Calculate roof pitch (X/12), slope angle in degrees, pitch multiplier, rafter length, and total roofing squares with shingle bundle estimates.
6 inches of vertical rise per 12 inches of horizontal run
A 6/12 roof pitch means the roof rises 6 inches vertically for every 12 inches of horizontal run (an angle of 26.6°). An 8/12 pitch rises 8 inches vertically per 12 inches of run (an angle of 33.7°).
The pitch multiplier is the ratio used to calculate true rafter length and roof surface area from flat horizontal dimensions. For example, a 6/12 roof has a multiplier of 1.118, meaning a 1,000 sq ft flat footprint requires 1,118 sq ft of roofing.
The minimum pitch for standard asphalt shingles is 2/12 (with special double underlayment). For conventional single underlayment installation, 4/12 or higher is recommended by most manufacturers.
There are typically 3 bundles of standard 3-tab or architectural asphalt shingles in 1 roofing square (100 square feet). Heavyweight designer shingles may require 4 or 5 bundles per square.
