AS 1684 Timber Framing Code Compliance — Last Updated: 8 August 2026
Pitching a conventional timber roof on an Australian job site requires precise trigonometric calculations and strict adherence to framing standards. Whether you are framing a standard gable roof for a residential build or cutting an extension, this calculator simplifies the calculations for common rafters, overhang tails, seat cuts, and notch limits. Before framing walls or sizing roof timber, refer to our Wall Framing Calculator and Timber Span Calculator.
Key definitions for Australian roof framing:
- Span: The total horizontal width between the outer faces of the load-bearing top plates.
- Run: Half of the total span (for a symmetrical gable roof). This is the horizontal base of the rafter triangle.
- Rise: The vertical distance from the top of the wall plate line to the top of the ridge board.
- Pitch: The roof incline angle measured in degrees (°), or expressed as a ratio of rise to run. In Australia, tiled roofs generally range between 15° and 35° (22.5° standard), while Colorbond steel roofs range between 2° and 15°.
- Rafter Line Length: The theoretical length from the centre of the ridge board to the outside edge of the top plate.
AS 1684 Birdsmouth & Notch Limits
Timber framing in Australia must comply with AS 1684.2 (Non-cyclonic) or AS 1684.3 (Cyclonic). One of the most important structural rules for carpenters cutting pitched roofs is the birdsmouth cut limit.
According to AS 1684 clause 7.2.5, the vertical depth of a birdsmouth notch cut into a rafter at the wall plate seat must not exceed one-third (1/3) of the rafter timber depth.
- 90x45 MGP10 Rafter: Max birdsmouth depth = 90 / 3 = 30.0 mm
- 120x45 MGP10 Rafter: Max birdsmouth depth = 120 / 3 = 40.0 mm
- 140x45 MGP10 Rafter: Max birdsmouth depth = 140 / 3 = 46.7 mm
- 190x45 MGP10 Rafter: Max birdsmouth depth = 190 / 3 = 63.3 mm
Cutting deeper than 1/3 reduces the effective cross-sectional depth of the timber member at the point of maximum shear stress, increasing the risk of structural failure under wind uplift or heavy tile dead loads.
Common Rafter Trigonometric Formulas
To verify rafter dimensions manually on site using a builder's calculator or framing square, apply these formulas:
1. Run (Half-Span)
2. Roof Rise Height
3. Rafter Line Length (Hypotenuse)
4. Eave Tail Length
5. Plumb Cut & Level Cut Angles
Real-World Worked Example: 6-Metre Span Gable Roof
Here is a practical step-by-step example for pitching a residential double-car garage gable roof in Melbourne:
- Building Dimensions: Total roof span = 6.0 metres (6000 mm).
- Roof Pitch: 22.5° (standard roof tile pitch).
- Timber Specification: 120x45 MGP10 structural radiata pine.
- Eave Overhang: 450 mm horizontal overhang.
- Ridge Board: 19 mm thick hardwood ridge.
Step 1 — Calculate Run:
Run = 6.0m / 2 = 3.0 metres (3000 mm).
Step 2 — Calculate Rise:
Rise = 3.0m × tan(22.5°) = 3.0 × 0.4142 = 1.243 metres (1243 mm).
Step 3 — Calculate Rafter Line Length:
Rafter Line Length = sqrt(3000² + 1243²) = sqrt(9000000 + 1544049) = 3247 mm.
Step 4 — Calculate Eave Tail Length:
Tail Length = 450mm / cos(22.5°) = 450 / 0.9239 = 487 mm.
Total Theoretical Length: 3247mm + 487mm = 3734 mm.
Step 5 — Ridge Deduction & Birdsmouth Limit:
- Ridge Deduction: Half ridge thickness = 19mm / 2 = 9.5mm horizontal. Subtract 9.5mm from the 3000mm run before setting top plumb cut.
- Max Birdsmouth: 120mm depth / 3 = 40 mm max vertical notch.
Pitch Angle to Rise & Ratio Reference Table
The table below provides quick reference conversions between roof pitch angles in degrees, pitch ratios, rise rates per metre, and secant rafter multipliers:
| Pitch Angle (°) | Pitch Ratio (in 12) | Rise per Metre Run (mm/m) | Rafter Multiplier (Secant) | Common Australian Roof Material |
|---|---|---|---|---|
| 5.0° | 1.05 : 12 | 87.5 mm/m | 1.0038 | Colorbond Klip-Lok / Trimdek (Low Slope) |
| 10.0° | 2.12 : 12 | 176.3 mm/m | 1.0154 | Colorbond Custom Orb (Corrugated Steel) |
| 15.0° | 3.22 : 12 | 267.9 mm/m | 1.0353 | Low-pitch Concrete / Terracotta Tiles |
| 18.5° | 4.00 : 12 | 334.6 mm/m | 1.0541 | Standard Metal Sheet Roof Incline |
| 22.5° | 4.97 : 12 | 414.2 mm/m | 1.0824 | Standard Australian Roof Tile Pitch (22.5°) |
| 26.6° | 6.00 : 12 | 500.8 mm/m | 1.1180 | Steep Slate & Decorative Tile Pitch |
| 30.0° | 6.93 : 12 | 577.4 mm/m | 1.1547 | High Pitch Architectural & Attic Truss |
| 35.0° | 8.40 : 12 | 700.2 mm/m | 1.2208 | Traditional Federation & Heritage Pitch |
Step-by-Step Pattern Rafter Layout on Site
Follow this site workflow when laying out and cutting your pattern rafter:
- Select a straight, square length of rafter timber free of heavy bow or edge knots.
- Set your circular saw bevel or framing square to the plumb cut angle (equal to the roof pitch angle, e.g. 22.5°).
- Mark the top plumb cut line at the ridge end. Subtract half of the ridge board thickness perpendicular to the plumb line.
- Measure along the rafter edge the exact line length calculated (3,247 mm in our worked example) to locate the point of the birdsmouth.
- Mark the level seat cut (90° minus pitch angle) ensuring the notch depth does not exceed 1/3 of the timber depth.
- Measure the eave tail length along the rafter line (487 mm) and mark the tail plumb cut for fascia board fixing.
- Cut the pattern rafter, hoist it onto the framed plates, and test-fit against the ridge board before batch cutting.
For prefabricated roof truss layouts or engineered timber, see our Roof Truss Calculator and Roof Area Calculator.