TL;DR, rafter length in 30 seconds
Rafter length = √(run² + rise²), or more usefully on site, run ÷ cos(pitch).
For a 22.5° pitch on a 3 m run, that is about 3.25 m before you add the eaves.
Memorise three multipliers: 22.5° = 1.082, 30° = 1.155, 45° = 1.414.
Three cuts per rafter: plumb cut (top, against the ridge), seat cut or birdsmouth (sits on the top plate), tail cut (eaves end).
AS 1684 caps the birdsmouth notch at one third the rafter depth. On a 190 mm rafter that is 63 mm. On a 240 mm rafter, 80 mm.
Or skip the trig: a rafter length calculator gives you plumb cut, seat cut and total length in one go, and the Built Simple Roof Rafter Calculator is free in the app.
Every Australian carpenter has stood on a top plate at 6:30 am with a tape in one hand, trying to remember whether the run gets multiplied by 1.082 or 1.155. Get it wrong and the ridge sits proud, the eaves line runs out, or the birdsmouth eats too much of the rafter and the frame gets pulled up at inspection. A rafter length calculator removes that guess, but it only helps if you know which number to feed it and why.
This guide covers the lot: which rafter you are actually cutting, the pitches you will meet on Australian residential work, the formula worth memorising, a fully worked 8.4 m gable, what AS 1684 says about the birdsmouth, the eaves and ridge adjustments that catch people out, and what a bad set-out costs once you price the recut. There is an FAQ at the end for the questions that come up on site.
Common, hip and jack rafters: what you are actually cutting
Get clear on the rafter type before you measure anything. The geometry changes, and a calculator output is only as good as the input you give it.
Common rafter. The workhorse. Runs from the top plate to the ridge at 90 degrees to the wall, in the same plane as a gable end. Every gable, every shed, every simple hip section uses them. The formula below applies directly: pitch, run and rise behave exactly as Pythagoras says they will.
Hip rafter. The diagonal from the building corner up to the end of the ridge. In plan view it travels at 45 degrees to the commons on an equal-pitch hip, so its run is the common run multiplied by 1.414. That longer run over the same rise means the hip sits at a shallower angle than the commons. A 22.5° common rafter gives a hip of roughly 16.3°. Cut a hip at 22.5° and it will not sit down on the corner. Hips get their own calculation every time.
Jack rafter. The short ones running from the top plate up to a hip (or down to a valley). Each successive jack is shorter than the last by the same amount, the common difference. On an equal-pitch hip at 600 mm centres and 22.5° pitch, that difference is 600 × 1.082 = 649 mm. At 450 centres it is 487 mm. Cut the longest one, step down by that figure, and you never recalculate from scratch.
Quick check: a skillion, a gable or a shed only needs common rafters. Hips and jacks arrive with a hip end or a pitch break.
Roof pitch ratios used in Australian residential roofing
We quote pitch in degrees here rather than the rise-over-run fraction the Americans use. These are the ones you will actually meet:
| Pitch | Rise per m of run | Rafter length per m of run | Typical AU use |
|---|---|---|---|
| 15° | 268 mm | 1.035 m | Sheds, skillions, low-slope sheet roofs |
| 17.5° | 315 mm | 1.049 m | Around the low limit for many concrete tile profiles |
| 22.5° | 414 mm | 1.082 m | The most common modern AU residential pitch |
| 30° | 577 mm | 1.155 m | Hamptons and period homes, terracotta tile |
| 35° | 700 mm | 1.221 m | Federation and steeper character builds |
| 45° | 1000 mm | 1.414 m | Steep gables, architectural features, slate |
Three pitches cover most residential work: 22.5°, 30° and 45°.
Two honest notes on the low end. Minimum pitch for profiled steel sheeting is set by the manufacturer's published spec for that profile, not by a single trade rule of thumb, and several common profiles run far lower than 15°. Read the sheet you are actually fixing. Tiles are the tighter constraint: minimums vary by profile and by whether sarking is installed, so check the tile maker's published figure alongside AS 2050 for installation. Above 45° the job gets expensive quickly. More rafter length per metre of run, harder to walk, harder to lift sheets, and usually scaffold rather than a ladder.
