Every Australian carpenter has stood on a top plate at 6:30 am, tape measure in hand, trying to remember whether to multiply the run by 1.082 (a 22.5 deg pitch) or 1.155 (a 30 deg pitch). Get it wrong and the ridge sits too high, the eaves end up uneven, or worse — the birdsmouth weakens the rafter and fails inspection.

This guide walks through exactly how to calculate rafter length the right way for an Australian residential roof. We’ll cover pitch ratios, the Pythagorean shortcut every chippy should have memorised, AS 1684 birdsmouth rules, plumb and seat cuts, eaves overhang, and a fully worked example. At the end, you can plug the same numbers into our free Roof Rafter Calculator and have the whole job set out in under a minute.

Common, hip and jack rafters — what you’re actually cutting

Before you measure anything, get clear on which rafter you’re building. The geometry changes based on type, and the calculator output is only useful if you tell it the right one.

Common rafter

The workhorse. Runs from the top plate to the ridge board at 90 degrees to the wall, in the same plane as a gable end. Every gable roof, every shed, every simple hip section uses common rafters. The length formula in this article applies directly to common rafters — pitch, run and rise behave exactly as Pythagoras predicts.

Hip rafter

The diagonal rafter that travels from the corner of the building up to the ridge end on a hip roof. Because it runs at 45 deg to the common rafters in plan view, its effective run is longer than the common rafter’s run. The unit length per metre of run for a hip is roughly 1.414 multiplied by the common rafter unit length — but the pitch in the plumb cut is shallower than the common rafter pitch. Hips need their own calculation; do not just lengthen a common rafter.

Jack rafter

The shorter rafters that run from the top plate to a hip rafter (or to a valley rafter). Each successive jack rafter is shorter than the last by the same increment — called the common difference. Once you know the spacing (usually 450 mm or 600 mm on centre) and the pitch, you can step them down without recalculating from scratch.

Pitch ratios used in Australian residential roofing

Roof pitch is the angle of the rafter measured from horizontal. In Australia we usually quote it in degrees rather than the rise-over-run fraction Americans prefer. Here are the pitches you’ll actually see on site:

Pitch (deg)	Rise per metre of run	Rafter length per m of run	Typical use in AU
15 deg	268 mm	1.035 m	Minimum for Colorbond Trimdek
17.5 deg	315 mm	1.049 m	Minimum for concrete tile roofs (AS 2050)
22.5 deg	414 mm	1.082 m	Most common modern AU residential (low-slope)
30 deg	577 mm	1.155 m	Hampton/period homes, terracotta tile
35 deg	700 mm	1.221 m	Federation, steeper character builds
45 deg	1000 mm	1.414 m	Steep gables, architectural feature, slate

The three pitches you’ll meet 80% of the time on AU residential work are 22.5 deg, 30 deg and 45 deg. Anything below 17.5 deg locks you out of tile and into metal sheet only. Anything above 45 deg gets expensive fast — more rafter material, harder to walk, harder to lift sheets, often needs scaffold rather than a ladder.

The Pythagoras formula every chippy should memorise

A rafter forms the hypotenuse of a right-angled triangle. The two legs are the run (horizontal distance from the outside of the top plate to the centre of the ridge) and the rise (vertical distance from the top plate to the underside of the ridge board).

If you know the pitch and the run, the rise comes from rise = run multiplied by tan(pitch). If you know two of the three sides, you can solve for the third every time.

Why use the trig version on site?

Because most of the time you know the run (half the building width plus or minus the ridge thickness) and the pitch the architect specified — not the rise. So the cleaner formula on the job is:

Rafter length = run divided by cos(pitch)

For a 22.5 deg pitch, cos(22.5) = 0.9239, so the multiplier is 1 / 0.9239 = 1.082. For 30 deg, the multiplier is 1.155. For 45 deg, it’s 1.414. Memorise those three and you can sanity-check any rafter on the fly.

Birdsmouth cut sizing — what AS 1684 actually says

The birdsmouth (or seat cut) is the triangular notch cut into the underside of the rafter where it bears on the top plate. It has two faces: a vertical plumb face against the outer edge of the top plate, and a horizontal seat face sitting on top of the plate. Get this notch right and the rafter transfers load cleanly into the wall. Get it wrong and the rafter splits or the eaves sag.

