Two numbers: how many sheets and how long
Steel roofing is ordered as a count of sheets cut to a length. The count comes from the roof dimension along the gutter divided by the cover width of the profile. The length comes from the slope distance between gutter and ridge, which is longer than the horizontal run because the roof is pitched.
sheets per plane = length ÷ cover width, rounded up
sheet length = run ÷ cos(pitch) + overhang
Cover width is the width a sheet actually covers once it is lapped onto its neighbour, not the width of the flat sheet. Corrugated is 762 mm cover from a sheet that measures about 850 mm. Get this wrong and you are two sheets short on a 12 m roof.
| Profile | Cover | Min. pitch | Fixing |
|---|---|---|---|
| Corrugated (Custom Orb) | 762 mm | 5° | Screws, every 2nd crest |
| Trimdek | 762 mm | 2° | Screws, each rib |
| Spandek | 700 mm | 3° | Screws, each rib |
| Klip-Lok 700 | 700 mm | 1° | Concealed clips |
| Klip-Lok 406 | 406 mm | 1° | Concealed clips |
Minimum pitches are the manufacturer's figures for short sheets without end laps. Longer sheets and lapped sheets need more pitch. Check the current data sheet for the profile you are buying.
Pitch to slope
A 4 m horizontal run at 22.5 degrees is 4 ÷ cos(22.5°) = 4 ÷ 0.924 = 4.33 m up the slope. Add 50 mm of overhang into the gutter and the sheet is 4.38 m. At 30 degrees the same run is 4.62 m. At 5 degrees it is 4.02 m. The steeper the roof, the more it costs in sheeting for the same floor plan, which is one reason low-pitch skillions are cheap to roof.
| Pitch | Slope factor | 4 m run becomes |
|---|---|---|
| 5° | 1.004 | 4.02 m |
| 10° | 1.015 | 4.06 m |
| 15° | 1.035 | 4.14 m |
| 22.5° | 1.082 | 4.33 m |
| 30° | 1.155 | 4.62 m |
| 45° | 1.414 | 5.66 m |
Capping, screws and battens
Ridge capping runs the length of the ridge on a gable, with a little extra for the end laps. Barge capping runs up each gable end on both sides. Both come in standard lengths and the calculator adds 5 percent for laps. Screws are counted per sheet per batten row: five for corrugated, fixed on every second crest, and four for Trimdek and Spandek, fixed in each rib. Klip-Lok is held by concealed clips and the count comes from the manufacturer's clip spacing. Batten rows come from the slope length divided by the batten spacing, plus one for the top. The spacing itself depends on sheet thickness, wind region and span tables, so treat 900 mm as a starting point, not a rule.
Worked example
A 12 × 8 m shed with a gable roof at 22.5 degrees and 300 mm eaves. Each plane runs 4.3 m horizontally from gutter to ridge and 12.6 m along the gutter with the barge overhang. Slope is 4.3 ÷ 0.924 = 4.65 m, so sheets are 4.70 m. Corrugated at 762 mm cover: 12.6 ÷ 0.762 = 16.5, so 17 sheets per side, 18 with a spare, 36 in total. Ridge capping about 13.2 m, barge capping about 19.6 m. With battens at 900 mm there are 7 rows per plane, so 36 × 7 × 5 = 1,260 screws, call it 1,325 with spares.
Common questions
What is the coverage of Colorbond roof sheets?
Corrugated and Trimdek 762 mm, Spandek and Klip-Lok 700 mm, Klip-Lok 406 mm. That is the lapped cover, not the flat sheet width.
How do I get sheet length from the pitch?
Run ÷ cos(pitch) plus 50 mm overhang. A 4 m run at 22.5° needs 4.38 m sheets.
What is the minimum pitch?
Corrugated 5°, Trimdek 2°, Spandek 3°, Klip-Lok 1°, for short unlapped sheets. Check the data sheet.
How many screws per sheet?
Corrugated 5 per sheet at each batten, Trimdek and Spandek 4. Klip-Lok is clipped.
Quantities only. Batten spacing, sheet thickness, fixings and minimum pitch for your wind region come from the manufacturer's current data sheets and the NCC. Roof work is work at height.