Calculate roofing squares, bundles of shingles, and materials needed. Accounts for roof pitch and waste.
Step 1: Calculate the building footprint area. Footprint = Length x Width
Step 2: Apply the pitch multiplier. The pitch factor accounts for the slope of the roof, which increases the actual surface area. Roof area = Footprint x Pitch factor
Step 3: Convert to roofing squares. One roofing square = 100 sq ft. Squares = Roof area / 100
Step 4: Calculate shingle bundles. Most shingles come 3 bundles per square. Bundles = Squares x 3
Step 5: Add waste factor. 10% for simple gable roofs, 15% for hip roofs, 20% for complex roofs with valleys and dormers.
The pitch multiplier converts flat footprint area to actual roof surface area:
4/12: 1.054 (most common residential). 5/12: 1.083. 6/12: 1.118. 7/12: 1.158. 8/12: 1.202.
9/12: 1.250. 10/12: 1.302. 11/12: 1.357. 12/12: 1.414 (45-degree pitch).
To find pitch: measure 12 inches horizontal from the edge of the roof, then measure vertical rise. That rise value is the pitch (e.g., 6 inch rise = 6/12 pitch).