How this roof pitch calculator works
Roof pitch is rise over run — in the US, inches of rise per 12 inches of run, so a 6:12 roof rises 6 inches per foot. The calculator converts that to an angle in degrees and a slope percentage, and computes the pitch multiplier: how much longer the sloped surface is than its horizontal footprint. Rafter length is half the building span times that multiplier — the structural length, before adding overhang.
The multiplier is the useful number
Multiply any horizontal (plan) area by the pitch multiplier to get true roof surface — essential for ordering shingles, underlayment or metal panels. A 6:12 roof has a multiplier of 1.118, so a 1,000 sq ft footprint carries about 1,118 sq ft of roofing. Steeper roofs grow fast: at 12:12 the multiplier is 1.414.
Pitch and material choices
Standard asphalt shingles are rated for 4:12 and steeper; between 2:12 and 4:12 they need special underlayment, and below 2:12 you are in membrane or standing-seam territory. Steeper pitches shed water and snow better and last longer, but cost more in material, staging and labor — walking a 10:12 roof is a job for harnesses.