Roof Pitch Calculator
Convert roof pitch between rise-per-12, degrees, and percentage grade, and get the rafter length, slope factor, and true roof area.
Cómo usar esta calculadora
- 1Choose what you already know: the pitch, the angle, or the rise and span.
- 2Enter the building width and length to get the roof area.
- 3Add the eave overhang so the rafter length and area include it.
- 4Use the slope factor to convert any other plan dimension to its true sloped length.
Cómo funciona
Roof pitch relationships
angle = arctan(rise ÷ 12) grade % = (rise ÷ 12) × 100 slope factor = √(rise² + 12²) ÷ 12 rafter = run × slope factor roof area = footprint area × slope factor ridge height = (span ÷ 2) × (rise ÷ 12)
Roofers express pitch as rise over a fixed 12-inch run, which makes framing arithmetic easy but is not an angle. Converting needs arctangent. The slope factor is the key working number: it is the hypotenuse of a rise-12 triangle divided by 12, so it tells you how much longer a sloped surface is than its shadow on the ground — multiply any plan measurement by it to get the real one.
Ejemplo resuelto
A 6:12 pitch is arctan(6/12) = 26.57°, a 50% grade, with a slope factor of √(36+144)/12 = 1.1180. A 24 × 40 ft building with 12-inch eaves has a 1,092 sq ft footprint, so the roof surface is 1,221 sq ft — just over 12 squares.
Roof Pitch Calculator: la guía completa
Three notations for the same thing
Roofers say 6:12. Architects and engineers say 26.57 degrees. Highway and drainage work says 50% grade. All three describe the identical slope, and confusing them is a real source of ordering errors.
The rise-per-12 convention exists because it makes framing practical: set a framing square to 6 and 12, and you can mark every rafter cut without touching a protractor. It is not an angle and does not behave like one — a 12:12 pitch is 45 degrees, but a 24:12 pitch is 63 degrees, not 90.
Pitch is also sometimes written as a fraction of the span rather than the run, so a 6:12 roof is called a "quarter pitch". That older usage still appears in some regions and is a frequent source of miscommunication.
The slope factor is the number to remember
Every roofing quantity that is measured on the ground has to be scaled up to the slope. The slope factor does exactly that: a 6:12 roof has a factor of 1.118, so a 1,000 sq ft footprint carries 1,118 sq ft of roofing.
It applies to more than area. Rafter lengths, underlayment, ice-and-water shield, and drip edge all scale by it. Ridge and hip lengths do not — those are measured directly.
Roofing is sold by the square, meaning 100 sq ft. Divide the sloped area by 100 and round up, then add waste: 10% for a simple gable, 15% or more for a roof with valleys, dormers, and hips, since every cut wastes shingle.
What pitch to build
Below 2:12 is a low-slope roof and asphalt shingles are not permitted — water moves too slowly and wind-driven rain gets under the courses. Those roofs need a membrane such as TPO, EPDM, or modified bitumen.
From 2:12 to 4:12, shingles are allowed but require a double layer of underlayment. From 4:12 to 9:12 is the standard residential range: it sheds water and snow well, is workable without staging, and is what most manufacturers assume.
Above 9:12 the roof becomes steep-slope work, which increases labour cost substantially and requires roof jacks and fall protection. Snowy regions favour steeper pitches so snow sheds rather than accumulating, which is why alpine architecture looks the way it does. Windy coastal areas favour shallower pitches to reduce uplift.
Measuring an existing roof safely
The safest method needs no ladder at all: from inside the attic, hold a level horizontally against a rafter, mark 12 inches along it, and measure straight up to the rafter's underside. That vertical distance is the rise per 12.
From outside, a smartphone level app held against a rake board gives the angle in degrees directly — enter it in the degrees mode above and the calculator converts it back to pitch.
If you only have the ridge height and the span, use the third mode. A gable roof rises over half the span, so a 24-foot-wide building with a ridge 6 feet above the plate is a 6:12 pitch — the calculator handles the halving for you.
Preguntas frecuentes
What does a 6:12 roof pitch mean?
The roof rises 6 inches vertically for every 12 inches of horizontal run. That works out to 26.57 degrees, a 50% grade, and a slope factor of 1.118 — so the roof surface is about 11.8% larger than the footprint it covers.
How do I convert roof pitch to degrees?
Take the arctangent of the rise divided by 12. For a 6:12 pitch, arctan(0.5) = 26.57°. Common conversions: 4:12 is 18.43°, 8:12 is 33.69°, and 12:12 is exactly 45°.
How much roofing material do I need?
Multiply the footprint area by the slope factor to get the true roof area, divide by 100 for squares, then add 10% waste for a simple roof or 15% for one with valleys and dormers. The calculator gives you the first two steps.
What is the minimum pitch for asphalt shingles?
2:12, and between 2:12 and 4:12 manufacturers require a double underlayment. Below 2:12 shingles will leak regardless of installation quality and most warranties are void — those roofs need a membrane system instead.