New Zealand · metal roofing
Corrugated iron roof calculator
Standard 16.5 mm corrugate is still the default Kiwi roof. It is also the profile people try to lay too flat.
NZMRM CoP 7.1.1A sets standard corrugate (16.5 mm rib) at 8° minimum cladding pitch, and tells you to check drainage after 15 m of run. That 8° is not the same number as the pitch on the building consent drawings. If purlins are wide, the sheet can pond below 8° even when the truss ticket says 8°.
Dimond lists 762 mm cover / 851 mm sheet width. Metalcraft and Stratco CGI commonly list 760 mm cover. This calculator counts sheets from cover width. Using overall width under-orders by about 10%. Metalcraft also publishes a longer-sheet rule: 9° once sheets go past 30 m (assuming ≤100 mm/hr rainfall).
If the roof is under 8°, stop. Switch the profile to a deeper trapezoidal (Six Rib at 4°, Hi Five or T-Rib at 3°) or change the framing. Do not paper over a 6° corrugate garage with a “bit of extra waste.”
FAQ
Can I lay corrugate at 5° if the run is short?
Not according to the generic CoP table. Short runs help drainage capacity, they do not lower the published minimum cladding pitch. Ask the manufacturer in writing if you think you have a special case.
What about 21 mm or 35 mm deep corrugate?
Deeper corrugate drops to 4° or 3° in the CoP table. The profiles in this library are the common 16.5 mm residential corrugate. Do not use this page for a 35 mm commercial corrugate without changing the min-pitch and cover figures.