Convert a pitch to degrees and slope, get the multiplier that turns footprint into actual roof area, and pull a rafter length from the building span — the three numbers every roof job starts with.
Roof numbers
Pitch—
Slope—
Pitch multiplier—
Rafter length (incl. overhang)—
The multiplier is the takeoff cheat code: footprint area × multiplier = true roof surface. A 2,000 sq ft footprint under a 6/12 roof is really 2,236 sq ft of shingles.
Pitch, angle, slope — same roof, three languages
Carpenters say 6/12, engineers say 26.6°, and site plans say 50% slope. They're all the same triangle: rise over a 12-inch run. The fourth number, the pitch multiplier, is the one that makes you money on estimates — it converts flat footprint into true sloped surface.
angle = atan(rise ÷ 12) · multiplier = √(12² + rise²) ÷ 12 · rafter = (span ÷ 2) × multiplier
Worked example
A 24-ft wide garage with a 6/12 roof and 12-in overhangs: multiplier = 1.118, so each rafter runs (12 × 1.118) + (1 × 1.118) = 14.53 ft — buy 16-footers. And the shingle order for a 24 × 30 footprint isn't 720 sq ft, it's 805 (× 1.118), or about 8.1 squares before waste.
What different pitches mean on the ground
4/12 and under is walkable and fine for most shingles (3/12 is the usual asphalt minimum, 2/12 with special underlayment); 6/12–9/12 is classic residential and needs footwear respect; 10/12 and up is staging-and-harness territory where labor bids climb with the angle.
Frequently asked questions
What does a 6/12 roof pitch mean?
The roof rises 6 inches vertically for every 12 inches it runs horizontally — about 26.6 degrees. It’s one of the most common residential pitches: steep enough to shed water well, shallow enough to walk carefully.
How do I calculate shingles from my house footprint?
Multiply the footprint area by the pitch multiplier from this calculator, then add roughly 10% for waste, ridges and starters. Divide by 100 to get "squares," which is how roofing is sold.
What is the minimum pitch for asphalt shingles?
Most manufacturers allow standard installation down to 3/12 (about 14°) — under that, down to 2/12, you need special double-coverage underlayment; below 2/12 shingles are out and you’re into membrane roofing.