Plan area is not roof area
Stand on the ground, tape out the footprint, order that many square metres of sheet, and you'll be short. What you measured is the roof's shadow. The actual surface runs up a slope, so it's longer than the shadow by a fixed amount — one divided by the cosine of the pitch.
At 22.5° that factor is 1.0824. So 80 m² of footprint is 86.6 m² of roof. Six and a half square metres, which on 762mm cover is most of two extra sheets. At 30° the factor is 1.1547 and the same footprint becomes 92.4 m².
Measuring it without getting on the roof
You only need two horizontal numbers and the pitch. Tape the length along the eaves, then the horizontal distance from the eaves line in to a point directly under the ridge. Both are ground-level measurements. The multiplier does the rest.
What you must not do is measure up the slope from the ground with a tape held at an angle — the fascia, the gutter and the overhang all throw that out. If you can get on the roof safely, measure the rafter run from gutter to ridge directly; that figure already has the slope in it, so enter it as the run and set the pitch to zero.
One plane at a time
This works a single roof plane. For a gable, the plan run is half the total span, and you run it twice. For a skillion, one pass does the whole roof. For a hip, break it into the rectangles and triangles you can see from above and add them up — a triangular hip end is half its base times its plan height, then the same multiplier applies.
A worked example
A 10m × 8m gable shed at 22.5°, corrugated at 762mm cover, 50mm of overhang into the gutter.
- One plane in plan: 10 × 4 = 40 m²
- Multiplier: 1 ÷ cos 22.5° = 1.0824
- True surface, one plane: 40 × 1.0824 = 43.3 m²
- Sheet run: 4 × 1.0824 = 4.33m, plus 0.05m overhang = 4.38m
- Sheets across the 10m: 10,000 ÷ 762 = 13.12 → 14 sheets
- Lineal metres, one plane: 14 × 4.38 = 61.3m
Both sides: 28 sheets, 122.6 lineal metres, 86.6 m² of roof — against the 80 m² the footprint suggested.
Count on cover width, never sheet width
A sheet is wider in the stack than the roof it covers, because its edge rib laps the sheet beside it. Order and count on the cover figure or you'll finish a sheet short on every second roof.
| Profile | Cover width | Worth knowing |
|---|---|---|
| Corrugated (Custom Orb style) | 762mm | The everyday Australian roofing profile; curves and follows a slightly uneven frame better than a rib profile |
| Trapezoidal (Trimdek style) | 762mm | Flatter ribs, stiffer sheet, works down to a lower pitch than corrugated |
| Concealed-fix (Klip-Lok style) | 700mm | Clipped rather than screwed, so there are no exposed fasteners to re-seal in ten years — but you lose 62mm of cover per sheet |
Minimum pitch varies by profile and by whether the run is continuous or end-lapped. Corrugated is generally published from about 5°, trapezoidal from about 2°, concealed-fix lower again. Check the manufacturer's own literature for the profile you're buying before you commit to a low-pitch roof.
Order sheets cut to length
Australian roll-formers cut to your length, so a 4.38m sheet is a normal order, not a special. Take the full run in one sheet wherever you can handle and cart it — a continuous sheet has no end lap, and an end lap is the thing that leaks in ten years' time.
Where a run genuinely has to be lapped, put the lap over a purlin and give it at least 150mm. Add that lap to your ordered length; the calculator's run figure is the covered run, not the cut length of a lapped sheet.
What else goes on the order
| Item | How you work it out |
|---|---|
| Ridge capping | Lineal metres of ridge, plus a lap at each joint |
| Barge capping | Lineal metres down each gable edge — that's the sloped length, so apply the multiplier |
| Valley gutter | Sloped length of each valley, which is longer again than a rafter |
| Sarking or anti-con blanket | Roof area plus laps — order about 10% over the true roof area |
| Roof screws | Roughly 6–8 per square metre for corrugated at normal purlin spacing |
| Foam closures | Lineal metres of ridge and eaves, per plane |
How much waste to allow
A plain gable is nearly waste-free — the sheets are cut to length and the only offcuts are at the ends. Five per cent covers it. A hipped roof is the opposite: every sheet landing on a hip is a raking cut, and the triangle you cut off is rarely long enough to use anywhere else. Ten to fifteen per cent is honest there. Valleys are worse again.
Add a spare full sheet to any order you can afford it on. A sheet damaged in the sling on a Friday afternoon is a week's delay if the roll-former has to make another one.
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Try Yamate free → Free to try · no card · sign in with a link, no password to rememberRoof area & sheets FAQ
How do I work out roof area from plan dimensions?
Multiply the plan length by the plan run to the ridge, then multiply by the pitch multiplier — one divided by the cosine of the pitch. At 22.5° that multiplier is 1.0824, so 80 m² of footprint is 86.6 m² of roof.
How many roof sheets do I need?
Divide the length along the gutter by the sheet cover width and round up. Corrugated and most trapezoidal profiles cover 762mm, so a 10m run needs 14 sheets per plane. Concealed-fix covers 700mm, so the same run needs 15.
What is the cover width of Colorbond roof sheeting?
Corrugated and standard trapezoidal profiles both cover 762mm. Concealed-fix profiles cover 700mm. Cover width is narrower than the sheet itself because the edge rib laps the next sheet — always order against cover.
How long should I order my roof sheets?
The horizontal run from gutter to ridge, times the pitch multiplier, plus your overhang past the fascia. Australian suppliers roll sheets to length, so ordering the exact run is normal and saves you an end lap.
Do I need an end lap in a roof sheet?
Not if you can take the run in one sheet, and that is always the better roof — a continuous sheet has nothing to leak through. If a run is too long to handle, lap it at least 150mm and land the lap on a purlin.
How much waste should I allow on a roof?
About 5% on a plain gable, where the only offcuts are at the ends. Ten to fifteen per cent on a hipped or valleyed roof, because every sheet at a hip is a raking cut and the offcut usually goes in the bin.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. The pitch multiplier is geometry and is exact. Cover widths, minimum pitches and fixing rates are typical published Australian manufacturer figures and vary by profile, gauge and roof length — check the technical literature for the product you're buying. This is a materials estimate for ordering and quoting, not a roof design.