. | 6 | 6 |
| 19.2 in o.c. | 5 | 5 |
| 24 in o.c. | 4 | 4 |
| 32 in o.c. | 3 | 3 |
| 48 in o.c. | 2 | 4 |
| Span rating | Min. thickness | With clips | Without | Clips required at |
|---|---|---|---|---|
| 16/0 | 3/8 in | 16 in | 16 in | never (same span either way) |
| 20/0 | 3/8 in | 20 in | 20 in | never (same span either way) |
| 24/0 | 3/8 in | 24 in | 20 in | over 20 in, up to 24 in |
| 24/16 | 7/16 in | 24 in | 24 in | never (same span either way) |
| 32/16 | 15/32 in | 32 in | 28 in | over 28 in, up to 32 in |
| 40/20 | 19/32 in | 40 in | 32 in | over 32 in, up to 40 in |
| 48/24 | 23/32 in | 48 in | 36 in | over 36 in, up to 48 in |
One clip in every rafter bay along the panel's long edge. A 4 × 8 sheet laid across rafters at 24 in on center spans four bays, so it takes 4 clips on its edge; at 16 in it would take 6, at 32 in 3, and at 48 in 4 (two per bay). Because the eave and ridge edges never get clips, a whole roof averages about 3 clips per sheet at 24 in centers.
Only when the panel's span without edge support is shorter than your rafter spacing. Under 2021 IRC Table R503.2.1.1(1), 3/8 in 24/0 panels on 24 in centers need edge support (they span 20 in without it), while 7/16 in 24/16 OSB spans 24 in either way and does not. Clips, tongue-and-groove edges or solid blocking all count as edge support. Local codes and some shingle makers ask for clips anyway, so check before you skip them.
The size stamped on the clip must match the panel thickness: 7/16 in clips for 7/16 in OSB, 1/2 in clips for 15/32 or 1/2 in plywood, 5/8 in for 19/32 or 5/8 in, and so on. A clip that is too big lets the edges move; one that is too small will not seat.
On the long horizontal joint between two courses of panels, midway between rafters or trusses: slide the clip onto the top edge of the course you just nailed, then drop the next course onto it. At 48 in spacing use two clips per bay, equally spaced. They do not go on the eave or ridge edge or on the short ends, which land on framing.
APA recommends about a 1/8 in gap at panel edges and ends so the sheathing can swell without buckling. Many panel clips are shaped to hold about that gap along the edge; at the short ends, which land on a rafter, set it yourself with a 1/8 in spacer.
Choosing the panel? The roof sheathing thickness calculator gives the code minimum for your spacing.
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.