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Skillion Roof Rise Calculator

Calculate the rise of a skillion (mono-pitch) roof from span and pitch in degrees. Returns rafter length, slope factor, and surface multiplier to AS 1684 and NCC 2022.

Skillion Roof Rise Calculator

Calculate from:
Rise (m)
1
Angle
14.04°
Rafter length (m)
4.432
Slope factor
1.031
Multiply footprint area by this for true roof surface area.

What this calculator does

This tool computes the vertical rise of an Australian skillion (mono-pitch) roof from the span and either the pitch in degrees or a target rise. It returns the rise, the slope angle, the rafter length (including eaves overhang), and the slope factor — the multiplier you apply to the plan area to get the true roof surface area for ordering Colorbond, zincalume, or insulated panel.

Inputs are in metres. Output is consistent with AS 1684.2:2010 (Non-Cyclonic Areas) and AS 1684.3:2010 (Cyclonic Areas) for residential timber construction, and AS 1562.1:2018 for metal roof and wall cladding.

How skillion roof maths works

A skillion roof is a right triangle in section. The horizontal leg is the span, the vertical leg is the rise, and the hypotenuse is the rafter along the slope. The relationships are:

  1. Rise = span × tan(angle) when working from degrees.
  2. Pitch = atan(rise / span) when working from rise.
  3. Slope factor = sec(angle) = 1 / cos(angle) — multiply plan area by this for actual roof area.
  4. Rafter length = (span + overhang) × sec(angle) — rafter material per piece.

The slope factor matters because Colorbond and zincalume sheet are cut and quoted per square metre of actual roof surface, not per square metre of footprint.

Reference test cases

SpanAngleOverhangRiseRafter lengthSlope factor
4.0 m14°0.6 m0.997 m4.74 m1.031
3.0 m5°0.45 m0.262 m3.46 m1.004
5.0 m22°0.6 m2.020 m6.04 m1.079
6.0 m10°0.75 m1.058 m6.85 m1.015

The first row is the canonical Australian backyard granny flat at 14° pitch — common because that angle suits Lysaght Custom Orb (5° minimum), provides positive drainage in NSW Hunter and Sydney rainfall events, and works as a low-pitch standing seam Klip-Lok 406 roof.

Materials and minimum pitches

AS 1562.1:2018 minimum slope for metal roof cladding:

  • 1° (1 in 60) — Standing seam concealed-fix (Lysaght Klip-Lok 406, Stramit Speed Deck Ultra, Fielders KingKlip 700, Metroll Metdek). Kingspan Optimo and Bondor SolarSpan insulated panels.
  • 2° (1 in 30) — Trapezoidal corrugated (Lysaght Custom Orb, Stramit Corrugated, Fielders Tuffline) with sealed end-laps and neutral-cure silicone.
  • 5° (1 in 11.4) — Symmetrical corrugated with standard 150 mm overlap, no sealant.
  • 10° — Concrete tile (Monier, Boral, CSR Bristile) with anti-ponding boards at the eaves.
  • 15° — Terracotta tile (Monier La Escandella, CSR Bristile, Boral Stratos) with sarking under.

Cyclonic regions (AS 1684.3, NCC 2022 Volume Two Part 2.1.5) require manufacturer’s cyclonic detail sheets — BlueScope Lysaght, Stramit, and Fielders all publish C1 (60 m/s) and C2 (70 m/s) fastener schedules with 14g × 65 mm Type 17 self-drillers at every corrugation crest, doubled at perimeter zones.

For a typical Sydney/Melbourne/Brisbane backyard granny flat (4 m × 6 m skillion) the cheapest compliant build is a 5° to 10° Lysaght Custom Orb Colorbond roof on 70 × 45 MGP10 rafters at 600 mm centres. Q1 2026 pricing from Bunnings Trade, Reece, and Tradelink runs A$95 to A$140/m² installed for Custom Orb in Surfmist or Monument finish, including paper-faced sarking, top hat purlins, and box gutter to AS/NZS 3500.3.

Code and authority references

  • NCC 2022 Volume Two — National Construction Code, Performance Provisions for residential.
  • AS 1684.2:2010 — Residential timber-framed construction — Non-Cyclonic Areas.
  • AS 1684.3:2010 — Residential timber-framed construction — Cyclonic Areas.
  • AS 1684.4:2010 — Residential timber-framed construction — Simplified — Non-Cyclonic Areas.
  • AS 1720.1:2010 — Timber structures — Design methods.
  • AS 1562.1:2018 — Design and installation of sheet roof and wall cladding — Metal.
  • AS/NZS 1170.0/1/2/3 — Structural design actions.
  • AS 3959:2018 — Construction of buildings in bushfire-prone areas (BAL-LOW to BAL-FZ).
  • AS 4055:2021 — Wind loads for housing (N1 to N6, C1 to C4).
  • AS/NZS 4859.1:2018 — Thermal insulation materials for buildings.

