Roof Type
Building Dimensions
Gable Roof Options
Material Options
Footprint Area—
Roof Slope Area—
Ridge Length—
Number of Sheets / Tiles—
Underlayment Area (×1.1)—
Approx. Material Weight—
—
Results are estimates. Add 10–15% for cuts and overlaps. Always verify with a licensed roofing contractor before purchasing materials.
How to Calculate Roof Area
Roof area differs from floor area because it accounts for the slope of the roof surface. For a gable roof the slope area is calculated by dividing the full width into two halves, finding the slant height of each side (using Pythagoras — horizontal run and vertical rise), then multiplying by the roof length plus overhangs. A 10m × 8m building with a 25° pitch has a slant length roughly 10% longer than the flat half-width, resulting in about 20% more roof area than floor area.
Key steps:
- Find the footprint: length × width of the building.
- Find the slant height: sqrt((half-width + eave overhang)² + (half-width × tan(pitch))²).
- Multiply by roof length: (building length + 2 × gable overhang) × slant height × 2 sides.
- Add wastage: always add 10–15% for cuts, overlaps and breakages.
Common Roof Pitch Angles and Their Uses
- 10–15°: Near-flat. Requires waterproof membrane. Common on commercial buildings.
- 18–25°: Low to standard pitch. Typical for metal sheet roofing in Australia and NZ.
- 25–35°: Standard residential. Suits most tile and metal products. Good water shedding.
- 35–45°: Steep pitch. Traditional terracotta and slate. Excellent drainage, higher wind load.
- 45°+: Very steep. Decorative or heritage styles. High material and labour cost.
Frequently Asked Questions
Roof pitch is the angle or steepness of a roof expressed in degrees. Common residential pitches range from 15° to 45°. A 25° pitch is typical for most Australian homes. Higher pitches shed water more effectively but use more material and increase wind load on the structure.
Standard eave overhang is 450–600mm (0.45–0.6m) and gable overhang is 200–400mm (0.2–0.4m). Overhangs protect walls from rain, reduce solar heat load in summer and are required by many building codes. In high-wind zones, overhangs may be restricted to reduce uplift forces — always check your local requirements and council regulations.
A gable roof has two sloping sides that meet at a central ridge, with vertical triangular gable ends at each end of the building. A hip roof has four sloping sides — two trapezoidal and two triangular — all meeting at a ridge or central apex. Hip roofs are more wind-resistant and require no gable end cladding, but they are more complex to frame and use significantly more roofing material due to the additional hip rafter runs.
A 100m² roof using standard 900×2400mm metal sheets (2.16m² each) with 10% wastage requires approximately 52 sheets. Tile roofs using 330×420mm clay or concrete tiles need around 800–850 tiles for the same area. Asphalt shingles are typically sold by the square (9.3m²), so a 100m² roof needs about 12 squares. Always add 10–15% for cuts, waste and future repairs.