Roof Sheathing Calculator (Plywood and OSB)

How the sheathing math works

Start with sheets. A 4 x 8 panel covers 32 sq ft, so the count is roof area (plus waste) divided by 32, rounded up. If you have a footprint instead of roof area, multiply it by the pitch factor, which is the square root of 1 + (pitch / 12) squared. At 6/12 the factor is 1.118.

• Roof area = footprint x pitch factor
• Sheets = roof area x (1 + waste) / 32, rounded up
• H-clips (24 in framing, recommended, not required) = sheets x 4, one per rafter bay along each sheet's long edge
• Nails per sheet = 2 panel-end rows x 9 (6 in spacing) + middle rows x 5 (12 in spacing)
• Underlayment rolls = roof area x (1 + waste) / coverage per roll
• Ice barrier slope distance = (overhang + wall + 24 in) x pitch factor
• Ice rolls = (eave ft x courses + valley ft) x (1 + waste) / 66.7
• Drip edge sticks = (eave + rake ft) x (1 + waste) / 10

Here are the default numbers worked by hand: a 60 x 30 ft gable house with an 1,800 sq ft footprint and a 6/12 pitch. The roof area is 1,800 x 1.118 = 2,012.4 sq ft. Adding 10 percent brings it to 2,213.64, and dividing by 32 gives 69.2, so you need 70 sheets. On 24 inch trusses that's 70 x 4 = 280 H-clips if you use them. At 33 nails a sheet (two end rows of 9, three middle rows of 5), you need 2,310 nails, or about 25 pounds. Synthetic underlayment is 2,213.64 / 1,000, rounded up to 3 rolls.

For the ice barrier, a 12 inch overhang plus a 6 inch wall plus the 24 inches the code requires is 42 inches measured level. Times 1.118, that's about 47 inches up the slope. One 36 inch course doesn't reach, so you need two, with the second lapping the first by about 3 inches. The house has 120 ft of eave, so two courses is 240 ft, plus 10 percent is 264 ft, and 264 / 66.7 = 3.96, or four rolls. Drip edge is 120 + 67 = 187 ft, plus 10 percent, which is 21 sticks. Materials come to about $2,068 to $3,552 with OSB.

What thickness of roof sheathing do you need?

A 7/16 inch panel with a 24/16 span rating is the everyday answer. Every rated panel is stamped with a span rating, and the first number is the farthest apart the roof framing can be. The APA roof sheathing tables (and the matching IRC span table) put a 7/16 inch 24/16 panel at 24 inches, with or without edge support. A 3/8 inch 24/0 panel only gets 20 inches without it.

On 24 inch trusses most crews still clip the long edges, because a deck that meets code can still feel bouncy and show ridges at the seams. In snow country with real loads, or under tile and slate, go to 1/2 inch (32/16) or 5/8. Per APA, the 24/16 panel is rated for only about 40 pounds per sq ft of live load at 24 inches.

H-clips are recommended, not required

APA rates a 24/16 panel for 24 inch spans with or without edge support, so clips are optional. Plenty of crews use them anyway, one midway between each pair of supports. On 24 inch framing an 8 ft sheet crosses four bays, so it takes four clips. Clips also hold the 1/8 inch gap between panels that gives them room to swell. At 16 inches you can usually skip them.

Nailing pattern for roof sheathing

Use 8d nails at 6 inches along supported panel ends and 12 inches at the middle supports. Hand-driven 8d commons run about 95 to a pound. Gun nails are thinner, so a pound goes further, and they come in coils anyway. Windy areas often tighten the spacing, so ask your inspector. Our roof sheathing guide covers OSB versus plywood if you haven't decided between them.

How far up the roof does ice and water shield go?

At least 24 inches inside the exterior wall line, wherever ice dams have a history. IRC R905.1.2 requires an ice barrier from the eave to that point, and the 24 inches is measured level, not along the roof. That is the part people miss. Fine Homebuilding works through it: a 6/12 roof with a 12 inch overhang and a 2x6 wall needs about 47 inches of membrane up the slope. One 3 ft roll won't get there, and steeper roofs need even more.

Valleys usually get a full 36 inch strip centered in them even where eave protection isn't required, which is why the tool still counts valley footage when you pick "No." Rolls are about 36 inches by 66.7 feet (200 sq ft), like GAF WeatherWatch. More on this in ice dam prevention.

Underlayment roll coverage

Synthetic underlayment usually covers 10 squares a roll. Felt is sold by its gross size: a #15 roll is 432 sq ft (sold as 4 squares once you take out the laps) and #30 is 216 sq ft (2 squares). The tool counts net coverage, the way your supplier will.

It also runs underlayment across the whole roof without subtracting the ice barrier strip, so the extra covers laps and mistakes. Our underlayment guide has details on the three types.

How much does roof sheathing cost?

At the lumberyard, 7/16 OSB costs about $15 to $25 a sheet and plywood about $25 to $45. Synthetic underlayment runs about $15 to $25 a square, #15 felt $5 to $10, #30 $10 to $18, and ice and water shield $50 to $100. Drip edge is $8 to $12 a 10 ft stick. Lumber prices swing (OSB rose to several times its usual price in 2021), so call around.

Paying a roofer to replace rotten sheets during a reroof is a different number, about $80 to $150 a sheet with labor and haul-off. The roof cost calculator prices the whole job from your measurements, shingles and tear-off included.