Practical notes for Australian skillion roofs

Bushfire Attack Level (BAL) compliance. AS 3959:2018 Section 9 sets construction requirements at BAL-12.5, BAL-19, BAL-29, BAL-40, and BAL-FZ. Skillion roofs in BAL-29+ require: (a) non-combustible cladding only — Colorbond or zincalume sheet, (b) ember-proof fascia and gutter system (Bushfire-Compliant FB-rated mesh in eaves vents), (c) FRL 60/60/60 ceiling system at BAL-FZ, (d) sealed eaves to AS 3959 Section 9.5. The BlueScope BAL-FZ Bushfire Detail Sheets and Stramit BAL Compliance Guide provide compliant skillion details.

Cyclonic regions. N4, C1, C2 wind classes per AS 4055 require AS 1684.3 cyclonic timber framing — typically 90 × 45 MGP10 rafters at 600 mm centres for C1 reduce to maximum 1.4 m span, vs 2.6 m N3, vs 1.9 m N2. Cyclone tie-down rods, top hat purlins fixed with 14g screws at every crest, and continuous structural ridge are mandatory.

Heritage and conservation. Properties listed on the State Heritage Register (NSW Heritage Council, Heritage Victoria, Queensland Heritage Council, Heritage SA, ACT Heritage, Heritage WA, Tasmanian Heritage Council, Northern Territory Heritage) require a Heritage Impact Statement before any roof works. Most heritage skillion lean-tos must retain the original pitch within ±2° and original cladding type — corrugated zincalume or terne-coated lead.

Insulation and condensation. NCC 2022 Volume Two Part 13.2 (formerly 3.12) sets minimum total R-value at R3.0 for climate zones 1 to 4, R4.1 for zones 5 to 7, and R5.0 for zone 8 (alpine). Skillion roofs are particularly susceptible to condensation because there is no attic space — NCC 2022 Part F8.5 (mandatory from 1 May 2024 in CZ 6, 7, 8) requires sarking with vapour permeance ≥ 3.0 µg/N·s and continuous eaves intake at 25,000 mm²/m. Use the Roof Vent Calculator to size eaves and ridge ventilation.

Sources and authority data

  • Master Builders Australia — Roofing Handbook 2024 edition.
  • ARC (Australian Roofing Contractors) — Tech Bulletin 21 on skillion roof minimum slopes.
  • BlueScope Lysaght — Custom Orb, Klip-Lok 406, Trimdek installation manuals 2025; BAL-FZ Bushfire Detail Sheets; Cyclonic Wind Detail Sheets.
  • Stramit — Corrugated, Speed Deck Ultra, Monoclad installation manuals 2025.
  • Fielders — Tuffline, KingKlip 700, Centenary installation manuals 2025.
  • hipages 2026 — Granny flat and lean-to cost data, A$1,800 to A$3,500/m² turnkey for granny flat builds.
  • Bunnings Trade, Reece, Tradelink — Q1 2026 retail pricing for Custom Orb sheet, MGP10 framing timber, top hat purlins, and box gutter to AS/NZS 3500.3.

