Calculate roof area, sheets, tiles, screws, battens, underlay and cost for gable, hip, flat and skillion roofs — with the pitch factor worked and shown.
Roofing Material
Fixings & Underlay (optional)
Cost (optional)
| Item | Value |
|---|
The most popular roofing materials in Australia are Colorbond steel (BlueScope Steel) and concrete/terracotta tiles. Colorbond is dominant in new construction due to its durability, lightweight, and low maintenance.
| Pitch | Use case | Pitch factor |
|---|---|---|
| 5°–15° | Carports, commercial (min for Colorbond) | 1.00–1.04 |
| 15°–25° | Most common residential (gable) | 1.04–1.10 |
| 25°–35° | Steeper residential, traditional | 1.10–1.22 |
| 35°+ | Heritage, steep cottage | 1.22+ |
Written and reviewed by Mohsin Iqbal · Last reviewed August 2026
A roofing calculator converts your building's footprint into the actual sloped roof area, then into the materials you need to buy. The roof area is always larger than the building footprint underneath it — a sloped surface simply has more area than the flat ground plan it covers, and how much more depends entirely on the pitch.
Confusing roof area with floor area is the single most common estimating mistake in roofing. A 120 m² house doesn't have a 120 m² roof — once eaves and pitch are accounted for, the true roof area is routinely 15–40% larger.
Plan area = (Length + 2 × eave) × (Width + 2 × eave)
Pitch factor = 1 ÷ cos(pitch)
Roof area = Plan area × Pitch factor
The pitch factor is the multiplier that converts flat ground area into sloped roof area. At 0° (flat) it's exactly 1.0; at 22.5° it's about 1.08; at 45° it's about 1.41. A steeper roof isn't just more dramatic — it genuinely needs more material per square metre of house underneath it.
The most common residential roof shape: two rectangular sloped planes meeting at a central ridge, with triangular gable-end walls at each end. Ridge length runs the full building length (plus eaves).
A "flat" roof is never actually dead flat — it needs a minimum fall for drainage, typically at least 1–2°, even though it reads as flat to the eye. At that shallow an angle, the pitch factor is so close to 1.0 that roof area and plan area are effectively the same for material-ordering purposes.
A single sloped plane running across the full width of the building — common on garages, sheds, and increasingly on contemporary homes as a design choice. The whole footprint is covered by one slope, so the pitch factor applies to the full width rather than half of it.
| Pitch (degrees) | Approx. rise per 12 (imperial) | Pitch factor |
|---|---|---|
| 0° (flat) | 0-in-12 | 1.000 |
| 10° | 2.1-in-12 | 1.015 |
| 15° | 3.2-in-12 | 1.035 |
| 22.5° | 5.0-in-12 | 1.082 |
| 30° | 6.9-in-12 | 1.155 |
| 45° | 12.0-in-12 | 1.414 |
Australian residential roofs commonly sit between 15° and 35°. Steeper pitches shed water and debris faster but need more material and are more expensive and hazardous to work on.
| Material | Typical lifespan |
|---|---|
| Colorbond steel | 30–50 years (up to 70 in favourable conditions; shorter within about 1km of the coast) |
| Zincalume (unpainted steel) | 20–40 years, coast-dependent |
| Concrete tile | 40–60 years (surface coating often needs attention well before the tile itself fails) |
| Terracotta tile | 50–100+ years |
| Asphalt shingles | 10–25 years |
These are broad planning ranges, not warranties — actual lifespan depends heavily on coastal exposure, installation quality, local climate, and ongoing maintenance. Estimates vary noticeably between sources for this reason; treat the ranges above as a starting point for planning, not a guaranteed figure for your specific roof.
| Material | Typical coverage | Notes |
|---|---|---|
| Colorbond / metal sheet | Common cover widths: 762mm, 820mm | Custom-cut to length in Australia — one sheet, eave to ridge |
| Corrugated steel | Cover width varies by profile | Check the specific profile's effective cover width, not its total sheet width |
| Concrete tile | ~7 tiles per m² | Varies by exact tile size and gauge |
| Terracotta tile | ~7-9 tiles per m² | Varies by profile |
| Shingles | ~3 bundles per 100 ft² (roughly one "square") | US/Canadian convention; less common in Australia |
Metal roofing is ordered by sheet count, not area — each sheet is custom-cut to the exact slope length and laid side by side along the eave line. This calculator works out how many sheets fit across the roof's length at your sheet's cover width, then multiplies by the number of slopes.
For standard residential corrugated or Colorbond roofing, a commonly cited planning figure is around 10–12 roofing screws per square metre, though the exact count depends on the profile, batten spacing and local wind zone — a coastal or cyclone-rated area will need more. Battens for metal roofing are commonly spaced around 900mm apart on timber battens, though this also varies by structural design. Both figures are editable in the calculator so you can match your specific product's fixing schedule.
