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Roofing Calculator
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Roofing Calculation Formula
True Roof Area: Plan Area × Pitch Multiplier
Area + Waste: True Roof Area × (1 + Waste ÷ 100)
Area + Waste: True Roof Area × (1 + Waste ÷ 100)
Pitch Multipliers: 5° = 1.004, 10° = 1.015, 15° = 1.035, 20° = 1.064, 22.5° = 1.082, 25° = 1.103, 30° = 1.155
Note: Waste allowance accounts for cutting, overlaps and hip/valley losses.
Roofing Calculator — Roof Area, Pitch & Materials
Australia approved 171,000 new dwellings in 2025 (ABS). Every one needs a roof, and getting material quantities right means fewer delays and lower costs. Calculate true roof area from plan dimensions using pitch multipliers below.
Pitch Multipliers — Plan Area to Roof Area
| Pitch (degrees) | Rise : Run | Multiplier | Example (200m² plan) |
|---|---|---|---|
| 5° | 1:11.4 | 1.004 | 201 m² |
| 10° | 1:5.7 | 1.015 | 203 m² |
| 15° | 1:3.7 | 1.035 | 207 m² |
| 20° | 1:2.7 | 1.064 | 213 m² |
| 22.5° | 1:2.4 | 1.082 | 216 m² |
| 25° | 1:2.1 | 1.103 | 221 m² |
| 30° | 1:1.7 | 1.155 | 231 m² |
Material Coverage Rates
| Material | Coverage | Sheets/Tiles per 100m² |
|---|---|---|
| Colorbond Trimdek (762mm) | 0.762m wide | ~18 sheets (7.2m long) |
| Colorbond Custom Orb | 0.762m wide | ~18 sheets (7.2m long) |
| Colorbond Klip-Lok | 0.700m wide | ~20 sheets (7.2m long) |
| Concrete Roof Tiles | ~9.5 per m² | ~950 tiles |
| Terracotta Tiles | ~13 per m² | ~1,300 tiles |
Tips for Roofing Estimates
- Always calculate from true roof area (plan area × pitch multiplier), not floor area.
- Add 5–10% for overlaps, ridge capping, and waste.
- Include hip and valley flashings — typically 1 linear metre per 3–4 m² of hip/valley roof.
- Don’t forget fascia, guttering, and downpipes in your material list.
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