Three families of profile
NZ Steel groups metal roofing as corrugated, trapezoidal (rib), and secret fix, which takes in trough, tray, standing seam and decking. Generally, it says, the taller the profile, the stronger it is. Local rollformers such as Dimond, Roofing Industries and Metalcraft roll them in ZINCALUME, galvanised steel and COLORSTEEL, and some in aluminium.
Ideally each sheet runs unbroken from ridge to spouting, because a join across the sheet (an end lap) is a weak point. Dimond says sheets over 25 m need special transport licences, and some tray profiles can be rolled on site instead.
How the three compare
| Corrugate | Trapezoidal rib | Tray and standing seam | |
|---|---|---|---|
| Look | The classic wave Dimond calls a firm Kiwi favourite | Flat pans between sharp ribs | Wide flat pans, slim seams, no visible screws |
| Typical minimum pitch | 8° for standard corrugate, lower for deeper versions | 3° on many, 4° on some | 3° on most, 5° on some |
| Fixing | Screws through the sheet | Screws through the sheet | Hidden clips under the seams |
| What sits underneath | Purlins | Purlins | Purlins, or plywood for some systems |
Corrugate
RANZ gives 8° as the minimum pitch for standard corrugate, and Roofing Industries says 10° where sheets are end-lapped. Below 8° the sheet ends need turning down into the spouting, which the Code of Practice says corrugate can't do satisfactorily.
Deeper corrugates go lower: Roofing Industries' True Oak, a return to the older rounder wave, has a 4° minimum and True Oak Deep 3°. Metalcraft also offers bullnose, drape and spring curving, which is how curved verandas get their shape.
Trapezoidal rib
Most trapezoidal profiles go down to 3°, so they suit lower-pitch modern homes. The pan matters to anyone working up there: Dimond says DP955's wide pan gives safe foot placement and V-Rib's symmetrical ribs tolerate foot traffic well.
Longer runs need more pitch. Roofing Industries adds 1 degree for Trimrib and RT7 on runs over 40 metres, and more where rainfall is intense.
Tray and standing seam
Tray hides its fixings in clips under the seams. Dimond says its Dimondek 630, with no screw holes through the roof, virtually eliminates leaks, and calls Dimondek 400 ideal for modern contemporary homes.
Some trays need a plywood base, such as Dimond's Eurotray Roll Cap on at least 12 mm ply; others, like Roofing Industries' spanlok, span between purlins. Aluminium, zinc and copper trays expand about twice as much as steel, so they need expansion clips.
Wind, snow, coast and geothermal
Wind. Fixings tighten up as the wind zone climbs. Dimond spaces Eurotray clips at 600 mm purlin centres from Low to Very High wind zones and 400 mm in Extra High. Your zone is worked out with NZS 3604 methods, so ask the roofer what it is.
Snow. Dimond asks for its technical team's advice on Eurotray pitch in snow zones N1 to N5, and spouting in snow areas needs closer brackets and snow straps.
Coast. The profile doesn't change the coastal rules; the coating and fixings do. Dimond wants at least Class 4 fasteners, Class 5 in Very Severe sites, and may need cut edges lacquered in severe marine areas.
Geothermal. The Code of Practice lists harsh geothermal areas among the places GRP (glass-reinforced plastic) sheeting gets used, and Dimond makes a GRP corrugate.
No roof is safe to stand on by default
The metal roofing Code of Practice says it must never be assumed that a roof is safe to walk on, and makers publish where people may step. Leave it to the roofer. Clear or translucent sheets must never be walked on unless they're designed for it.
What makes a longrun job bigger or smaller
Pitch and run length come first, because they rule profiles in or out. Then the roof's shape: every valley, hip, chimney and skylight needs flashing, curved or very long sheets need special handling, and a tray roof on plywood adds a layer of work. E2/AS1 requires underlay under all profiled longrun.
Changing profile can matter for consent. Wairoa District Council, for one, says like-for-like re-roofing with no change to the purlins doesn't need consent unless the old roof failed within 15 years, but changing the roofing system does. See building consent for a re-roof.
Questions for the roofer
- What's my roof's actual pitch, and does the profile's minimum leave room to spare?
- What are my wind and snow zones?
- Will each sheet run in one length, or do you need end laps?
- Are the flashings and accessories from the same maker as the sheets?
Your part, and the roofer's
You can
- Photograph the roof from the ground so a roofer can identify the profile
- Check the council property file for past re-roof consents
- Note any leaks or rust around screws after heavy rain
Leave to a roofer
- Measuring pitch and choosing the profile
- Replacing screws, washers or sheets
- Flashings around chimneys, skylights and valleys
How we'd pick a profile
We'd start from the roof's real pitch, not what the plans say, because the minimums apply to the finished cladding. On a steeper older home, corrugate is hard to beat for looks. On a low-pitch modern roof, trapezoidal or tray is the safer bet, and we'd choose a profile the roofer has laid many times, from a rollformer who'll back it in writing.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.