Roof Area Calculator
Your roof is bigger than your floor plan — here's how much.
Find the true roof surface area from your building's footprint and roof pitch — for tiles, sheets, sarking or solar. Enter the length, width and pitch and the calculator applies the slope factor, adds your eaves, and can estimate the tiles or sheets you'll need.
Estimates from roof geometry — the slope-factor method. Confirm material coverage on the product data sheet and get a roofer's take-off for complex roofs.
How the roof surface area is worked out
Roof surface = footprint area × slope factor
The calculator takes your building's footprint — length × width — and multiplies it by the slope factor, which is 1 ÷ cos(pitch). A pitched roof is a sloping plane, and a sloping plane is always longer than the flat base beneath it (the long side of a right-angled triangle), so the true roof surface is always more than the floor area. If you add eaves, they enlarge the footprint first, then the slope factor is applied to the whole overhanging outline.
Worked example
A 10 × 8 m building is 80 m² of footprint. At a 22.5° pitch the slope factor is ×1.082, so the actual roof surface is 80 × 1.082 ≈ 86.6 m² before any eaves — about 8% more roof than floor. Roofing, sarking, battens and solar space are all ordered to this surface area, never the floor plan.
Pitch, slope factors & eaves
The steeper the roof, the more surface for the same footprint. The slope factor is 1 ÷ cos(pitch):
- 15° — ×1.035 (adds 3.5%)
- 20° — ×1.064
- 22.5° — ×1.082 (typical tiled)
- 25° — ×1.103
- 30° — ×1.155 (adds 15.5%)
Metal roofs often sit low at 10–15°, tiled roofs usually run 15–25°, and heritage or steep-gable designs are 30° and above. Eaves matter too: the roof overhangs the walls — typically 450–600 mm on Australian homes — and that overhang is real roof. Add the eaves to the footprint before the slope factor, or you'll under-order.
Tiles, sheets and complex roofs
Tiles and sheets
Once you have the roof surface area, multiply by the coverage. Concrete and terracotta tiles run about 10 per m² (varies 10–16 by profile), so a 120 m² roof needs roughly 1,200 tiles plus about 10% waste for cuts, hips, valleys and breakage — around 1,320. Metal roofing is ordered in linear metres of sheet at the product's cover width instead. Always confirm the exact coverage on the product's data sheet.
Gable, hip and complex roofs
For a simple gable or hip roof, the total surface is footprint × slope factor — the two planes both tilt at the pitch. Split L-shaped or multi-pitch roofs into rectangles and calculate each part at its own pitch, then sum them; add dormers and verandahs separately. This tool estimates the area, not a full take-off — for anything complex, get a roofer to measure it.
❓ Frequently asked Frequently asked questions
How do I calculate roof area?
Roof surface area = footprint area × slope factor, where the slope factor is 1 ÷ cos(pitch). Take the building's length × width, add the eaves each side, then multiply by the slope factor for your pitch. A 10 × 8 m footprint is 80 m² of floor, and at a 22.5° pitch the slope factor is ×1.082, so the actual roof surface is about 86.6 m² before eaves.
Why is the roof bigger than the floor plan?
A pitched roof is a sloping plane, and the sloping surface is always longer than the flat base beneath it — like the long side of a right-angled triangle. The slope factor 1 ÷ cos(pitch) captures exactly how much longer. That's why roofing, sarking, battens and solar space are always ordered to the roof surface, never the floor plan, and the steeper the roof the bigger the gap.
How do I find my roof's pitch?
Roof pitch is the angle from horizontal, in degrees, or as a rise-over-run ratio. Hold a level horizontally against the roof line, mark 300 mm along it, and measure straight down to the roof — that drop in mm ÷ 300 gives the ratio, and its arctangent is the angle (for example a 125 mm rise over 300 mm ≈ 22.6°). A phone level app on the underside of a rafter also works. If you truly can't tell, 22.5° is a fair typical tiled-roof default.
Do I need to include the eaves?