Frequently asked questions

What is a typical skillion roof pitch in Australia?
Most Australian skillion roofs sit between 5° and 22°. The NCC 2022 Volume Two Part 7.2 references AS 1562.1 for metal roof and wall cladding, which sets minimum slope at 1° (1 in 60) for fully welded metal roof, 2° (1 in 30) for trapezoidal corrugated profile (Lysaght Custom Orb, Stramit Corrugated, Bondor Mini Orb), and 5° for symmetrical corrugated under standard end-lap. Bushfire Attack Level (BAL) zones FZ, BAL-29, and BAL-40 under AS 3959:2018 effectively require non-combustible Colorbond or zincalume sheet, which performs reliably from 2° upward. For a typical backyard granny flat or carport, a 5° to 10° Colorbond skillion is the most common build.
How do I calculate the rise on a skillion roof from span and angle?
Rise = span × tan(angle). For a 4 m span at 14° pitch, the rise is 4 × tan(14°) = 0.997 m. To convert pitch to angle, take the inverse: angle = atan(rise / span). The rafter length along the slope is span / cos(angle), and the slope factor (multiplier from plan area to true roof area) is 1 / cos(angle), or equivalently sqrt(1 + (rise/span)²). For 14° that factor is 1.031, so a 4 m × 6 m granny flat has a roof surface of 24 × 1.031 = 24.7 m² of Colorbond to order.
What is the minimum pitch for a Colorbond skillion roof?
AS 1562.1:2018 Table 3.1 sets the minimum roof slope for trapezoidal corrugated metal sheet (Lysaght Custom Orb, Stramit Corrugated, Fielders Tuffline) at 5° (1 in 11.4) for symmetrical end-lap with 150 mm overlap, or 2° (1 in 30) with sealed end-laps using neutral-cure silicone and minimum 200 mm overlap. Standing seam concealed-fix (Lysaght Klip-Lok 406, Stramit Speed Deck Ultra, Fielders KingKlip 700) goes to 1° (1 in 60) per the manufacturer's BAL-rated Bushfire Detail Sheets. Bondor SolarSpan insulated panels and Kingspan Optimo go to 1° (1 in 60). For BAL-FZ (Flame Zone) bushfire areas, AS 3959:2018 Section 9.5 effectively requires standing seam at 1° to 3° because fastener heads must be concealed.
How does the eaves overhang affect the rafter length?
The overhang adds to the horizontal span, so the rafter timber grows in proportion. For a 4 m span at 14° with a 600 mm eaves projection, the rafter length is (4 + 0.6) × sec(14°) = 4.6 × 1.0306 = 4.74 m. AS 1684.2:2010 requires a minimum 300 mm overhang at the eaves on N1, N2, N3 wind classifications and 600 mm minimum on N4, C1, C2 cyclonic regions to throw water clear of the wall, but BAL-29 to FZ bushfire zones cap the overhang at the manufacturer's BAL detail (typically 750 mm with non-combustible eaves lining like FibreCement). The Bushfire Verification Method AS 3959 Section 7.5 requires non-combustible eaves construction in BAL-29+.
What is the slope factor and why does it matter for skillion roofs?
The slope factor is sec(angle) — the multiplier from plan area to true roof surface area. For 5° it is 1.004, for 10° it is 1.015, for 14° it is 1.031, for 22° it is 1.079, for 30° it is 1.155. Multiply the building footprint by the factor to get the actual area of Colorbond, zincalume, or insulated panel to order. A 4 m × 6 m granny flat at 10° pitch has 24 m² of footprint and 24.36 m² of roof — round up to standard sheet lengths from BlueScope Lysaght (3.0 m, 3.6 m, 4.2 m, 5.4 m, 6.0 m for Custom Orb) or Stramit (cut to length up to 14 m).
Can I build a skillion roof in a cyclonic region?
Yes, but AS 1684.3:2010 (Cyclonic Areas) imposes substantially shorter spans and additional cyclone-resistant detailing. C1 (cyclonic, 60 m/s gust) and C2 (severe cyclonic, 70 m/s gust) regions require: (a) increased rafter sizing per AS 1684.3 Table 7.6 — typically 90 × 45 MGP10 to 1.4 m for C1 vs 2.6 m N3, (b) cyclone tie-down rod from rafter to bottom plate per AS 1684.3 Section 9, (c) Colorbond sheet fixed at every corrugation crest with 14g x 65 mm Type 17 self-drillers, double on perimeter, (d) compliance with manufacturer's cyclonic Bushfire and Cyclonic Wind Detail Sheets (BlueScope, Stramit, Fielders all publish C1/C2 fastener schedules).
What rafter size do I need for a skillion roof span?
AS 1684.2:2010 Table 7.5 (Wind Class N2, single-span common rafter, 600 mm centres, 0.30 kN/m² roof load) gives: 90 × 35 MGP10 to 1.6 m, 90 × 45 to 1.9 m, 120 × 45 to 2.6 m, 140 × 45 to 3.1 m, 190 × 45 to 4.4 m, 240 × 45 to 5.7 m. F11 hardwood adds about 18 percent to spans, F17 about 28 percent. Wind Class N3 reduces spans by about 15 percent, N4 by 28 percent. The [Roof Rafter Calculator](/calculators/roof-rafter-calculator/) handles species, grade, spacing, wind class, and snow zone (sub-alpine NSW Snowy and Vic Alps) together.
How do I find the high wall height for a skillion lean-to extension?
The high wall is taller than the low wall by exactly the calculated rise. For a 4 m span at 14° pitch attached to a 2.4 m high outer wall, the abutment to the existing house sits at 2.4 + 4 × tan(14°) = 3.4 m AGL. NSW BCA Building Code single dwelling exempt-development thresholds limit attached lean-to extensions to 6 m maximum height, 1 m setback from boundary, and 4.5 m projection per the State Environmental Planning Policy. Always confirm with the local council before relying on exempt or complying development under SEPP (Exempt and Complying Development Codes) 2008.

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