Calculating sheet lengths and counts
Long-run steel is cut to length, so what you are ordering is a set of lengths and quantities — not an area. Here is how both numbers are worked out, and the two places they go wrong.
Sheet length is the plan run from ridge to gutter, multiplied by the pitch
factor 1 / cos(pitch), plus the overhang into the gutter, rounded up.
Sheet count is the face width divided by the profile's cover width — not its
sheet width — rounded up. On corrugate, cover is 762 mm against a nominal 860 mm sheet, and
using the wrong one leaves you short by about one sheet in nine.
The two numbers
Every roof face reduces to a length and a count. Do those per face, then group identical lengths together for the order.
1. Sheet length
Start with the plan run: the horizontal distance from the ridge line to the gutter line, measured on the flat. Then:
- Multiply by the pitch factor,
1 / cos(pitch), to get the true slope length - Add the overhang into the gutter — typically 50 mm, and it is your call, not a constant
- Round up to the next 100 mm, or to whatever increment your merchant rolls to
| Step | Worked example |
|---|---|
| Plan run, ridge to gutter | 4.20 m |
| Pitch | 25° |
Pitch factor, 1 / cos 25° | 1.1034 |
| True slope length | 4.20 × 1.1034 = 4.634 m |
| Overhang into the gutter | + 0.050 m |
| Sheet length, rounded up to 100 mm | 4.70 m |
A gable roof has two of those runs. A mono-pitch has one, running the full width. A hip roof has a rectangular face and two triangular ones, and the triangular faces are a set of different lengths rather than one repeated length — which is why hip roofs take longer to take off and why the cut list matters more.
The trap: measuring to the wrong gutter
A sheet run must 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 survives right through to the delivery, and you find out when the sheets are on the ground.
2. Sheet count — cover width, not sheet width
A sheet covers less than it measures, because it side-laps into the one beside it. The number that matters is the cover width, and it is on the manufacturer's datasheet for every profile.
New Zealand corrugate is the one everybody knows: a nominal 860 mm sheet with 762 mm of cover. Divide by 860 instead of 762 and you order 88.6 per cent of the sheets you need — about one sheet short in every nine. On a 100-sheet roof that is eleven sheets, which is a second delivery and a day lost.
| Face width | ÷ 0.762 m cover | Sheets, rounded up |
|---|---|---|
| 6.00 m | 7.87 | 8 |
| 9.60 m | 12.60 | 13 |
| 12.00 m | 15.75 | 16 |
| 15.20 m | 19.95 | 20 |
Trapezoidal profiles — 5-rib, and the wider decks like Multidek — cover differently again, and the cover width is a property of the profile rather than something to remember. Look it up once per profile and keep it with your price book. Getting it from the sheet width is the mistake; getting it from the datasheet is the method.
Round up, always, and round per face rather than on a total. Half a sheet does not arrive.
The datasheets: the profiles we order
Cover width, rib height, minimum pitch and the lengths a profile rolls to are the manufacturer's numbers, not ours. These are the product pages for the profiles we buys, and the ones RoofMap ships with — check your own profile against its page before the first order, and keep the cover width with your price book.
- Corrugate — Roofing Industries
- Trimrib — Roofing Industries
- Multirib — Roofing Industries
- Ribline R-800 — Roofing Industries
- Multidek — Roofing Industries
- Dimondek 400 — Dimond Roofing
What this guide assumes. The worked examples use NZ corrugate at 762 mm cover, a 50 mm gutter overhang, and lengths rounded up to 100 mm. The pitch factor applies to sheet runs, which follow the slope from ridge to gutter; hips and valleys run diagonally across the slope and carry a larger factor — see Roof pitch explained. Wastage is not in the sheet count at all: it lives in the flashing and fixings allowances, and on a hip roof the triangular faces produce offcuts that a good cut list reuses.
3. Laps, and when a sheet is not full length
Long-run steel side-laps by one rib, which is what the cover width already accounts for. End-laps only come into it when a run is longer than the sheet you can get on site — then the lap follows the profile and pitch rules, and on low pitches it is longer or sealed.
Aim for single lengths wherever the run allows. Two sheets end-lapped is two chances of a leak and an extra row of fixings, and on a re-roof it is rarely worth it.
4. What the counts then give you
Once you have lengths and quantities, the consumables are arithmetic off them:
- Underlay by roof area at the true pitch, with the lap allowed
- Screws by sheet metres and purlin spacing — count the ridge and eave rows separately, because they are denser
- Sealant by the number of laps and penetrations
- Rivets by flashing metres, which is a separate count entirely
Fixing centres themselves depend on the profile, the wind zone and the purlin layout, per the manufacturer and the code. An estimate assumes typical residential spacing; the roof gets fixed to what is actually up there.
5. Wastage on sheets is small
Because long-run is cut to length, sheet wastage is the off-cut at the end of a run and the occasional sheet ruined in handling. Two to five per cent covers most roofs. It is flashings, not sheets, that need a real allowance — and those are allowed per piece. That is covered in roof flashings explained.
Do it once, then check it
The roofing sheet calculator runs the whole thing — pitch factor, overhang, rounding, cover width — and shows the working, so you can check it against your own take-off rather than trusting it.
What comes up most.
What is the cover width of NZ corrugate?
762 mm, from a nominal 860 mm sheet. The 98 mm difference is the side lap.
That is the number to divide a face width by. Dividing by 860 gives you 88.6 per cent of the sheets you need — roughly one short in every nine.
How much overhang into the gutter should I allow?
Around 50 mm is typical, but it depends on the gutter, the fascia and how the eave is detailed. It is a decision, not a constant — set it once for the way you detail your roofs and use it consistently.
Whatever you choose, add it after the pitch factor. The overhang is a real horizontal-ish distance at the bottom of the sheet, not something to be multiplied up.
Do I apply the pitch factor to the sheet count as well?
No. Pitch makes sheets longer, not more numerous.
The face width across the roof — the direction you count sheets in — is level, so it is the same in plan as it is on the roof. Multiplying the count by the pitch factor as well as the length double-counts the pitch and over-orders the whole roof.
What length sheets can I actually get?
Long-run is rolled to order, so length is limited by transport and handling rather than by the machine. Very long sheets are awkward to get onto a roof and easy to damage; discuss anything unusual with the merchant before you price it.
Rounding up to the next 100 mm is a safe default for an estimate. Confirm the increment your supplier rolls to before the order goes.
How do I count sheets on a hip roof?
The rectangular faces count normally. The triangular hip ends are a series of decreasing lengths, each cut on the rake, so the count is the same — face width divided by cover — but the cut list is a set of individual lengths rather than one repeated one.
That is where hand take-offs get slow, and where an off-plan cut list earns its keep.
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