A roof’s pitch determines almost everything else about it — the rafter length, the snow and wind loads, the choice of roofing material, the cost of timber, and the height of the building. Get it right at the framing stage and the eaves line up, the gable ends close cleanly, and the tiles or sheets sit at their designed angle. Get it wrong and you spend three days re-cutting birdsmouths. The Chippy Tools roof pitch calculator does the math for you in metric or imperial — punch in the rise and run, and the app returns the pitch ratio, the pitch angle in degrees, and the rafter length for any straight roof slope.
What the pitch calculator does
The Chippy Tools pitch calculator takes two inputs — the vertical rise and the horizontal run — and returns three outputs: the pitch ratio (e.g. 6/12), the pitch angle in degrees (e.g. 26.6°), and the rafter length (the diagonal distance from the wall plate to the ridge). The same calculator works for full roof rafters, verandah pitches, dormer rafters, lean-to roofs, and any other rise-over-run problem on a build — including stair stringers and ramp slopes.
The app runs locally on iOS and Android with no internet required, so the calculation works whether you’re on a windy ridge with no signal or in a basement with concrete walls. Switch between metric and imperial without restarting and save common pitches (4/12, 6/12, 12/12) as presets if you build the same style of roof regularly.
Roof pitch and rafter anatomy
A common rafter forms a right triangle. The horizontal edge is the run (half the building width, measured at the top of the wall plate). The vertical edge is the rise (from the wall plate up to the underside of the ridge). The hypotenuse is the rafter — the timber that carries the roofing from ridge to wall plate.
In the diagram: (1) rise, (2) run, (3) rafter, (4) pitch angle, (5) birdsmouth.
The pitch ratio is rise/run expressed against a standardised run (12 inches or 300mm). A roof that rises 6 inches over 12 inches of run is a “6/12” or “six in twelve” pitch. The pitch angle is the same slope expressed in degrees: arctan(rise / run). A 6/12 pitch is about 26.6°.
The birdsmouth is the notch cut into the rafter where it meets the wall plate — it has a horizontal seat that bears on the plate and a vertical heel that sits flush against the outer face. The eave overhang is the section of rafter that projects past the birdsmouth — typically 300–450mm for a domestic roof. Ridge thickness halves the effective run because the rafter actually stops at the centerline of the ridge board, not the outer face.
How to calculate roof pitch (rise over run)
The standard rise-over-run workflow is straightforward:
- Measure the rise — the vertical distance from the top of the wall plate to the underside of the ridge board.
- Measure the run — the horizontal distance from the centerline of the wall plate to the centerline of the ridge.
- Express as rise/run against a 12-inch or 300mm run — multiply or divide both numbers as needed.
- Confirm with Chippy Tools — enter the raw rise and run, and the app returns the standardised pitch and the angle in degrees.
For a worked imperial example: a roof rises 4 feet (48 inches) over a 12-foot (144-inch) run. Standardise to a 12-inch run: 48 ÷ 12 = 4 inches per 12. That’s a 4/12 pitch, equivalent to arctan(4/12) ≈ 18.4°. Metric example: a roof rises 1.5m over a 3m run. Standardise to a 300mm run: 1.5m × (300/3000) = 150mm rise per 300mm run. That’s a 1/2 pitch (or “150 in 300”), equivalent to arctan(150/300) = 26.6°.
How to calculate rafter length
Rafter length is the hypotenuse of the rise/run right triangle, calculated using Pythagoras:
rafter length = √(rise² + run²)
For a 6/12 pitch over a 4.0m run: rise = 4.0 × (6/12) = 2.0m, so rafter = √(4.0² + 2.0²) = √(16 + 4) = √20 ≈ 4.47m. Add the eave overhang to that figure when ordering — a 450mm overhang takes the order length to ~4.92m, round up to the next stocked length (5.4m or 6.0m).
For an imperial example with a 9/12 pitch over a 16-foot run: rise = 16 × (9/12) = 12 feet. Rafter = √(16² + 12²) = √(256 + 144) = √400 = exactly 20 feet. Round up to a stocked 22-foot length to allow for end cuts and the eave overhang.
The Chippy Tools pitch calculator returns the rafter length directly so you can quote material before you cut.
Calculate Pitch and Rafter Length On-Site
Download Chippy Tools and use the pitch calculator on iOS or Android. Offline, fast, and accurate — no signal required to size a roof.
Common roof pitches and what they’re used for
| Pitch | Angle | Common use |
|---|---|---|
| 1/12 (and lower) | 4.8° | Industrial flat-pan metal, built-up commercial roofing, modern minimalist homes |
| 2/12 | 9.5° | Modern flat-look architectural homes; metal-only roofing |
| 3/12 | 14.0° | Low-slope verandahs, awnings; standing-seam metal |
| 4/12 | 18.4° | Common minimum for asphalt shingles; entry-level Australian project homes |
| 6/12 | 26.6° | The most common North American residential pitch; works with shingles, metal, tile |
| 8/12 | 33.7° | Steep-look traditional homes; better snow shedding |
| 9/12 | 36.9° | Heritage and Victorian-era homes |
| 10/12 | 39.8° | Steep architectural homes; some Australian period styles |
| 12/12 | 45° | “Full-pitch” classic — slate, very steep architectural detail |
The 4/12 minimum for asphalt shingles is a common code reference but specific minimums vary by underlayment system — always check the manufacturer spec. Tile and slate typically need 5/12 minimum to keep rain from being driven up under the laps in a strong wind.