The Pythagoras formula every chippy should have memorised
A rafter is the hypotenuse of a right-angled triangle. The two legs are the run (horizontal, from the outside of the top plate to the centre of the ridge) and the rise (vertical, top plate to the underside of the ridge).
The formula
Rafter length = √(run² + rise²)
or, when you know the pitch instead of the rise:
Rafter length = run ÷ cos(pitch)
and rise = run × tan(pitch)
The cosine version is the one that earns its keep, because on site you usually know the run and the pitch off the plans, not the rise. cos(22.5°) = 0.9239, so 1 ÷ 0.9239 = 1.082. For 30° it is 1.155. For 45° it is 1.414. Learn those three and you can sanity-check any rafter on the fly, including one a calculator hands you.
That last part matters. The value of doing the arithmetic once by hand is not nostalgia, it is that you can spot a wrong answer. If the app says a 4.2 m run at 22.5° gives a 5.9 m rafter, you know immediately that someone has typed the eaves in twice.
A worked example: 8.4 m gable at 22.5 degrees
Say the plans show a gable, 8400 mm wide over the frames, 12.0 m long, 22.5° pitch, sheet roof, 600 mm eaves, rafters at 600 centres onto a 35 mm ridge board.
1. Find the run. Half the width is 4200 mm. Deduct half the ridge thickness, 17.5 mm, because the run is measured to the centre of the ridge and the rafter stops at its face. Run = 4182.5 mm.
2. Rafter length, heel to plumb cut. 4182.5 × 1.082 = 4527 mm.
3. Rise, for setting the ridge height. 4182.5 × 0.4142 = 1732 mm above the top plate, measured to the underside of the ridge.
4. Add the tail. A 600 mm horizontal eaves overhang is not 600 mm of timber. It runs on the rake, so 600 × 1.082 = 649 mm. Total timber from tail cut to plumb cut = 4527 + 649 = 5176 mm.
5. Order length. 5176 mm plus the plumb tail cut and a bit of waste puts you on 5.4 m lengths, 42 of them for both sides at 600 centres.
One thing the arithmetic will not tell you: a 4.18 m single span is long. Check the section size against the AS 1684 span tables for your wind classification, roof mass and rafter spacing, because most sheet roofs at that run pick up an underpurlin and struts. That changes nothing about the length calculation and everything about the timber you order.
Birdsmouth sizing: what AS 1684 actually says
The birdsmouth is the notch cut into the underside of the rafter where it bears on the top plate. Two faces: a vertical plumb face against the outer edge of the plate, and a horizontal seat face on top of it. Cut right, it transfers load cleanly into the wall. Cut wrong, the rafter splits at the notch or the eaves sag.
The rule that matters: the birdsmouth must not exceed one third the depth of the rafter.
- 240 mm rafter, maximum notch 80 mm
- 190 mm rafter, maximum notch 63 mm
- 140 mm rafter, maximum notch 46 mm
That is a maximum, not a target. AS 1684 also sets a minimum bearing length onto the plate, so read both figures together rather than cutting to whichever one you remember. AS 1684 Residential Timber-Framed Construction is called up by the National Construction Code as a deemed-to-satisfy pathway for timber-framed housing, which is why a certifier can knock back a frame on this one detail alone.
The practical trap is the deep rafter on a wide plate. A 90 mm top plate with a 22.5° seat cut needs a notch deep enough to seat properly, and on a 140 mm rafter you can find yourself at the one third limit before the rafter is fully bearing. The answer is a deeper rafter, not a deeper notch.
The three cuts, and the two angles that set them
Every common rafter carries three cuts, and only two angles.
Plumb cut. At the ridge, cut to the pitch angle off square. At 22.5° pitch, the plumb cut is 22.5°.
Seat cut. The horizontal face of the birdsmouth, cut at 90 minus the pitch. At 22.5° pitch, that is 67.5°. Its plumb face is at the same 22.5° as the ridge cut.
Tail cut. At the eaves. Plumb if you are running a fascia, square or bevelled if the detail calls for it. Same 22.5° when plumb.
So a 22.5° roof is a saw set to 22.5° for two of the three cuts. Set your circular saw once, cut every plumb and tail cut on all 42 rafters, then reset for the seats. That single sequencing decision saves more time than any other on a rafter day.
Key takeaway: the pitch is the plumb cut. The seat cut is 90 minus the pitch. If you only remember one thing off this page, remember that pair.
