Roof sheathing calculator
Enter the size of the house, the overhangs and the roof pitch. You get the true roof area, the sheets of decking with waste, whether the panels need H-clips at your rafter spacing, the nails, the weight and the cost.
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Roof sheathing spans
IRC Table R503.2.1.1(1): the farthest a roof panel may span between rafters or trusses, by its span rating, with and without edge support (H-clips, blocking or tongue-and-groove edges). Thickness is the usual one for each rating.
| Span rating | Usual thickness | With edge support | Without |
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How it is worked out
A 40 x 28 ft house with 12 in overhangs is 42 x 30 ft in plan, 1,260 sq ft. At 6/12 the pitch factor is 1.118, so the roof is 1,409 sq ft: just over 44 sheets of 4 x 8, 49 to buy with 10% waste. 7/16 in OSB rated 24/16 spans 24 in without edge support, so trusses at 24 in need no clips by the table, though many builders use them anyway.
Do I need H-clips?
H-clips support the long edges of the panels between the rafters, so the edges do not sag or step. The IRC requires edge support where the rafter spacing is more than the panel's span without it: for example, a 3/8 in panel rated 24/0 spans 20 in without edge support but 24 in with it. The code's note asks for one clip midway between each pair of supports, two where they are 48 in apart. Tongue-and-groove panels or blocking under the edges do the same job.
Nailing
IRC Table R602.3(1) calls for 8d nails at 6 in on the panel edges and 12 in on the intermediate supports for roof sheathing. In high-wind regions the code and many local amendments tighten this to 6 in everywhere, with ring-shank nails; check yours. Leave a 1/8 in gap at panel ends and edges for expansion.
For shingles, underlayment and drip edge, use the roofing calculator; for the rafters themselves, the rafter span chart.
Questions
How many sheets of plywood do I need for my roof?
Work out the roof area, footprint with overhangs times the pitch factor, and divide by 32 for 4 x 8 sheets, then add 10 to 15% waste. A 1,409 sq ft roof is just over 44 sheets, 49 with 10% waste.
How many sheets of OSB are in a roofing square?
A square is 100 sq ft, so about 3.1 sheets of 4 x 8, or 3.4 with 10% waste.
What thickness of OSB do I need for a roof?
7/16 in OSB rated 24/16 is the common choice over trusses at 24 in on centre, and the IRC table allows it at 24 in without clips. Use 1/2 or 5/8 in under tile, slate or where you want a stiffer deck.
Are H-clips required on roof sheathing?
Only where the rafter spacing is more than the panel's span without edge support in IRC Table R503.2.1.1(1). For 7/16 in rated 24/16, that span is 24 in, so clips are not required at 24 in, but many builders and some local codes use them anyway.
How many nails per sheet of roof sheathing?
At 6 in on the panel ends and 12 in in the field, a 4 x 8 sheet over trusses at 24 in takes about 33 nails: 9 on each end and 5 on each of the three trusses between.
How much does a sheet of 7/16 OSB weigh?
About 46 lb for 4 x 8, using the APA's 1.3 lb per sq ft for 7/16 in panels plus 10% for OSB.
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