AS 1684 Residential Timber-Framed Construction gives the rule that matters: the birdsmouth notch must not exceed one third the depth of the rafter. For a 240 mm deep rafter, the maximum vertical cut is 80 mm. For a 190 mm rafter, the max is roughly 63 mm. Cut deeper than that and you’ve removed too much section — the rafter loses bending strength right at the support, which is the worst possible place to weaken it.

Birdsmouth horizontal bearing length

The seat face needs enough horizontal bearing on the top plate to spread load. AS 1684 sets a minimum of 35 mm of seat length for stress grade F5 framing. For F7 or higher, 30 mm can be acceptable, but most carpenters keep 35 mm as the standard. The full top plate thickness (usually 90 mm or 70 mm) is the maximum — don’t extend the seat past the inner edge or you’ve cut a useless step.

Plumb cut and seat cut angles

The two angles on a rafter are simple once you remember which is which:

• Plumb cut — the angle that makes the rafter sit vertical (plumb) when in place. Equal to the pitch angle. For a 22.5 deg roof, the plumb cut is 22.5 deg off square. Used at the top end (against the ridge) and again on the bottom of the birdsmouth.
• Seat cut — the angle that makes the rafter sit horizontal (level) on the top plate. Equal to 90 deg minus the pitch. For a 22.5 deg roof, the seat cut is 67.5 deg off square — but most chippies set the saw to the complement, which is 22.5 deg, just rotated.
• Tail cut — the cut at the eaves end. Usually a plumb cut so the fascia sits vertical; sometimes a square cut if there’s no fascia.

On a Stanley or DeWalt sliding compound saw, you bevel the blade to the pitch angle for plumb cuts and to 90 deg minus pitch for seat cuts. A bevel gauge or a roofing square gets the same job done with hand tools.

Eaves overhang — don’t forget to add it

The rafter length you calculated from the run is the distance from the ridge to the outside of the top plate. You still need to add the eaves overhang to get the total stock length you’ll cut from.

Typical Australian residential eaves are 450 mm to 600 mm, measured horizontally from the wall. To convert that to additional rafter length, divide by cos(pitch). For a 600 mm horizontal overhang on a 22.5 deg roof, the extra rafter length is 600 / 0.9239 = 649 mm.

Add another 50 to 100 mm of waste at the tail end so you’ve got room to mark and cut. So for that 600 mm overhang at 22.5 deg, order rafters with at least 700 mm of extra length past the birdsmouth.

Worked example: 6 m span, 22.5 deg pitch, 240 mm rafter

Let’s walk through a complete calculation for a typical Australian project room — say a 6 m wide extension with a 22.5 deg gable roof, 600 mm eaves, and 240 mm deep MGP10 pine rafters.

That’s the complete cut sheet for a single common rafter. Multiply across the roof, account for the gable-end and barge rafters, and you’ve got your timber order. For the hips on this roof, you’d repeat the process with the hip-rafter unit length (about 1.500 per metre of common run at 22.5 deg).

Common mistakes that cost time and timber

1. Confusing span with run

Span is the full distance between the two top plates. Run is half that, less the ridge half-thickness. If you punch span into a formula expecting run, you’ll cut every rafter twice as long as it needs to be. Always check which value the calculator is asking for.

2. Forgetting roof load width when speccing rafter size

AS 1684 span tables for rafters are indexed by roof load width (RLW), not just span. RLW is the horizontal distance the rafter carries — for a simple gable, that’s half the span plus the eaves overhang. Use the wrong RLW column and your 190 mm rafter might actually need to be 240 mm. AS 1684.2 Table 2.5 (sheet roof) and Table 2.7 (tile roof) are the ones to reference.

3. Cutting the birdsmouth too deep

“It’s only a few extra mil, she’ll be right.” No — and certifiers know exactly what to look for. A 100 mm cut into a 240 mm rafter is non-compliant and will be flagged on inspection. Stick to one third max.

4. Ignoring the ridge board thickness

The run is from the top plate to the centre of the ridge, not to the outside face. On a 35 mm ridge that’s only 17.5 mm of difference — easy to absorb in trim. On a 70 mm laminated ridge, it’s 35 mm. Across both sides of a gable, that’s 70 mm of accumulated error — your ridge will sit too high or your eaves won’t line up.

5. Not allowing for sheathing or sarking thickness

If the architect specced rise from finished sarking or finished tile line, you need to subtract the sarking and batten thickness from the rise before back-calculating the rafter. Most residential jobs measure to the top edge of the rafter, but check the detail drawing.

