Roof pitch explained
Pitch is the one number that changes almost every other number on a roof — and it does not change all of them by the same amount, which is where take-offs go wrong.
Roof pitch is the angle of the slope, given in degrees in New Zealand and as
rise-over-run elsewhere. It makes every sloping length longer than it looks on a
plan, by 1 / cos(pitch) — 1.035 at 15°, 1.103 at 25°, 1.221 at 35°. It does
not change level lengths: ridges, gutters and the width you count sheets
across are the same in plan as on the roof.
Two ways of saying the same angle
New Zealand roofing works in degrees. Plans, pitch gauges and manufacturers' minimum-pitch tables are all in degrees. You will still meet rise over run — a ratio like 1:3, meaning the roof rises one for every three it runs — on older drawings and in imported detailing.
They convert: pitch = atan(rise ÷ run), and rise ÷ run = tan(pitch).
| Pitch | Rise per 1 m of run | As a ratio | What it looks like |
|---|---|---|---|
| 5° | 0.087 m | 1 : 11.4 | Near-flat — check the profile's minimum pitch |
| 10° | 0.176 m | 1 : 5.7 | Low-pitch modern, trapezoidal territory |
| 15° | 0.268 m | 1 : 3.7 | Common on newer NZ housing |
| 20° | 0.364 m | 1 : 2.7 | The everyday residential pitch |
| 25° | 0.466 m | 1 : 2.1 | Also very common; walkable |
| 30° | 0.577 m | 1 : 1.7 | Getting steep underfoot |
| 35° | 0.700 m | 1 : 1.4 | Roof ladders and harnesses |
| 40° | 0.839 m | 1 : 1.2 | Steep; work slows noticeably |
| 45° | 1.000 m | 1 : 1 | Rise equals run |
What pitch changes, and by how much
This is the part worth knowing cold. Two different multipliers apply, and using the wrong one is not a rounding error.
| Pitch | Rafter, sheet run, barge, area1 / cos θ | Hip or valley on a square corner√(1 + tan²θ ⁄ 2) |
|---|---|---|
| 5° | 1.004 | 1.002 |
| 10° | 1.015 | 1.008 |
| 15° | 1.035 | 1.018 |
| 20° | 1.064 | 1.033 |
| 25° | 1.103 | 1.053 |
| 30° | 1.155 | 1.080 |
| 35° | 1.221 | 1.116 |
| 40° | 1.305 | 1.163 |
| 45° | 1.414 | 1.225 |
Read that second column carefully, because it surprises people. At 25° a rafter is 10.3 per cent longer than its plan length, but a hip is only 5.3 per cent longer than its plan length. The hip is already running diagonally across the plan, so it has picked up most of its extra distance before the pitch is applied at all. Multiply a hip by the rafter factor and you over-order it; apply nothing and you under-order it.
The hip column assumes the two planes meet at a square corner and share a pitch, which is the normal case. An odd-angled corner or two different pitches needs the geometry doing properly rather than a table.
What pitch does not change
- Ridge length. Level.
- Gutter and fascia length. Level.
- The face width you count sheets across. Level, so the sheet count is unaffected — only the sheet length moves.
- The building footprint. Obviously, but worth saying: plan area is not roof area. A quote priced off the footprint at 35° buys 82 per cent of the roof.
Roof area at the true pitch
Roof area uses the same 1 / cos θ factor, because the roof is only stretched
in one direction. A 120 m² footprint at 30° is 120 × 1.155 = 138.6 m² of
roof. That is 18.6 m² of steel and underlay that a plan-area estimate does not buy.
Measuring pitch on site
- A pitch gauge or a phone level laid on a sheet or a rafter. Take it in a couple of places — old roofs sag, and a verandah is often a different pitch from the main roof.
- Rise over run with a level and a tape: hold the level horizontal
against the slope, measure a metre along it, drop to the roof, and that vertical is the
rise.
atan(rise ÷ 1)is the pitch. - Off the plans, if there are plans and the roof was built to them.
Record the pitch per roof, not per job. A house with a lean-to at a different pitch has two pitch factors, and averaging them is a way of being wrong twice.
Low pitch is a specification question
Long-run steel goes down to low residential pitches, but the lower it gets the more the laps, the underlay and the fixing detail matter. Below the profile's stated minimum it is not a judgement call — check the manufacturer's minimum pitch for that profile and follow the code of practice. Confirm the site pitch before you specify the profile, not after.
Steepness costs hours
Above about 35° the work slows noticeably, the access requirements change, and the labour rate that worked on a 20° gable stops working. Pitch belongs in the labour calculation as well as the material one.
The roof pitch calculator converts between degrees and ratios and gives both multipliers for any pitch, with the formulas on the page.
What comes up most.
What is a normal roof pitch in New Zealand?
Most residential long-run steel roofs sit somewhere between about 15° and 30°, with 20–25° the everyday range. Low-pitch modern designs run down towards 10° and below, usually on a trapezoidal profile.
There is no single right answer — it is set by the design, and your job is to measure what is actually there rather than assume.
How do I convert a 1:3 pitch to degrees?
Take the arctangent of rise divided by run. 1 ÷ 3 = 0.333, and atan(0.333) = 18.4°.
The other direction: tan(18.4°) = 0.333, so the roof rises 333 mm for every metre it runs.
Does pitch change the number of sheets?
No — only their length. You count sheets across the face of the roof, and that direction is level, so it measures the same in plan as it does up on the roof.
Applying the pitch factor to the count as well as the length is a double-count, and it over-orders the entire roof by the pitch factor.
Why is the hip multiplier smaller than the rafter one?
Because a hip is already travelling diagonally in plan. On a square corner its plan length is √2 times the run, so most of its extra distance is horizontal, not vertical, and the pitch adds proportionally less.
At 25° the rafter factor is 1.103 and the hip factor is 1.053. Using 1.103 on a hip over-orders it by about five per cent, on the longest and most expensive flashing on the roof.
Can I measure pitch from an aerial photo?
Not reliably. An aerial gives you the plan shape, which is everything except the pitch — the one number a photo taken from directly above cannot show you.
Measure it on site with a gauge, or take it off the plans. Then the plan shape and the pitch together give you every length on the roof.
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Try it on one of your own roofs.
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