Roof Pitch Calculator

How to Use the Roof Pitch Calculator

This roof pitch calculator works three ways. Enter the rise and run in millimetres to get the pitch in degrees; enter an angle and the run to work out the rise and rafter length; or use fall mode for flat and skillion roofs, where you know how many millimetres the roof drops over a length in metres. Open the optional section to add an eave overhang, the plan area of the house for a roof area estimate, and a gable span for the ridge height.

Results include the pitch angle, 1-in-X ratio, percentage grade, X/12 pitch, rafter (sloping) length, pitch factor and roof surface area, a to-scale diagram, and a minimum pitch guide showing which common roofing materials typically suit your slope — everything you'd want from a roof slope calculator or roof pitch estimator in one place.

Roof Pitch Formulas

Worked Example

A gable-roofed house has a span of 8,000 mm between the walls, so the run to the ridge is 4,000 mm. The ridge sits 1,650 mm above the wall plate. That gives a 22.42° roof pitch — 1 in 2.42, a 41.3% grade, or about 4.9/12.

Each rafter from wall to ridge is 4,327 mm. Add a 450 mm eave overhang (measured horizontally), which is 487 mm along the slope, and the full rafter length is 4,814 mm. The pitch factor is 1.0817, so a plan area of 180 m² (including eaves) becomes about 194.7 m² of actual roof surface — the figure to start from when ordering sheeting or tiles, before allowing for laps and waste. At 22.42°, the pitch meets every typical minimum in the guide, including concrete and terracotta tiles.

How to Measure Rise and Run

The easiest on-site method uses a 1000 mm spirit level and a tape measure. Hold the level horizontally with one end touching a rafter, the underside of the sheeting or a barge board, and measure vertically from the other end of the level to the roof line. Because the run is exactly 1000 mm, that measurement is the rise per metre: 87 mm is roughly 5°, 268 mm about 15°, 364 mm 20.00° and 577 mm about 30°. For the whole roof, measure the span between the outside of the wall frames and the height from the top plate to the ridge — or check the truss or framing drawings, which usually state the pitch.

Degrees, Ratios, Percent and X/12 — Why Australia Uses Degrees

Australian roofing products, truss designs and installation guides almost always specify pitch in degrees, which is why this pitch calculator leads with the angle. Flat roofs, gutters, box gutters and drainage are more often described as a fall ratio such as 1:100, or as millimetres per metre (1:100 is 10 mm per metre). Percentage grade is common for driveways and ramps. The X/12 format comes from the United States, where pitch is quoted in inches of rise per foot of run — useful if you're reading an American plan or video, but rarely used on Australian drawings.

Minimum Roof Pitch by Material

Each roofing material needs enough slope to shed water past its laps and joints. As a typical guide: some trapezoidal or ribbed metal decking profiles go as low as about 1°–2°, corrugated metal is typically around 5°, concrete tiles typically need about 15°, terracotta tiles about 15°–20°, and slate around 20° or more. Low-pitch tile systems, special underlays and long sheet runs can all shift these figures, so treat them as a starting point only and always check the manufacturer's installation guide for the exact product. Wind region, bushfire attack level and council requirements may also limit which roofing you can use.

Flat Roof Fall

So-called flat roofs aren't truly flat — they need a fall so water drains to the gutter or outlet. Use the roof fall calculator mode to check: 60 mm of fall over 6 m is 1 in 100 (0.57°), and 30 mm over the same length is only 1 in 200 (0.29°). A fall of around 1:100 is often cited as a typical minimum for some waterproof membrane systems, but requirements vary by product and by the relevant standard, so confirm the fall needed for your membrane or sheeting before building. Allow extra for sag and construction tolerances.

Frequently Asked Questions

How does a roof pitch calculator work in Australia?

A roof pitch calculator turns the rise (vertical height) and run (horizontal distance) of a roof into an angle using trigonometry: pitch = arctan(rise ÷ run). Australian builders, roofers and manufacturers usually quote roof pitch in degrees, so that's the headline figure here, with the 1-in-X ratio, percentage grade and US-style X/12 shown alongside.

How do I figure out my roof pitch on site?

Hold a 1000 mm spirit level horizontally against a rafter or the underside of the roof sheeting, then measure straight down (or up) from the end of the level to the roof surface. That vertical measurement is the rise per 1000 mm of run — for example, 364 mm gives a 20.00° pitch. Measure from inside the roof space or from a ladder at the gable where possible rather than walking on the roof.

Is there a roof pitch calculator for Sydney, or is it different in each state?

The pitch maths is the same everywhere — 26.57° is 26.57° in Sydney, Perth or Hobart. What can differ by location is which roofing products and fixings are suitable: wind classification, bushfire attack level (BAL) and local council or NCC requirements can all affect material choice and installation. Your builder, roofer or certifier will confirm what applies to your site.

What's the difference between degrees, a 1 in X ratio, percent grade and 6/12 pitch?

They're all the same slope written differently. Degrees is the angle (the Australian convention), 1 in X means the roof rises 1 unit for every X units of run, percent grade is rise ÷ run × 100, and X/12 is the US convention — inches of rise per 12 inches of run. For example, a 26.57° roof is 1 in 2, a 50% grade and 6/12.

What is the minimum roof pitch for metal roofing and tiles?

As a typical guide only: some trapezoidal or ribbed metal decking profiles can go as low as about 1°–2°, corrugated sheeting is typically around 5°, concrete tiles are typically about 15° minimum and terracotta tiles about 15°–20°, depending on the profile and underlay. Always check the manufacturer's installation guide for the exact product, because sheet length, laps and site exposure all change the minimum.

How do I use a flat roof slope or roof fall calculator?

Switch to Fall mode and enter how many millimetres the roof drops over its length in metres. A fall of 60 mm over 6 m is 1 in 100, or 0.57° — a figure often quoted as a typical minimum for some flat-roof membranes, though the actual requirement depends on the product and the applicable standard. A "flat" roof still needs some fall so water can drain.

How do I calculate rafter length and roof area for ordering materials?

Rafter length is √(rise² + run²), plus any eave overhang measured along the slope (horizontal overhang ÷ cos θ). For roof area, multiply the plan area — the footprint seen from above, including eaves — by the pitch factor (1 ÷ cos θ). Suppliers usually add an allowance for laps, cutting and waste on top of the calculated area.

What is a roof pitch factor?

The pitch factor is 1 ÷ cos(pitch angle), and it converts flat plan measurements into the true sloping length or area. At 30° it's 1.1547, so 100 m² of plan area becomes about 115.5 m² of roof surface. It applies to a roof whose planes all share the same pitch.

Is this a roof pitch estimator or an exact measurement?

The maths is exact for the numbers you enter, but the result is only as accurate as your measurements, so treat it as an estimate. Real roofs aren't always perfectly straight or true, so confirm critical dimensions on site and let your builder or roofer check before ordering trusses, sheeting or tiles.