My daughter bought a 1970s weatherboard house in Brooklyn, Wellington, in 2022, on the ridge above Ohiro Road where the southerly comes straight off Cook Strait. The first winter storm took a run of ridge capping off the garage and left three sheets on the main roof drumming loud enough to hear from the kitchen. When I went up afterwards the screws had not snapped: they had pulled through the purlins, spaced as though the roof lived in a sheltered Christchurch suburb. It did not, and the wind zone missing from the fixing schedule was the whole story.
What a wind zone is and why Wellington is mostly at the top
NZS 3604 sorts every site into Low, Medium, High, Very High or Extra High wind zones, with design gust speeds of 32, 37, 44, 50 and 55 metres per second. The zone comes from region, exposure, topography and elevation, and Wellington’s hill suburbs collect penalties on every count: lee slopes, spur sites, no shelter in the prevailing quarters. Brooklyn, Kingston, the western edge of Karori and most of the south coast sit in Very High or Extra High, and 50 metres per second is 180 kilometres an hour.
Wind does not push a roof down. It lifts it. Air accelerating over the ridge drops in pressure and the roof becomes a wing. Suction is fiercest along the edges, at the ridge and eaves in a band about a tenth of the building width wide, which is exactly where my daughter’s sheets let go.
Fixing schedules for long-run steel
Every roofing manufacturer publishes a fixing table for each profile and wind zone, and it is the document a council inspector wants to see. For 0.40 millimetre corrugate on 900 millimetre purlin spacing the pattern changes like this:
- Low to High zones: screws on every second or third crest across the body of the roof, every crest at the eaves and ridge.
- Very High: every second crest in the body, every crest along all edges and both ends, and purlin spacing often dropped to 700 or 800 millimetres.
- Extra High: every crest everywhere, load-spreading washers under every screw head, and 0.55 millimetre steel or a stiffer profile rather than standard 0.40.
A 12 gauge by 65 millimetre roofing screw into a 45 millimetre timber purlin gives around 1.5 kilonewtons of pull-out in dry radiata pine. In wet timber or a split purlin that figure can halve. Under a 50 metre per second gust the uplift on a single edge-zone sheet can pass 2 kilonewtons, which is why the tables crowd fixings so close. The garage had 10 gauge screws at every third crest with no washers, fitted to a Christchurch habit rather than a Wellington table.
The parts that fail first
Ridge caps and barge flashings go before the body sheets nearly every time. They are narrow, sit at the highest suction point, and the screws often go only through the flashing and sheet crest, not into timber. A Wellington ridge cap should be fixed through to the purlin at every second crest with a screw long enough to bite 35 millimetres into wood.
Screws also back out. Each thermal cycle works the screw a fraction. After fifteen or twenty years the neoprene washers have hardened and the screws stand proud. Re-screwing a 150 square metre roof with new Class 4 fixings and washers runs NZ$1,800 to NZ$3,500, which is cheap against one sheet through a neighbour’s window. Wind-driven rain is the quieter problem: Wellington rain travels sideways and climbs under laps and ridge caps. Sheets in Very High zones want a turned-up back edge at the ridge, stop-ends at the spouting, and underlay across the whole roof.
Getting the zone confirmed
The wind zone for an address is not guesswork. The council property file and hazard maps give a starting point, and a licensed building practitioner in roofing or an engineer can assess a site for around NZ$350 to NZ$700. Re-fixing a whole roof to a higher zone is a job for a professional, and on a steep Brooklyn section with edge protection required the labour will match the hardware. Having gone up there once in a lull with a screw gun and a knot in my stomach, I would give that job to a Wellington roofing contractor and stand at the bottom holding the ladder. A quote should name the wind zone, the fixing pattern, the screw gauge and length, and whether load-spreading washers are included. If it does not, ask.
Frequently asked questions
How do I find the wind zone for my house? Check the property file or LIM held by the council, which often records the NZS 3604 zone from the original consent. If it is missing or the house predates 1999, a licensed building practitioner or engineer can assess the site for about NZ$350 to NZ$700.
How many screws should a long-run roof have in a Very High wind zone? Every second crest across the body of the sheet and every crest at the eaves, ridge and barges, on purlins no more than 900 millimetres apart, using at least 12 gauge screws with sealing washers. Extra High zones need every crest fixed and load-spreading washers throughout.
Can old roofing screws just be tightened instead of replaced? Only if the washer is still soft and the thread still bites. After fifteen years most neoprene washers have hardened and cracked, and a screw that has backed out has usually enlarged its hole, so replacement with a longer or larger gauge screw is the reliable fix at roughly NZ$3 to NZ$6 each installed.