Roof Sheathing Calculator: Plywood and OSB Sheets for Gambrel Roofs
Total 4×8 sheets, with waste factor and edge blocking estimate for gambrel roofs.
Sheets (no waste)
41
Sheets (with waste)
45
Edge Blocking
82 lf
Plywood vs OSB
Plywood and OSB both meet IRC R803 for roof sheathing. OSB is cheaper and consistently flat. Plywood survives water exposure better, important if the gambrel will be partially exposed during framing. For the gambrel knuckle, both materials require an H-clip between rafters at unsupported edges. Stagger sheets so vertical joints do not align across courses.
Sheet count adds ~10% waste on gambrels because the pitch transition forces a horizontal cut on every sheet that crosses the knuckle. Order full bundles and return unopened sheets if your supplier allows it.
Material Tip
Choose plywood over OSB any time the roof will sit exposed to weather during framing, which is common on a gambrel because the knuckle takes longer to dry in than a single-pitch roof. OSB is fine once the roof is dried in fast, but a few rainy weeks on an open deck is exactly where OSB's edges swell and telegraph through the finished roofing.
Frequently Asked Questions
›Plywood or OSB for gambrel roof sheathing?
Both meet code. OSB is 20% cheaper and equally strong in shear. Plywood handles repeated wet/dry cycles better. For a gambrel that will see standing seam metal, either works; for a gambrel that will see asphalt shingles in a humid climate, plywood is the safer bet.
›What thickness sheathing do I need?
7/16 in OSB or 1/2 in plywood for rafters at 16 in on center; 5/8 in for 24 in on center. Steep gambrel lower slopes can use the thinner option because snow loads slide off.
›Why does sheet count differ from area/32?
Because of cuts, staggering, and waste at the knuckle. A pitch transition forces a horizontal cut on every sheet that crosses it. Allow 10% waste on a gambrel, 15% if there are dormers.