New Zealand · metal roofing
Trapezoidal roof calculator
When the pitch drops below 8°, corrugate is the wrong product. Trapezoidal profiles exist for that job.
CoP generic trapezoidal asymmetrical: 20 mm rib → 4° and check after 20 m; 27 mm → 3° and 40 m; 36 mm → 3° and 65 m. Dimond Six Rib sits in the first bucket, Hi Five and Metalcraft T-Rib in the second. Cover widths are around 760–765 mm — always tighter than the overall sheet.
Low-pitch trapezoidal roofs fail on flashings and run length more often than on sheet count. Extra High wind zone forces Category B covers at any pitch. A 22 m Six Rib run at 4° should not get a silent green tick; it should tell you to open the NZMRM maximum-run calculator with a HIRDS rainfall figure.
Tray / secret-fix is a different geometry (narrow pans, 3° typical). The library includes a generic 300 mm tray so you can see the CoP row; override the cover if you know the pan.
FAQ
Six Rib vs Hi Five?
Six Rib is a shallower six-rib trapezoidal (about 20 mm, 4°). Hi Five is a deeper five-rib (about 27 mm, 3°) with a longer allowable run. Appearance and purlin span also differ — read the span tables, which this takeoff does not replace.
Is T-Rib the same as Hi Five?
They occupy the same CoP row (deeper five-rib, 3°) but they are different rolls. Cover on Metalcraft T-Rib is listed at 760 mm; Dimond Hi Five at 765 mm (some sheets say 755 mm). Never mix the two on one roof without checking.