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Roof Sheathing Calculator

Calculate required plywood or OSB sheets for roof decking based on pitch, area, and waste factors. Optimize material orders for construction.

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Category Roofing
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About

Estimating roof sheathing materials requires accounting for both the surface area and the slope (pitch) of the roof. As the steepness increases, the actual surface area exceeds the flat footprint area significantly. This tool calculates the total count of 4x8 sheets (Plywood/OSB) needed to cover a deck. It incorporates a pitch multiplier and a customizable waste factor to ensure builders order sufficient material to cover cuts, mistakes, and overlap seams.

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Formulas

The calculator first determines the Pitch Multiplier M based on the Rise R:

M = √1 + R122

Total Sheathing Area (A_total) is the flat footprint (L × W) times the multiplier:

A_total = L × W × M

Sheet count is determined by dividing by 32 (sq ft per 4x8 sheet) and adding Waste w (%):

Sheets = A_total32 × 1 + w

Reference Data

Roof Pitch (Rise/12)Angle (°)Multiplier Factor
0/12 (Flat)0.0°1.000
4/12 (Low)18.4°1.054
6/12 (Common)26.6°1.118
9/12 (Steep)36.9°1.250
12/12 (45 Deg)45.0°1.414

Frequently Asked Questions

Common code requirements (like IRC) specify 7/16-inch OSB or 1/2-inch CDX plywood for rafters spaced 24 inches on center. Always check local building codes for snow load requirements.
For simple gable roofs, 10% is usually sufficient. For complex hip roofs with valleys and dormers, calculate 15-20% waste due to the high number of diagonal cuts.
The inputs should represent the total deck size. If you have a 2-foot overhang (soffit/eaves), ensure you add that to your Length and Width inputs before calculating.