Roof Sheathing Calculator

Roof Sheathing Calculator. Roof sheathing (also called roof decking) is the layer of panels — usually plywood or OSB — nailed across the rafters before shingles go on, and buying the right number of sheets upfront saves trips to the lumber yard. Enter your roof length, roof width, roof pitch, eaves overhang, and waste factor into the Roof Sheathing Calculator to get the sheets required for your project. Secondary outputs include total roof area, area with waste, and the exact (unrounded) sheet count. Also try the Snow Load Calculator.

Roof Length *

Roof Width *

Length Unit *

Width Unit *

Roof Pitch *

Eaves Overhang *

Roof extension beyond the walls

Waste Factor *

Extra material for cuts and waste

Results

Sheets Required

Total Roof Area

Area with Waste

Exact Sheets

How do I calculate roof sheathing requirements?

Calculate the roof area by multiplying length × width, then adjust for roof pitch using the pitch multiplier. Add overhang and waste factor, then divide by 32 sq ft (one 4×8 sheet) to get the number of sheets needed. See also our calculate Lower Rafter Length, Upper Rafter Length & Total Roof Height — Gambrel Roof.

What size plywood sheets are used for roof sheathing?

Standard roof sheathing uses 4×8 foot plywood or OSB sheets, which cover 32 square feet each. Common thicknesses are 1/2 inch, 5/8 inch, and 3/4 inch depending on rafter spacing and local codes.

How does roof pitch affect sheathing calculations?

Steeper roof pitches require more material because the actual surface area is larger than the horizontal footprint. The pitch multiplier increases the base area calculation to account for the slope.

What waste factor should I use for roof sheathing?

A 10% waste factor is typical for roof sheathing to account for cuts, damaged pieces, and odd angles. For complex roofs with many hips, valleys, or dormers, consider 15-20% waste factor.

Should I include roof overhang in my calculations?

Yes, always include eaves overhang in your sheathing calculations. Overhangs typically extend 12-24 inches beyond the wall and require additional sheathing material to properly cover.

What's the difference between plywood and OSB for roof sheathing?

Both plywood and OSB work for roof sheathing. Plywood is more moisture-resistant and stronger, while OSB is typically less expensive. Both come in 4×8 sheets and provide adequate structural support when properly installed.

How do I measure my roof dimensions for sheathing?

Measure the length and width of each roof section at the building footprint level. Don't try to measure the actual sloped surface - the calculator will adjust for pitch automatically. Include any dormers or additions as separate calculations. You might also find our Roof Shingle Calculator useful.