What can a roofers calculator work out, and what can’t it?
Published · By Sai Nath
A roofers calculator handles the geometry: pitch in degrees, X in 12 and per cent, the rafter line with its overhang, the ridge height and gutter fall. At 20°, a 7200 mm span rises 1310.3 mm to the ridge line; an Australian eaves gutter falls at least 1:500, 2 mm per metre. Cuts, hips and sheet counts need other tools.
Which roof sums can a calculator do?
The geometry of one roof plane: its slope, the lengths along it, the heights and the falls. Every one is a right triangle, so two numbers give the rest.
| Job | The sum | In MeisterCalc |
|---|---|---|
| Pitch from a rise and a run | slope = rise ÷ run; degrees = arctan(slope); X in 12 = slope × 12; per cent = slope × 100 | Pitch |
| Rafter line, wall line to ridge line | run ÷ cos(angle) | Pitch (Hypotenuse) |
| Rafter line with the eaves | (run + level overhang) ÷ cos(angle) | Pitch (Hypotenuse) |
| Ridge height above the wall plate | run × tan(angle) | Pitch (Rise) |
| Gutter fall | gutter run ÷ 500 at 1:500 | Pitch (Run + Grade, 0.2%) |
| A gable, a brace, a run from a sloped length | any two sides, or a side and an angle | Triangles |
| Studs under a skillion or a gable end | low height + distance × tan(pitch) − plates | Raked Walls |
MeisterCalc has no calculator for rafter plumb and seat cuts, birdsmouths, hips, valleys or creepers, roof area, or counts of sheets, tiles and battens. If you set out hip and valley roofs every week, you'll want an app built for roof framing as well. The free roof pitch and slope calculators below run the same formulas as the app. For other trades, see calculators by trade and country, or all the construction calculators.
How do I get a pitch in degrees, X in 12 and per cent?
Divide the rise by the run, then convert: arctan for degrees, × 12 for X in 12, × 100 for per cent. The run is level, from the wall line to the ridge centre line, which is half the span on a gable roof.
Worked example: a re-roof with an 8200 mm span, so a 4100 mm run, and 1450 mm from the top of the wall plate to the ridge line.
- Slope: 1450 ÷ 4100 = 0.3537.
- Degrees: arctan 0.3537 = 19.48°.
- X in 12: 0.3537 × 12 = 4.24, about 4 1/4 in 12.
- Per cent: 0.3537 × 100 = 35.37%.
- Rafter line: √(1450² + 4100²) = 4348.9 mm.
Degrees are what Australian sheet roofing is written in. Under the NCC 2025 Housing Provisions, Part 7.2, metal sheeting must meet the minimum pitch for its profile (clause 7.2.3 and Figure 7.2.3), and where a full-length sheet can't be laid, end laps are 150 mm on slopes above 15° and 200 mm between 5° and 15° (clause 7.2.6(2)(b)). At 19.48° this roof takes the 150 mm lap. Read your profile's minimum in Figure 7.2.3 and the sheet maker's literature. Checked on 1 November 2026. To measure a pitch with a 1 m level when you can't reach the ridge, see how to calculate roof pitch.
How long is the rafter line, and how high is the ridge?
Rafter line = run ÷ cos(angle), with the level overhang added to the run first. Ridge height above the plate = run × tan(angle).
Worked example: a 7200 mm span at 20°, with eaves sticking out 600 mm past the wall, measured level.
- Run: 7200 ÷ 2 = 3600 mm.
- Ridge height above the wall plate: 3600 × tan 20° = 3600 × 0.3640 = 1310.3 mm. With the plate 2700 mm above the floor, the ridge line is 2700 + 1310.3 = 4010.3 mm up.
- Rafter line, wall line to ridge line: 3600 ÷ cos 20° = 3600 ÷ 0.9397 = 3831.0 mm.
- With the eaves: (3600 + 600) ÷ 0.9397 = 4469.5 mm. The overhang alone is 600 ÷ 0.9397 = 638.5 mm along the slope.
These are line lengths along the top of the rafter, from the ridge centre line to the tail. They don't allow for the ridge board, the plumb and tail cuts, or the battens and sheets on top, which also add height above the 4010.3 mm. The method in more detail is in rafter length from pitch and span and ridge height from span and pitch.
How much fall does a gutter need?
In Australia an eaves gutter falls at least 1:500, which is the gutter run ÷ 500: 2 mm for every metre to the outlet.
Worked example: a 15 m eaves gutter.
- Downpipes: each may serve no more than 12 m of gutter, so 15 m needs two.
- Runs: with a downpipe at each end and the high point in the middle, each run is 7500 mm.
- Fall: 7500 ÷ 500 = 15 mm each way. Set the middle bracket high and each end 15 mm lower, then string the brackets between.
With one downpipe at one end the gutter would be a single 15 m run: over the 12 m limit, and needing 30 mm of fall. Sources: NCC 2025 Housing Provisions, Part 7.4: clause 7.4.4(1)(a), a fall “not less than 1:500, unless designed in accordance with AS/NZS 3500.3” (Western Australia's variation drops the exception), and clause 7.4.5(1) for the 12 m, checked on 1 November 2026. New Zealand's E1/AS1 (5.3.1) says only that gutters fall to an outlet, and England's Approved Document H (H3, 1.6) asks for “any fall towards the nearest outlet”: there, the gutter maker and the plumber set the figure. Box and valley gutters follow other rules; see the minimum fall for a gutter.
Where do roofers go wrong with the numbers?
- Mixing degrees, X in 12 and per cent. 20° is 36.40% and 4 3/8 in 12. A 20% grade is only 11.31°, and 4 in 12 is 18.43°, not 4°.
- Measuring the run to the wrong point. Use the full 7200 mm span with the 1310.3 mm rise and you get 10.31°, about half the real 20°. Measure to the fascia, 4200 mm, and you get 17.33°.
- Adding the overhang straight on. 3831.0 + 600 = 4431.0 mm, 38.5 mm short of the real 4469.5.
- A fall too small for the length. 10 mm over 15 m is 1 in 1500, a third of the 1:500 minimum, which needs 30 mm.
- Reading a centre-line height as the finished height. The ridge figure is to the ridge line; the ridge board, battens, sheets and capping sit above it.
What else in the app helps on a roof job?
Triangles solves a right triangle from any two values: give it the 3831.0 mm rafter line and the 20° angle and it returns the 3600 mm run and the 1310.3 mm rise, a quick check on a set-out. Raked Walls lists the studs for a skillion or gable-end wall, worked through on the page for carpenters. Equal Spacings divides a gutter run into equal bracket spacings; it rounds to the nearest whole number of spaces, so check the result is inside the maker's maximum. The plans, the roofing maker's instructions, the code and your certifier decide what's needed; MeisterCalc does the arithmetic.
Which MeisterCalc calculators are free for roof work?
MeisterCalc runs on Android 7.0 or later, in millimetres by default, with feet and inches under Settings ▸ Units ▸ Default units ▸ Imperial (ft/in). Calculations run on the phone, so it works on a roof with no signal. Pitch, Triangles and Raked Walls are paid calculators; Concrete, Stairs and Check Square are free. What Pitch does, mode by mode, is in the roof pitch calculator app guide.