Roof Rafter Calculator (Australia)

Roof rafter calculator — what this returns

This tool sizes cut-roof common rafters for Australian residential builds and gives you the order quantity for the timber yard:

• Rafter length per piece, including the eave overhang tail
• Count per slope and total (both sides combined)
• Total linear metres of timber
• Recommended section (90×140 / 190 / 240 / 290 mm) auto-selected for your snow load
• Timber cost in AUD at 2026 MGP10 KD pricing, with an 8% waste allowance

The math follows AS 1684.2 Supplement 4 (light timber framing — domestic span tables) for prescriptive rafter spans, with snow-load and spacing de-rating applied so the size recommendation tracks your actual ground snow load (Pg from AS/NZS 1170.3) rather than a generic non-alpine 0 kPa default.

How rafter sizing works under Australian practice

Australian residential rafter design lives at the intersection of three documents: the prescriptive tables in AS 1684.2 Supplement 4 (covering most flat ground in non-cyclonic regions), AS 1684.3 for cyclonic regions C1/C2/C3, and AS 1684.4 (simplified non-cyclonic) for owner-builders. NCC 2022 Vol 2 Part 3.2.4 endorses all three as Deemed-to-Satisfy solutions. For atypical loads (alpine snow, structural ridge beams, vaulted ceilings), only the engineered route to AS 1720.1 applies.

Loads. Use AS/NZS 1170.1 for permanent (dead) loads — typical light-frame roof with Colorbond Trimdek + sarking + R3.5 batts + gypsum ceiling = 0.45 kPa. Tile roof (Bristile concrete) = 0.55 kPa. Slate Welsh = 0.70 kPa. AS/NZS 1170.3 nominates regional snow per Section 5; sub-alpine regions take 0 kPa, alpine zones reach 2.5 kPa above 1700 m. Wind to AS/NZS 1170.2 — N1/N2/N3/N4 in non-cyclonic, C1/C2/C3 in cyclonic.

Span direction. AS 1684 spans are horizontal projection — wall plate to ridge measured flat. The calculator divides full building span by 2 and adds the eave overhang for the actual rafter length, then sizes the section against the horizontal half-span only.

Section choice. Australian timber merchants stock MGP10 (Machine Graded Pine) and MGP12 in metric sizes 90×45, 90×90, 90×140, 90×190, 90×240 and 90×290 mm. F7 KD pine (most common nominal grade) corresponds to MGP10 strength. F17 hardwood (spotted gum, blackbutt, ironbark) lets you go down a section size in BAL-29 zones where the eaves see fire embers.

Spacing. 450, 600, or 900 mm centres in line with AS 1684.2 Section 7. The base table is 600 mm; tighter spacing (450 mm) gives a ~10% bonus to allowable span; 900 mm is permitted for Colorbond and other lightweight metal roofs in non-cyclonic regions and drops allowable span by ~15%.

Worked example — 9.5 m × 12 m four-bedroom, 22.5° pitch, N2 wind, BAL-12.5

Inputs: 9.5 m span, 12 m long, 22.5° pitch, 600 mm centres, 0 kPa snow (Geelong VIC), MGP10, 0.6 m overhang.

Computation:

slopeFactor      = √(1 + tan(22.5°)²) = 1.082
half-span run    = 4.75 m
rafter length    = (4.75 + 0.6) × 1.082 = 5.79 m each
count per side   = ceil(12 / 0.6) + 1 = 21
total rafters    = 42
total timber     = 42 × 5.79 = 243 m
90×290 MGP10 max ≈ 4.95 m at 0 kPa, 600 mm — exceeds 4.75 m, just OK
                  → in practice you'd specify 90×290 F17 hardwood or LVL ridge beam
peak height      = 4.75 × tan(22.5°) = 1.97 m above plate

For a 9.5 m clear span at the prescriptive limit, most builders specify a structural ridge beam (steel UB 250×130 or LVL 360×63 mm) reducing the rafter span to 4.75 m. This drops the rafters to 90×190 mm MGP10 — much cheaper timber package overall.

Section table — AS 1684.2 Supp 4 at 0 kPa snow, 600 mm centres, MGP10, N2 wind

Source: AS 1684.2 Supplement 4 (Tasman version, current). Values for L/300 instantaneous deflection, R10 ceiling load reduction, no rafter-tie reduction. Above 4.95 m, engineered LVL or steel UB ridge beams are mandatory.

