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Roof Area Calculator (Australia)

Free roof area calculator for Australian projects. Get on-slope area in m², Colorbond sheet count, tile quantities and sarking rolls based on footprint and pitch in degrees. AS 2050 compliant.

Roof Square Footage Calculator

Roof surface area
1503
square feet
Roofing squares
15.03
1 square = 100 sq ft
Bundles needed
50
3 bundles per square + 10% waste

How to use this calculator

Measure your home’s exterior footprint at the wall plate or just outside the gutter line:

  1. Footprint length — the longest external dimension in metres
  2. Footprint width — the shorter external dimension in metres
  3. Pitch — enter degrees (Australian convention)
  4. Eaves overhang — typically 0.45 to 0.75 m on Australian homes

The calculator returns:

  • Roof on-slope area in square metres (the actual area you sheet or tile)
  • Sheet count for Lysaght Custom Orb / Trimdek / Klip-Lok at common sheet widths
  • Tile count at standard concrete and terracotta gauges
  • Sarking rolls at standard 1500 mm × 30 m roll size

Why this calculation gets done wrong on Australian quotes

Two common errors on Aussie roofing quotes:

Error 1: Using plan area instead of on-slope area. A 200 m² project home at a 22.5 degree pitch has an on-slope area of 216 m². Quoting from plan area underorders by 16 m². For Custom Orb at A$32 per m² installed that is a A$510 shortfall — and the supplier charges premium freight on a top-up order.

Error 2: Ignoring the deep Australian eaves. Climate-appropriate Australian designs use 600 to 900 mm eaves to shade the walls in summer. On a 15 m × 9 m footprint, going from a 300 mm overhang to a 600 mm overhang adds 14 m² to the plan area — that is 7 extra Custom Orb sheets at 6 m length.

The formula

Plan area (m²) = (length + 2×overhang) × (width + 2×overhang)
Slope factor = 1 / cos(pitch in degrees) = √(1 + (rise/run)²)
On-slope area = plan area × slope factor

The calculator does all of this; you supply the four inputs.

How material counts work in Australia

Lysaght Custom Orb (corrugated): 762 mm cover width, supplied in any length up to 16 m. For a 12 m run you order 12 m sheets — count = roof length / 0.762, rounded up. End laps on pitches under 10 degrees per BlueScope guide.

Lysaght Trimdek: 762 mm cover width, similar count to Custom Orb. Concealed-fixed Klip-Lok 700 has 700 mm cover, so count is roof length / 0.700.

Concrete tiles (Boral, Monier, Brikmakers): 9.7 to 10.5 per m² at standard gauge. Boral Cambridge uses 9.5 per m² at 320 mm gauge. Monier Centurion uses 10.5 per m² at 295 mm gauge.

Terracotta tiles (Bristile, La Escandella, Boral): 10 per m² for Marseille profile, 12 per m² for Madison.

Sarking (anti-condensation foil, Bradford Thermoseal, Fletcher Sisalation): standard roll is 1.5 m × 30 m = 45 m² gross, 40 m² net allowing 100 mm side lap. Mandatory under AS/NZS 4200 in Climate Zones 4 to 8.

Battens (timber 38×50 H2 treated, or steel TopHat): metres = on-slope area / gauge in metres. At 295 mm gauge concrete tile that is 3.4 linear metres of batten per m² of roof.

Ridge cap, barge cap and gutter: linear metres of perimeter elements. The calculator outputs perimeter so you can order Lysaght accessories separately.

Adjusting for complex Australian roofs

  • Hip roof (very common in project homes): use the same calc — on-slope area is the same as a gable for the same pitch and outline.
  • L-shape or T-shape: split into rectangles, calculate each separately, sum the results.
  • Skillion (single slope): still uses the slope factor; just use the building footprint without dividing.
  • Queenslander steep pitch with verandahs: calculate the main roof and verandah separately. Verandah pitches are often half the main pitch.
  • Gambrel or Dutch barn: use the gambrel roof calculator.

Sources used in this calculator

Coverage rates are taken from current Lysaght technical manual (BlueScope), Boral and Monier tile data sheets, and AS 2050:2018 (Installation of roof tiles). Sarking widths and laps follow AS/NZS 4200. Waste allowances are calibrated against Master Builders Australia rate guides and ARC technical bulletins.

For installed pricing per m² in your area, hipages and ServiceSeeking publish state-level averages; confirm with at least three local quotes before committing.

Frequently asked questions

How do I work out roof area in square metres for an Australian house?
Take the building footprint length by width to get plan area, then multiply by the slope factor for your pitch in degrees. A 15 m by 9 m project home with a 22.5 degree roof has a 135 m² footprint and a slope factor of 1.082, giving roughly 146 m² of on-slope area before adding eaves.
What is a typical roof pitch in Australia?
Most Australian project homes are between 15 and 25 degrees. Colorbond and Zincalume corrugated metal works down to 5 degrees. Concrete tiles need 15 degrees minimum per AS 2050, terracotta tiles 17.5 degrees minimum. Steeper pitches (30+ degrees) are common in Queenslander and Federation styles for cyclone shedding.
How many Colorbond sheets do I need per square metre?
Lysaght Custom Orb at 762 mm cover width: 1 linear metre of sheet covers 0.762 m². For a 6 m sheet length you cover 4.57 m² per sheet (less side and end laps). BlueScope recommends 150 mm minimum end lap on pitches under 10 degrees, so always order 5 percent extra and round up to standard sheet lengths.
How many tiles per m² for an Australian roof?
Concrete tiles such as Boral Cambridge or Monier Centurion run about 9.7 to 10.5 per m² depending on the profile. Terracotta tiles like Bristile Marseille are similar at around 10 per m². Always confirm the batten gauge from the manufacturer datasheet — this varies with pitch under AS 2050.
Should I include eaves overhang?
Yes. Australian eaves typically project 450 to 750 mm beyond the wall (deeper than UK or US homes for shading). On a 15 m by 9 m house a 600 mm overhang adds about 30 m² to the footprint — roughly 22 percent more roof material. The calculator offsets all four sides automatically.

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