What a rafter length calculator saves you in real money
Here is the part the trig articles skip. A set-out error is not one mistake, it is one mistake multiplied by the rafter count.
On the 8.4 m example above, a 20 mm error in the run puts every one of 42 rafters out by about 22 mm on the rake. If it is found before you cut, it costs ten minutes. If it is found after the first side is up, you are pulling rafters, recutting birdsmouths that are now in the wrong place, and either scarfing tails or dropping to a shorter eaves line the client did not ask for.
Price that properly rather than absorbing it. A day lost to a recut is a day of crew time you cannot re-sell, so cost it at your real rate, not at your out-of-pocket wages. If you are not sure what that number should be, our breakdown of carpenter day rates across Australia for 2026 has the market ranges by state. And remember the GST split when you compare it against materials: the timber you buy carries GST you claim back if you are registered, while the rate on the client's invoice is GST-inclusive. Compare supplier quotes ex-GST and quote the client inc-GST, or the margin you think you have is not the margin you have.
The material side is the smaller half but not nothing. Forty-two rafters at 5.4 m is a lot of timber to re-order at short notice, and the second order rarely lands at the first order's price.
Turning rafter length into a take-off
Once the length is right, the numbers flow straight into the quote: rafter count, length, section size, the ridge, the underpurlins and struts, the fascia and the barge. Doing that on the back of a plan set is where an accurate calculation quietly turns into an accurate price.
This is the same reason we build the calculators into the estimating side of the product rather than leaving them as a separate toy. If you are still moving figures from a phone calculator into a spreadsheet by hand, our comparison of construction estimating software in Australia covers what the options actually do with a take-off once you have one, and the wider construction project management software rundown covers where that estimate goes next.
Frequently asked questions
How do I work out rafter length without a calculator?
Multiply the run by the figure for your pitch: 1.035 at 15°, 1.082 at 22.5°, 1.155 at 30°, 1.221 at 35°, 1.414 at 45°. That gives the length from the birdsmouth heel to the plumb cut. Add the eaves overhang multiplied by the same figure to get the timber length.
What is the difference between rafter span and rafter length?
Span is the horizontal distance the rafter bridges between supports, and it is what the AS 1684 tables are indexed on when you pick a section size. Length is the timber on the rake. On a 22.5° roof the length is about 8 per cent longer than the span, which is exactly the sort of gap that turns into 42 short rafters.
Do I measure the run to the outside of the wall frame or the top plate?
To the outside face of the top plate, then deduct half the ridge thickness. Measuring to the outside of the cladding or the slab edge is a common source of a 20 to 40 mm error that carries through every rafter.
How deep can a birdsmouth be?
No more than one third the rafter depth under AS 1684. 63 mm on a 190 mm rafter, 80 mm on a 240 mm. If the seat will not sit properly within that, go up a rafter size rather than deeper into the one you have.
What is the standard roof pitch in Australia?
There is no single standard, but 22.5° is the most common on modern residential work and 30° is usual on Hamptons and period-style homes. Tiles push you to the steeper end, sheet lets you go lower.
Does a rafter length calculator handle hips and jacks?
Our Roof Rafter Calculator works on common rafters, which is what a gable, a skillion or a shed needs. For a hip end, remember the hip run is the common run × 1.414 and the hip's own pitch is shallower, and step your jacks down by the common difference (spacing × the pitch multiplier) rather than measuring each one.
How much eaves overhang is normal?
450 mm and 600 mm are the two you will see most often on AU residential. It is a design and shading decision rather than a structural one, but check the rafter tail is supported within the span table limits and that the overhang does not conflict with fire separation requirements near a boundary.
Get the cut sheet in one go
The maths above is worth knowing because it lets you catch a wrong number. It is not worth doing 42 times at 6:30 am. The Built Simple Roof Rafter Calculator takes your run, pitch, rafter depth and eaves and gives you the plumb cut, the seat cut, the rise and the total length ready for the saw.
It is one of the 45 calculators in the Built Simple app, free on iOS and Android, no signup needed to use them. If you want the numbers to keep going after the frame is up, into a take-off, a quote and a schedule, the same app does that too. Have a play with the calculator on your next roof and see whether it matches what you would have worked out on the plate.
