Roof Pitch Calculator

From the attic: hold a level horizontally against the underside of a rafter, mark 12 inches along the level from where it touches, and measure straight down from that mark to the rafter — that vertical distance is your rise per 12. No attic access? A speed square and level on a gable-end rake board works from a ladder, and several phone level apps read roof angle from a photo well enough for estimates. Measuring the shingle surface directly adds the shingle thickness to your error, so the rafter method wins.

It's the hypotenuse per foot of horizontal run — √(12² + rise²) ÷ 12 — and it does two jobs. Rafter length: a 14-ft run at 8/12 pitch needs 14 × 1.2019 = 16.83 ft of rafter, before adding overhang and subtracting the ridge board thickness. Roof area: multiply the building's footprint (plus eave overhangs) by the same factor to get true surface area for shingles — skipping this is the classic way DIY estimates come up two squares short.

Asphalt shingles are rated for 4/12 and steeper as standard; from 2/12 to 4/12 they're allowed only with doubled underlayment or ice-and-water membrane per manufacturer specs; below 2/12 shingles are out entirely — water moves too slowly and wicks under the courses — and you're in membrane territory (EPDM, TPO) or standing-seam metal, which can go as low as ¼/12. Climate pushes the numbers: heavy snow country favors 6/12+ for shedding, and hurricane zones have their own fastening schedules at every pitch.

Trade-offs, honestly presented: steeper roofs shed water and snow faster, hide less debris, ventilate attics better, and tend to outlast shallow roofs because water spends less time on them — but they cost more (more material per footprint via the multiplier, plus steep-slope labor rates kick in around 8/12 where staging and harnesses become mandatory) and DIY work above 6/12 stops being reasonable. The sweet spot for most climates and budgets sits at 4/12–8/12, which is exactly where most houses live.