For higher snow loads (alpine zones):

• 0.6 kPa (Mt Hotham VIC, lower altitude): multiply allowable span by 0.84
• 1.5 kPa (Falls Creek, Thredbo Top Stations): multiply by 0.62
• 2.0 kPa (>1700 m elevation): multiply by 0.55 — engineer mandatory

For cyclonic regions (C1/C2/C3), use AS 1684.3 — table values reduce by 15–25% depending on region.

Pricing — Q1 2026 reference data

Big-box (Bunnings) and pro-yard (Mitre 10, Stratco, Trade Tools Direct) pricing for MGP10 KD pine, mainland eastern states:

• 90×140 mm MGP10: A$9.50–$10.20/m at Bunnings, A$8.90–$9.50/m at pro yards
• 90×190 mm MGP10: A$13.20–$14.00/m
• 90×240 mm MGP10: A$18.00–$19.00/m
• 90×290 mm MGP10: A$23.50–$24.50/m

Specials and engineered:

• LVL 240×63 mm (Lewis Timber): A$32–$36/m
• LVL 360×63 mm structural ridge: A$48–$54/m
• Glulam 90×240 mm GL13 (Hyne): A$42–$48/m
• F17 spotted gum 90×240 mm: A$58–$66/m

For the 9.5 × 12 m worked example with 90×290 mm MGP10, lumber alone is about 243 m × A$24/m = A$5,830, plus A$320–$420 for ridge board (75×290 mm), framing brackets, cyclone ties (where N3+) and bracing. Crew time for a cut roof on this scale: 2 carpenters × 22–28 hours.

Trussed-rafter alternative for the same building (Mitek or Multinail prefab): roughly A$3,800–$4,400 for the truss order delivered, plus A$650–$900 for crane and crew on set day. Trusses are clearly cheaper for new-build standard houses; cut roofs win on heritage extensions and complex hip-and-valley roofs that don’t suit prefab profiles.

Code requirements (Australia)

• NCC 2022 Vol 2 Part 3.2.4 — Light timber framing for Class 1 and 10 buildings. AS 1684.2/.3/.4 are the cited Deemed-to-Satisfy standards.
• AS 1684.2:2010 Section 7 — Rafter span tables for non-cyclonic regions (N1–N4). Supplement 4 is the residential subset.
• AS 1684.3:2010 — Cyclonic regions C1/C2/C3. Tie-down is more onerous: cyclone straps at every rafter-to-plate connection plus rod tie-down to the slab.
• AS 1684.4:2024 — Simplified design for owner-builders, conservative limits.
• AS 1720.1:2010+A1 — Engineered timber design. Required when prescriptive tables don’t cover your case.
• AS/NZS 1170.0/1/2/3 — Structural design actions for permanent, imposed, wind and snow loads.
• AS 3959:2018 — Bushfire construction. Drives species and treatment selection for rafters in BAL-FZ, BAL-40 and BAL-29 zones (typically need H3 treated or hardwood F17).
• AS 4055:2021 — Wind loads for housing. Maps the building site to N1–N4 (non-cyclonic) or C1–C3 (cyclonic) wind regions.
• AS/NZS 4859.1:2018 — Thermal insulation materials. Drives the depth of insulation between/over rafters for compliance with NCC J1.2 (thermal performance).

Cut roof vs trussed rafter decision

Build rafters on site when:

• Your span is under 4.5 m and within prescriptive AS 1684.2 tables — no engineer needed
• You’re framing a vaulted ceiling with exposed rafter feet (Federation, Hamptons styles)
• Heritage Council consent requires retention of original cut-roof construction
• The roof has irregular hips, valleys and dormers that don’t fit standard truss profiles

Order trussed rafters when:

• Span is over 5 m — you’re outside the prescriptive tables either way
• Building length is over 10 m — labour savings dominate
• You want a clear-span loft (study, room-in-roof) with no internal bearing wall
• The build is on a fixed-price contract — trusses lock in the framing budget cheaper

For typical Australian new-build single-storey homes (9–12 m span, 12–18 m long, 22.5°–25° pitch), the timber package costs about the same and trusses install in a third of the labour — the roof truss calculator gives you the side-by-side cost comparison.

Pair with these calculators

• Roof pitch calculator — convert between degrees, percentage and slope factor before specifying.
• Roof truss calculator — get a trussed-rafter quote to compare against your cut-roof estimate.
• Roof area calculator — total roof surface for sheeting, sarking and tile or Colorbond counts on the same job.

When you change any input above, the output updates immediately. Print the page with your inputs and take it to the timber merchant for a take-off quote — Bowens, BlueScope and Tilling Timber all honour pre-priced take-off lists for 14 days at Trade-portal pricing.