How to Calculate Roof Sheeting

Roofing sheeting, or sheathing, covers a house's roof rafters and supports the shingles. It is usually either plywood or a manufactured wood product known as oriented strand board. Plywood is considerably more expensive than OSB and is typically only used on high-end houses. Calculating the amount of sheeting needed to cover a roof is an important step in building a house. Ordering too much or too little is a waste of time and money.

Stepladder
Framing square
Carpenter's pencil
Calculator

Instructions

- 1
  Measure the length and width of the building, and multiply the two numbers to find its area of the roof.
- 2
  Place the ladder beneath the rafter at the end of the house. Mark a spot on the rafter. Measure along the soffit with the long end of your framing square and mark a second point 12 inches from the first.
- 3
  Measure the distance from the 12-inch mark up to the rafter. This distance over 12 is your roof pitch and is used to determine the roofing multiplier: Pitches below 4/12 use a multiplier between 1.15 and 1.25; pitches from 5/12 to 9/12 use a multiplier between 1.25 and 1.4; pitches above 9/12 use a multiplier between 1.4 and 1.7.
- 4
  Multiply the area of the roof by the appropriate multiplier to determine the total area to be covered.
- 5
  Divide the total area by the area of a piece of sheeting, typically either 32 or 40 square feet, to determine the number of sheets needed to cover the roof.

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