Roof Pitch Calculator
Rise and run in, standard x/12 pitch, angle and rafter multiplier out.
Reading roof pitch
US roof pitch is inches of rise per 12 inches of run: a 6/12 roof climbs 6 inches for every foot of horizontal distance, which is 26.6 degrees or a 50% slope. Anything below 2/12 counts as flat (and needs membrane roofing, not shingles); 4/12 to 9/12 is the normal walkable-to-steep range; above 9/12 needs staging to work on.
The rafter multiplier
Multiply the horizontal run of your roof by the multiplier to get the sloped rafter length (before adding overhang). A 6/12 pitch has a multiplier of 1.1180, so a 14-foot run needs a 15 ft 8 in rafter. It's just the hypotenuse: √(1 + (rise/run)²).