Yes. The roof overhangs the walls at the eaves — typically 450–600 mm on Australian homes — and that overhang is real roof you have to cover. Add the eaves to the footprint before the slope factor is applied. On a 10 × 8 m building, 600 mm eaves all round grow the footprint from 80 m² to about 103 m², which becomes roughly 111 m² of roof at 22.5°.
How many roof tiles will I need?
Concrete and terracotta roof tiles run about 10 per m², though it varies from roughly 10 to 16 depending on the profile. A 120 m² roof needs about 1,200 tiles, plus around 10% for cuts, hips, valleys and breakage — roughly 1,320. Metal roofing is ordered in linear metres of sheet at your cover width instead. Always confirm the exact coverage on the product's data sheet.
Does it work for hip and complex roofs?
For a simple gable or hip roof, the total surface area is the footprint area × slope factor — this calculator's method — because both roof planes tilt at the pitch. Complex roofs with different pitches, dormers or multiple sections should be split into rectangles and each part calculated at its own pitch, then added. For a roughly rectangular home the single-footprint estimate is close enough to order materials with a waste allowance.
Where these figures come from
This is a geometry-based estimate — the roof surface area comes from your building's dimensions and pitch, not from any external rate table. The slope factor is a standard trigonometric result, and the material guidance reflects typical Australian trade practice.
- Slope factor — the roof surface is the footprint area × 1 ÷ cos(pitch). This is exact geometry: at 15° the factor is 1.035, at 22.5° it is 1.082, at 25° 1.103 and at 30° 1.155.
- Eaves — Australian homes typically overhang the walls 450–600 mm; that overhang is real roof and is added to the footprint before the slope factor.
- Tile and sheet coverage — concrete and terracotta tiles run about 10 per m² (10–16 by profile), and metal roofing is ordered in linear metres of sheet at the product's cover width. Coverage varies by product, so confirm it on the data sheet you buy.
Last checked: July 2026. The geometry is exact, but material coverage varies by product and complex roofs need a proper take-off. Always confirm coverage on the product's data sheet and get a roofer's measure-up for hips, valleys, dormers and multi-pitch roofs.
Select the question that matches where you are right now.
The headline number is your actual roof surface area — the footprint (including eaves) multiplied by the slope factor for your pitch. It's what you order roofing, sarking or insulation to, and it's always larger than the floor plan.
Order tiles, sheets, sarking or battens to this surface area, and add the waste allowance for cuts and breakage. For solar, it's the usable roof plane you're fitting panels onto.
It's not a full take-off. It doesn't measure individual hips and valleys, flashings, ridge capping, or a multi-pitch roof section by section — for those, get a roofer to measure it.
The slope-factor geometry is exact for a simple gable or hip. Real quantities depend on the tile or sheet profile, so confirm coverage on the product's data sheet before you order.
Three things move the roof area: the footprint, the pitch, and the eaves you add around it.
A shallow 15° roof adds only 3.5% over the footprint; a steep 30° adds 15.5%. Get the pitch right — measure it as rise over a 300 mm run — because it sets how much extra material the slope demands.
450–600 mm of overhang each side enlarges the footprint before the slope factor. On a 10 × 8 m house, 600 mm eaves add about 23 m² of footprint — skip them and you under-order by a fifth.
A simple gable or hip is footprint × slope factor. L-shaped, multi-pitch or dormered roofs need splitting into rectangles at their own pitches and summing — the single-footprint estimate is best for a roughly rectangular home.
A few habits stop you over- or under-ordering roofing.
Use the outline the roof actually covers — the walls plus the eaves overhang — not the internal floor area. Check the overhang on all four sides; verges and gable ends overhang too.
Cuts around hips, valleys, penetrations and breakage all consume material. About 10% is typical for tiles; leaner for simple gables, more for cut-up roofs. Round the order up.
Tiles per m² and sheet cover widths vary by product. The area is the constant; the number of tiles or metres of sheet depends on the profile you choose, so check the manufacturer's figure.
The roof is one line in a bigger renovation. Model the surfaces and the finishes around it too.