Roof Area Calculator
The real surface area of a pitched roof from the building's footprint, eaves and pitch — and how many tiles or sheets it takes with a waste allowance.
The footprint is length × width, each side extended by the eaves.
| Flat footprint | 80 |
|---|---|
| 22.5° roof surface | 86.59 |
How the roof area calculator works
The footprint is length × width, each side extended by the eaves. A pitched roof is larger than its footprint by the slope factor 1 ÷ cos(pitch): a 22.5° roof adds about 8%, a 35° roof about 22%. Tiles are roof area × tiles per m² × (1 + waste%), rounded up.
Formula: Footprint = (L + 2 × eaves) × (W + 2 × eaves) · roof area = footprint ÷ cos(pitch) · tiles = ⌈area × per m² × (1 + waste%)⌉
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 & slopePitch, 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 & complex roofsTiles, 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
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.
- Further guidance — ACCC — consumer rights on quotes, contracts and guarantees.
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.
Understanding your resultSelect 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.
What to do with itOrder 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.
What it is notIt'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.
AccuracyThe 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.
Pitch drives the slope factorA 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.
Eaves are real roof450–600 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.
Roof shape and complexityA 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.
Measure the real footprintUse 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.
Add a waste allowanceCuts 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.
Confirm coverage on the data sheetTiles 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.
Tiling elsewhereWorking out floor or wall tiles for a finished area?
Tile calculator → Pouring a slabSizing the concrete for a slab, driveway or shed floor?
Concrete slab calculator → Painting the jobEstimate the paint and cost for walls, ceilings and trim.
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A 10 × 8 m house with a 22.5° roof About 87 m² of roof
A building footprint of 10 × 8 m is 80 m² of floor, but the roof is bigger because it's sloped. At a 22.5° pitch the slope factor is ×1.082, so the actual roof surface is about 86.6 m² — before you add any eaves. Roofing (tiles, sheets, sarking, insulation) is ordered to this surface area, never the floor plan.
| Footprint | 10 × 8 = 80 m² |
|---|---|
| Pitch | 22.5° |
| Slope factor | ×1.082 |
| Roof surface | ≈86.6 m² |
- Add eaves They enlarge the footprint
- Order to Surface area, not floor area
How much does pitch change the area? 15° adds 3.5%, 30° adds 15.5%
The steeper the roof, the more surface for the same footprint. On a 100 m² footprint: a shallow 15° roof is 103.5 m², 22.5° is 108.2, 25° is 110.3, and a steep 30° is 115.5 m² — over 15% more material than the floor area. Metal roofs often sit at low 10–15° pitches, tiled roofs usually 15–25°, and heritage or steep-gable designs 30°+.
| 15° pitch | ×1.035 |
|---|---|
| 22.5° | ×1.082 |
| 25° | ×1.103 |
| 30° | ×1.155 |
- Metal roofs Often 10–15° (low)
- Tiled roofs Usually 15–25°
Don't forget the eaves 600 mm eaves add ~23 m² here
The roof overhangs the walls at the eaves — typically 450–600 mm in Australian homes — and that overhang is real roof you have to cover. On a 10 × 8 m building, 600 mm eaves all round grow the footprint from 80 m² to about 103 m² (11.2 × 9.2), which at 22.5° becomes ~111 m² of roof. Skip the eaves and you'll under-order by a fifth.
| Typical eaves | 450–600 mm |
|---|---|
| Footprint 10×8 | → 11.2 × 9.2 with 600 mm |
| Adds | ≈23 m² footprint |
- Set it At the Standard level
- Gables/verges Also overhang — add a little
How many tiles or sheets? ≈1,320 tiles for a 120 m² roof
Once you have the roof surface area, multiply by the coverage: concrete/terracotta roof tiles run about 10 per m² (varies 10–16 by profile), so a 120 m² roof needs about 1,200 tiles plus ~10% for cuts, hips, valleys and breakage — roughly 1,320. For metal sheeting, work in linear metres of sheet at your cover width instead. Always confirm the exact coverage on the product's data sheet.
| Concrete tiles | ≈10/m² (10–16 by profile) |
|---|---|
| 120 m² roof | ≈1,200 tiles |
| +10% waste | ≈1,320 |
- Metal roofs Work in linear metres of sheet
- Confirm Coverage on the data sheet
How do I find my roof's pitch? Degrees, or rise-over-run
Roof pitch is the angle from horizontal, in degrees, or as a rise-over-run ratio. To measure it: 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 (e.g. a 125 mm rise over 300 mm ≈ 22.6°). Or use a phone level app on the underside of a rafter. If you truly can't tell, 22.5° is a fair typical-tiled-roof default.
| Measured as | Degrees or rise:run |
|---|---|
| Method | Rise over 300 mm run |
| Typical default | 22.5° (tiled) |
- Phone app Level on a rafter underside
- Custom pitch Enter degrees at Simple level
Gable, hip or complex roofs Same footprint × slope holds
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 with its own pitch, then added. For a roughly rectangular home, the single-footprint estimate is close enough to order materials with a waste allowance.
| Simple gable/hip | footprint × slope |
|---|---|
| Complex/multi-pitch | Split & sum |
| Dormers, verandahs | Add separately |
- L-shaped home Split into rectangles
- This tool Great for a simple footprint
Why the roof is bigger than the house A sloped surface is always longer
Look at a right-angled triangle: the sloping side (the roof) is always longer than the flat base beneath it. The slope factor 1/cos(pitch) captures exactly how much longer. That's why a builder's 'roof area' always exceeds the floor plan — and why estimating roofing, sarking, battens or solar-panel space from the floor area alone leaves you short. The steeper the roof, the bigger the gap.
| Roof plane | The triangle's long side |
|---|---|
| Factor | 1 ÷ cos(pitch) |
| Always | Roof > footprint |
- Solar too Usable area follows the surface
- Never Order to the floor plan
Worked examples
| Inputs | Roof surface area | Note |
|---|---|---|
| 10 m × 8 m at 22.5° | 86.6 m² | 86.6 m² — 8% more than the 80 m² footprint |
| With 600 mm eaves and a 30° pitch | 119 m² | 11.2 × 9.2 m footprint, ×1.155 |
| Tiled at 11 per m² with 10% waste | 86.6 m² | 1,048 tiles |
FAQFrequently 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. Sources
The sources behind this calculator
- American Association of Feline Practitioners — Feline Life Stage Guidelines — the six feline life stages the cat-age page uses
- Wang et al. 2020, Cell Systems — Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome — the 16·ln(age)+31 epigenetic formula on the dog-age page
- Fair Work Ombudsman — sets hospitality award wages — why tipping is optional in Australia
Last checked: July 2026. Where a page states a convention (a tipping custom, a pet life stage), the page names the body it follows.