10% is a reasonable starting point for a simple gable or skillion roof. Hip roofs typically need a bit more — commonly 12% or higher — because every hip and valley line needs angled cuts that a straight gable roof simply doesn't have.
Total project cost combines material cost (units needed × price per unit) and labour (roof area × hours per square metre × hourly rate). Labour rates and hours vary significantly by region, roof complexity and pitch — steep roofs take longer and often carry a rate premium for the additional risk and access equipment required.
| Material | Typical installed cost | Avg. lifespan |
|---|---|---|
| Asphalt (3-tab) shingles | $4–$7 per sq ft | 15–20 years |
| Architectural/dimensional shingles | $5.50–$9 per sq ft | 25–30 years |
| Metal roofing (standing seam) | $9–$16 per sq ft | 40–70 years |
| Clay/concrete tile | $10–$20 per sq ft | 50+ years |
For a typical 2,000–2,500 sq ft roof (about 20–25 squares), that puts a full US asphalt-shingle replacement around $8,000–$17,500, and a metal roof replacement around $18,000–$40,000, before accounting for tear-off, structural repairs, steep-pitch premiums or regional labour rates — all of which can move the final number substantially. Get several local quotes; this calculator's cost fields are for planning, not a binding price.
| Material | Typical installed cost |
|---|---|
| Corrugated steel / Colorbond | $45–$90 per m² |
| Concrete tile | $55–$100 per m² |
| Terracotta tile | $70–$130 per m² |
A typical 200 m² roof (about 22 squares) in Colorbond runs roughly $9,000–$18,000 installed. As with the US figures, this is a planning range only — get quotes from licensed local roofers for anything binding.
| Step | Working | Result |
|---|---|---|
| Plan area | 12.9m × 8.9m | 114.81 m² |
| Pitch factor | 1 ÷ cos(25°) | 1.1034 |
| Roof area | 114.81 × 1.1034 | 126.68 m² |
| With 10% waste | 126.68 × 1.10 | 139.35 m² |
| Tiles needed (0.144 m² each) | 139.35 ÷ 0.144, rounded up | 968 tiles |
Identical footprint and pitch — roof area is exactly the same 126.68 m² as the gable example above (confirming the identity explained earlier), but with a much shorter 4.0m ridge (versus the gable's 12.9m) and four 6.9m hip lines. At 762mm sheet cover width: 38 sheets, roughly 1,394 roofing screws.
A shallow single-slope roof over a shed: 28.16 m² plan area becomes 28.59 m² of actual roof (the pitch factor at just 10° is only 1.015, barely more than flat). In metal sheet: 10 sheets, about 315 screws, 1 roll of underlay.
Returning to the 12m × 8m gable tile roof: at $4.50 per tile and $75/hr labour (0.6 hours per m² before the complexity adjustment), that's $4,356 in tiles and 95.0 labour hours at the default "medium" complexity (1.25× multiplier) for $7,125.68, giving a total estimated project cost of $11,481.68. Switching the roof complexity to "simple" removes the multiplier and brings labour down to 76 hours ($5,700.55), for a total of $10,056.55. Get an actual quote from a licensed roofer before committing — this is a planning estimate, not a fixed price.
Roofing work in Australia is generally governed by the National Construction Code and relevant Australian Standards for roof cladding and fixing (including wind classification requirements under AS 4055 and AS 1562). Structural roof work typically requires a licensed builder or roofer, and council approval may be required depending on the scope of work and your state or territory. This calculator provides planning estimates only — always confirm compliance requirements with a licensed professional or your local council before starting work.
Multiply your building's plan area (length × width, including eave overhang) by the pitch factor for your roof's slope (1 ÷ cos(pitch)). A 15m × 10m building with 0.6m eaves and a 22.5° pitch has a plan area of about 181 m², giving a roof area of about 196 m² once the pitch factor is applied.
Divide your building length by the sheet's cover width to find sheets per slope, then multiply by the number of slopes (2 for gable and hip, 1 for skillion). A 12.9m gable roof slope at 762mm cover width needs about 19 sheets per slope, or 38 total for both slopes.
Australian residential roofs commonly sit between 15° and 35°, chosen based on architectural style, local rainfall, and the specific roofing material's minimum pitch requirement — some profiles need a minimum slope to shed water properly. Check your chosen material's technical specifications.
This varies enormously by material, roof complexity, region and access — this calculator gives a planning estimate from material and labour rates you enter, but always get quotes from multiple licensed roofers for an accurate, binding price.
10% is a reasonable starting point for a simple gable or skillion roof. Hip roofs typically need more — commonly 12% or higher — because of the extra angled cuts at every hip and valley line.
Measure the building's length and width including eave overhang, then apply the pitch factor to get the total area of both sloped planes. The ridge runs the full building length.
Exactly the same total-area formula as a gable roof of the same footprint and pitch — verified geometrically, the two need identical roof area. What differs is the ridge length (shorter than a gable's) and the four hip line lengths, both of which this calculator estimates separately.
