Roof sheathing sheets = roof area in sq ft × (1 + waste) ÷ 32. A 40 × 30 ft house with a 6/12 gable roof and 1 ft overhangs has 1,503 sq ft of roof, so with 10% waste it needs 52 sheets of 4 × 8 ft plywood or OSB. At 24 in rafter spacing a 7/16 in panel with a 24/16 span rating is the thinnest that the APA table lists.
How roof sheathing is calculated
Sheathing is sold in 4 × 8 ft sheets, 32 sq ft each. Take the roof area (the sloped surface, not the floor plan), add waste for the cuts at the ridge, hips, valleys and rakes, and divide by 32.
Roof area = footprint × pitch multiplier · Sheets = roof area × (1 + waste) ÷ 32, rounded up
Worked example: the footprint with overhangs is 42 × 32 = 1,344 sq ft, and the 6/12 multiplier is 1.1180, so the roof is 1,502.6 sq ft. Ten percent waste makes 1,652.9 sq ft, and 1,652.9 ÷ 32 = 51.65, which rounds up to 52 sheets. If you already know the roof area, use the Roof area tab and skip the geometry. For the area of odd roofs see the hip roof calculator and the gambrel roof calculator.
Plywood or OSB thickness for roof sheathing
The APA Engineered Wood Construction Guide (Form E30) ties the thickness to the span rating and the distance between rafters or trusses. Its Table 33 lists, with edge support:
| Span rating | Panel thickness | Maximum support spacing |
|---|---|---|
| 24/16 | 7/16 in | 24 in |
| 32/16 | 15/32 in (1/2 in) | 32 in |
| 40/20 | 19/32 in (5/8 in) | 40 in |
The calculator reads your rafter spacing and names the thinnest panel that clears it. That is a minimum: many builders use 1/2 in plywood or OSB on 24 in spacing for stiffer decking and better nail holding, and shingle makers or local codes can ask for more. Plywood and OSB carry the same span ratings, so the same table applies to both. If you are pricing other sheet goods, the plywood calculator handles general panel counts.
Nails, clips and spacing
The APA schedule (Table 35) is 8d nails 6 in on center at panel ends and edges and 12 in at the intermediate rafters, except 6 in at every support when the spacing is 48 in or more. The guide recommends 8d as the minimum for roofs because of wind uplift, and it asks for 1/8 in between panels for expansion. Where panels meet between rafters, edge support is needed: tongue-and-groove edges, blocking, or one panel clip midway in each span. At 16 in spacing that is 6 spans across a 96 in sheet, so 6 clips per sheet.
On this roof the calculator counts 43 nails per sheet at 16 in spacing (9 along each end, 5 at each of the 5 rafters in between), so 52 sheets take 2,236 nails and 312 clips. At 24 in spacing there are 5 supports, 33 nails and 4 clips per sheet. High-wind areas and roofs built as a diaphragm need tighter nailing than this: follow the plan set.
Roof sheathing questions
Do I count the overhang? Yes. The deck runs out to the fascia, so the footprint includes the eave and rake overhangs.
Which way do the sheets go? With the long side across the rafters, staggered from course to course, as in the drawing. APA's guide also limits continuous deck sections to 80 ft during construction so wet panels can expand.
How much waste? Ten percent on a gable; raise it for hip roofs, dormers and valleys.
What to work out next
With the deck counted, the covering is next. Turn the same area into shingle bundles with the roof square calculator, then add underlayment, ice barrier and drip edge in the roofing calculator. If the roof is being framed, the rafter calculator gives the lengths and cuts. A tear-off also sends the old roofing to a dumpster: size it with the dumpster size calculator. Everything for the roof is in the roofing and siding calculators.