Roofing sheet calculator
One roof face at a time: how wide it is, how far it runs, and at what pitch. Out comes the sheet length, the count, the lineal metres and the true area — with the working.
Sheet length is the plan run × 1 / cos(pitch) + the gutter
overhang, rounded up. Sheet count is the face width ÷ the profile's cover width,
rounded up — 762 mm on New Zealand corrugate, from a nominal 860 mm sheet.
Do it per face, then group identical lengths for the order.
One roof face. A gable has two; a hip roof has two rectangular faces and two triangular ones, and the triangles are a set of different lengths rather than one repeated length.
The worked example, by hand
Those defaults are a 9.60 m wide face running 4.20 m in plan at 25°, in corrugate. Here it is without the calculator:
| Step | Working | Result |
|---|---|---|
| Pitch factor | 1 / cos 25° | 1.1034 |
| True slope length | 4.20 × 1.1034 | 4.634 m |
| Plus overhang | 4.634 + 0.050 | 4.684 m |
| Sheet length, rounded up | up to the next 100 mm | 4.70 m |
| Sheets, before rounding | 9.60 ÷ 0.762 | 12.60 |
| Sheets to order | rounded up | 13 |
| Lineal metres | 13 × 4.70 | 61.1 m |
| Roof area at the true pitch | 9.60 × 4.634 | 44.49 m² |
Note that the area uses the true slope length, not the rounded sheet length. The rounding is an ordering allowance; it is not roof.
The two inputs people get wrong
Cover width, not sheet width
A sheet covers less than it measures, because it side-laps into its neighbour. New Zealand corrugate is a nominal 860 mm sheet with 762 mm of cover. Put 860 in the box above and you will order 88.6 per cent of the sheets you need — about one short in every nine.
| Face width | Right: ÷ 762 mm cover | Wrong: ÷ 860 mm sheet | Sheets short |
|---|---|---|---|
| 6.00 m | 8 | 7 | 1 |
| 9.60 m | 13 | 12 | 1 |
| 18.00 m | 24 | 21 | 3 |
| 30.00 m | 40 | 35 | 5 |
| 76.20 m | 100 | 89 | 11 |
Trapezoidal profiles cover differently again. The cover width is on the manufacturer's datasheet for the profile; look it up once and keep it with your price book rather than working it out from the sheet width.
Plan run, not slope length
The run this calculator wants is the horizontal distance from the ridge line to the gutter line — what you would measure off a plan or an aerial. If you have already climbed up and measured along the slope, that number is the slope length: skip the pitch factor rather than applying it twice.
Measuring to the right gutter
A sheet run has to be measured to the gutter that actually catches it. On a roof with a wing or a step-down, the nearest gutter in a straight line is often not the one the water reaches, and a run measured to the wrong one comes out metres long. It is a mistake that survives all the way to the delivery.
What this does not include
- Wastage. Sheet wastage on long-run is small — two to five per cent covers the off-cuts and the odd damaged sheet — but it is a judgement about the roof, so it is not built in here.
- Flashings. A separate count entirely, and done as pieces at lengths rather than metres. See roof flashings explained.
- Consumables. Screws, underlay, sealant and rivets all follow from the sheet and flashing counts, and from the purlin layout and wind zone on the day.
- End laps. This assumes single-length sheets, which is what you want wherever the run allows it.
The full method, including all of the above, is in calculating sheet lengths and counts.
Using this calculator.
What cover width should I use for corrugate?
762 mm, from a nominal 860 mm sheet. The 98 mm difference is the side lap.
Using 860 instead gives you 88.6 per cent of the sheets you need — roughly one short in every nine, or eleven short on a hundred-sheet roof.
Should I add the overhang before or after the pitch factor?
After. Multiply the plan run by the pitch factor first, then add the overhang.
The overhang is a fixed distance past the gutter line, not a plan measurement waiting to be stretched. Adding it first multiplies it up as well, which quietly lengthens every sheet on the roof.
How do I use this on a hip roof?
Run it once for each rectangular face. The triangular hip ends do not reduce to a single length — each sheet is cut on the rake and gets shorter as it goes, so the count is the same but the cut list is a series of individual lengths.
That is exactly the part that makes hand take-offs slow, and where a cut list generated off the plan shape saves the most time.
Why is the area smaller than sheets × sheet length?
Because the sheet length has been rounded up for ordering and it includes the gutter overhang, and because the last sheet is usually cut down the length.
The area figure uses the true slope length and the actual face width, so it is roof — which is what underlay and labour should be priced off.
Does it handle a mono-pitch roof?
Yes — a mono-pitch is a single face, so run it once with the full plan run from the high side to the gutter. A gable is two faces; run it twice, or once and double the count if both sides are identical.
The thing to avoid on a mono-pitch is treating it like a gable and halving the run. That gives you twice as many sheets at half the length, which is the same lineal metres and completely the wrong order.
Next
Try it on one of your own roofs.
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