Calculate the roof area, add your waste allowance, then divide by your material's coverage — area per tile for tiles, area per bundle for shingles, or building length divided by cover width for metal sheets.
The angle of a roof's slope, expressed in degrees in Australia or as a rise-per-12-units ratio (e.g. 5-in-12) in the US and Canada. A steeper pitch sheds water faster but needs more roofing material per square metre of building footprint.
Divide your roof area (including waste allowance) by the coverage of one tile — commonly around 0.14–0.15 m² per tile for standard concrete or terracotta tiles, though this varies by exact tile size and profile.
Floor area is the flat internal floor space of a building. Roof area is the actual sloped surface area of the roof covering it, which is always larger once pitch and eave overhang are accounted for — routinely 15–40% larger depending on the pitch.
Colorbond is ordered by sheet count rather than area. Divide your building length by the sheet's cover width (commonly 762mm or 820mm) to find how many sheets span each slope, then multiply by the number of slopes on your roof.
Full roof replacement costs vary widely by material, roof size, pitch, and whether structural work is needed underneath. This calculator estimates material and labour cost from figures you enter, but a licensed roofer's on-site quote is the only reliable figure for a real project.
Calculating area and material quantities is straightforward with the right formula, which is what this calculator does. Actually installing a roof is a different matter — it's physically hazardous work that in most cases requires a licensed roofer, and structural or waterproofing mistakes are costly to fix later.
A commonly cited planning figure for standard residential metal roofing is around 10–12 screws per square metre, though the exact count depends on the roofing profile, batten spacing, and local wind zone. Confirm the exact fixing schedule with your roofing supplier or a licensed roofer for the final order.
The same plan-area-times-pitch-factor formula as any other roof shape, but the pitch factor applies to the full building width in one slope rather than being split across two slopes like a gable. A 6m × 4m shed at 10° pitch has a roof area of about 28.6 m², only slightly more than its 28.2 m² plan area, since a shallow pitch adds very little extra surface.
Underlay (sarking) is a waterproof membrane installed under the roofing material, providing a secondary weather barrier and, for metal roofs, condensation control. It's sold by the roll with a fixed coverage area — divide your total roof area (with waste) by the roll's coverage to get the number of rolls needed.
Insulation requirements depend on your climate zone and local building code — check the National Construction Code's minimum R-value for your region. The area needed is generally the same as the roof area itself, since insulation is typically installed across the full ceiling or roof plane.
Roughly the building's perimeter, including eave overhang — this calculator estimates it as twice the sum of the effective length and width. The exact figure depends on your specific gutter layout, downpipe positions, and any internal box gutters, which a guttering installer will confirm on site.
It varies by profile — some corrugated and trimdek-style profiles have a manufacturer-specified minimum pitch (often around 5° or less for purpose-designed low-pitch profiles), below which water can't shed reliably. Check the specific product's technical data sheet before finalising a very low-pitch design.
Tile roofs generally need a steeper minimum pitch than metal roofing — commonly cited minimums sit around 15°–17.5° depending on the tile profile and manufacturer, since tiles rely more on overlap and gravity than a continuous sealed metal sheet. Confirm the exact minimum with your tile manufacturer.
Rise-per-12 = tan(pitch in degrees) × 12. A 22.5° pitch works out to roughly 5-in-12; a 30° pitch is roughly 6.9-in-12. This calculator shows the rise-per-12 equivalent automatically alongside the degree figure you enter.
Corrugated refers to the wave-shaped profile of the sheet itself. Colorbond is a specific pre-painted steel product from BlueScope, available in corrugated and other profiles — so a sheet can be both "corrugated" (its shape) and "Colorbond" (its coating and brand) at the same time.
Ridge vents, whirlybirds and other ventilation fittings are typically priced and ordered separately from the roofing material itself, and don't materially change the sheet, tile or screw counts this calculator produces — they're a small number of point fixtures rather than an area-based material.
Eave overhang extends the roof beyond the building's wall line on every side, so it adds directly to both the length and width used in the plan area calculation — on a typical 0.6m eave, that's an extra 1.2m added to both dimensions, which is easy to forget if you only measure the building itself.
As a rough national planning range, asphalt shingle replacement runs about $4–$7 per sq ft installed, architectural shingles $5.50–$9, metal roofing $9–$16, and tile $10–$20+. For a typical 2,000–2,500 sq ft roof, that's roughly $8,000–$17,500 for shingles or $18,000–$40,000+ for metal or tile, before tear-off, structural repairs or regional labour differences. Always get several local quotes — this is a planning range, not a quote.
As a rough planning range, Colorbond/corrugated steel runs about $45–$90 per m², concrete tile $55–$100, and terracotta tile $70–$130 installed. A typical 200 m² roof in Colorbond costs roughly $9,000–$18,000. Get quotes from several licensed local roofers for an accurate figure, since region, roof complexity and access affect price significantly.