Birdsmouth, ridge thickness, and overhang
When you cut a rafter, three subtractions and one addition modify the raw Pythagorean rafter length:
- Subtract half the ridge-board thickness from the run before calculating the rafter — the rafter actually stops at the ridge centerline, not the inner face. For a 38mm ridge, subtract 19mm from the run.
- Subtract the vertical depth of the birdsmouth from the rise effective at the wall plate. The seat cut sits at the top of the plate; the heel of the rafter rests against the plate face. The bearing height is the rafter depth minus the seat depth.
- Add the eave overhang to the rafter length when ordering. A 450mm overhang adds 450mm of timber that projects past the birdsmouth. Don’t add it before calculating Pythagoras — add it to the cut length once you have the in-triangle rafter length.
- Round up to the next stocked length when ordering.
The Chippy Tools pitch calculator returns the in-triangle rafter length. Apply the ridge/birdsmouth/overhang adjustments manually based on the rafter timber and ridge dimensions you’re using.
Hip roof framing
Hip roofs slope on all four sides — the gable ends are replaced with a triangular hip plane. Hip framing uses three rafter types:
- Common rafters run perpendicular from the wall plate up to the ridge, just like a gable roof. Calculate them with the standard rise/run/rafter formula in Chippy Tools.
- Hip rafters run diagonally from the building corner up to the king post (where the ridge ends). They have a longer hypotenuse: hip rafter length = √(common rafter² + half-building-width²). The hip rafter sits at a different pitch than the common rafters because it crosses the corner diagonally.
- Jack rafters run from the wall plate to the hip rafter, getting progressively shorter as they approach the corner. Jack-rafter length is the common rafter length × (distance from corner / total hip run).
A dedicated hip roof calculator is on the Chippy Tools roadmap. For now, use the pitch calculator for the common rafter and back-calculate the hip and jack rafter lengths from the geometry of your specific roof.
Roof pitch and roofing materials
The pitch determines what materials you can install, what underlayment you need, and how the laps behave under driven rain.
Low pitch (1/12 to 3/12) — built-up bituminous, EPDM membrane, standing-seam metal with a sealed seam, screw-down corrugated metal with extra lap. Asphalt shingles work down to 2/12 only with a special double underlayment. Tile is generally not viable.
Mid pitch (4/12 to 7/12) — asphalt shingles in any region, all metal profiles, concrete and clay tile from 5/12, slate from 6/12. The most flexible range for material choice and the cheapest framing.
Steep pitch (8/12 and above) — anything goes. Snow regions favour 8/12+ for shedding; architectural styles drive 10/12 and 12/12 choices. Tile and slate look their best at this slope.
Roof Maths That Just Work
Chippy Tools handles roof pitch, rafter length, deck stairs, balustrade spacing, and decking layout in one app. Download once, calculate anywhere — no internet required.
Building code and minimum pitch requirements
Pitch minimums vary by region and material:
- Australia (NCC and NASH) — metal roofing typically requires a minimum 5° (≈ 1/12); tile typically 15° (≈ 3/12). Cyclonic regions tighten these.
- United Kingdom (Approved Document C, BS 5534) — minimum 17.5° for clay/concrete tile underlayed; 12.5° for some interlocking concrete tiles.
- United States (IRC and IBC) — asphalt shingle minimum 2/12 (with double underlayment) or 4/12 (single); tile minimum 4/12 with engineered underlayment.
- New Zealand (NZBC E2) — metal roof minimum 3°; tile minimum 15°. High-wind zones add stricter limits.
Always confirm with your local council, building authority, and the roofing manufacturer’s installation manual before fixing on a pitch. Codes change and some councils impose stricter local minimums than the national standard.
Imperial and metric units
Chippy Tools accepts millimetres, centimetres, metres, feet, inches, or feet and inches in any combination. Enter rise in inches and run in metres in the same calculation — the app converts internally. The pitch ratio itself is unitless, so 6/12 inches and 150/300mm produce the same pitch and the same angle.
For Australian and metric-country builders working with US-published roofing data, the easiest workflow is to enter measurements in mm, read off the pitch in degrees (which is universal), and pick a pitch ratio that matches the rounded number you’d see on a US drawing.
Why use a roof pitch calculator on your phone
The Chippy Tools app is built for tradespeople who need calculations on-site without internet. The pitch calculator is paired in the same app with the Stair Calculator, Decking Calculator, and Triangle Calculator — calculate the roof, the verandah deck, the deck stairs, and the trim miters in one workflow without re-entering measurements.
Web pitch calculators break in remote new-build sites where 4G is patchy. The Chippy Tools calculator runs locally — the answer is on screen in under a second. Switch between metric and imperial without restarting, save common 4/12 / 6/12 / 8/12 presets, and pull up the calculator straight from your home screen widget when a customer asks for a quick price during the quote.
Related calculators
- Stair Calculator — same rise-over-run math applied to stairs and stringers
- Triangle Calculator — for hip-rafter geometry, miter angles, and any general right-triangle problem
- Raked Wall Calculator — for cutting wall studs to follow a raked roof line under a gable
- Decking Calculator — for verandah and deck sizing where the roof pitch ties into a deck