AS 1684 — the standard you cannot skip

AS 1684 Residential Timber-Framed Construction is the deemed-to-comply standard for residential timber framing in Australia. For rafters specifically, you’ll use:

• AS 1684.2 for non-cyclonic areas (most of Australia south of the Tropic of Capricorn)
• AS 1684.3 for cyclonic areas (FNQ, NT coast, parts of WA Kimberley)
• Table 2.5 in 1684.2 — single-span rafters, sheet roof (Colorbond, zincalume)
• Table 2.7 in 1684.2 — single-span rafters, tile roof
• Table 2.9 for continuous-span rafters where the rafter passes over an internal support

The tables give maximum allowable span by stress grade (F5, F7, MGP10, MGP12), section size (mm wide x mm deep) and roof load width. Cross-reference all three before cutting timber. Most importantly, the tables assume an AS 1684-compliant birdsmouth — the one-third-depth rule we covered earlier. Cut deeper and the span table no longer applies.

If your design sits outside the 1684 envelope — span too long, RLW too wide, pitch unusual — you’ve stepped into engineered timber territory. That means LVL, glulam or steel, and an engineer’s spec sheet. Don’t try to extrapolate the table.

Skip the trig — let the calculator do it

If you cut rafters every week, you probably have most of this memorised. If you cut rafters every few months, you don’t, and pulling out a roofing square at 7 am isn’t anyone’s idea of fun. The Built Simple Roof Rafter Calculator takes four inputs — span, pitch, rafter depth, overhang — and gives you:

• Total rafter length (with eaves and waste allowance)
• Plumb cut and seat cut angles, ready for the saw
• Birdsmouth depth checked against AS 1684 one-third rule
• Rise at ridge — useful for setting the prop or strongback height
• Common difference for jack rafters (if you’re cutting a hip)

It’s free, no signup, AU-compliant, and lives alongside 50+ other free trade calculators built specifically for Australian builders, carpenters and owner-builders. Bookmark it, save it to the home screen on your phone, and the next time the apprentice asks “what’s the plumb cut for 22.5?” you’ll have it in three taps.

Frequently asked questions

What’s the most common roof pitch in Australia?

For new residential builds, 22.5 degrees is the most common — it works with both Colorbond and concrete tile, sheds water reliably in AU rainfall conditions, and stays under the height limits set by most council planning schemes. Character and Hampton-style homes typically push up to 30 degrees, and steep architectural gables can hit 45 degrees.

How do I calculate rafter length without a calculator?

Use Pythagoras: rafter length = sqrt(run squared + rise squared). Or memorise the unit length per metre of run for the three common AU pitches — 1.082 at 22.5 deg, 1.155 at 30 deg, 1.414 at 45 deg — and multiply by your run in metres. Then add the eaves overhang (also adjusted for pitch).

What is the maximum birdsmouth cut depth under AS 1684?

AS 1684 limits the birdsmouth notch to a maximum of one third the depth of the rafter. For a 240 mm deep rafter that’s 80 mm; for a 190 mm rafter it’s roughly 63 mm. Anything deeper compromises the rafter at the support and voids the span table.

Do I cut the birdsmouth before or after the plumb cut?

Most carpenters cut the top plumb (against the ridge) first, then measure down the rafter to mark the birdsmouth, then cut the tail last. Cutting the birdsmouth first means you’ve locked in the bearing point without confirming the overall length — if you’ve miscalculated, you waste the whole rafter.

When do I need to consult a structural engineer for rafters?

Any time the rafter falls outside AS 1684 span tables — long spans, wide roof load widths, unusual pitches, point loads from skylights or AC units, or a birdsmouth detail that needs to be deeper than one third. Also any time you’re using LVL, glulam, steel or a structural ridge beam carrying point loads. When in doubt, get the engineer’s sheet before timber arrives.

Next steps

If you’re building right now, jump into the Roof Rafter Calculator and run your numbers — it’ll save you 15 minutes of trig. If you’re putting together a quote and need to spec timber across multiple rafters and joists, the full Built Simple calculator hub has tools for stair stringers, truss spacing, decking joists, pier footings and 45 more trades. And if you’re a carpenter looking for a way to estimate, schedule and track jobs without spreadsheets and group chats, take a look at Built Simple for carpenters — built by builders, in Melbourne, designed to keep admin out of your way.

Built Simple is Australian-made construction project management software for builders, tradies and owner-builders. Melbourne, 2025. More building. Less admin.