The nor’wester of 10 September 2013 came through Christchurch with gusts recorded above 150 km/h, and in the five weeks after it I priced 63 roofs. Fifty-one of them had lost material at the ridge, not at the eaves where most owners expect wind to get under a sheet. A house on Hoon Hay Road had every ridge cap on the north-west side gone and four long-run sheets folded back over the ridge like an open tin, while the eaves fixings were all still holding. That pattern is not luck. It is how wind loads a pitched roof.
Where the suction is highest
Wind hitting the windward face of a roof pushes on it. Once the airflow reaches the ridge it separates and speeds up, and the pressure over the ridge and the first metre of the leeward face drops sharply below atmospheric. That is suction, pulling sheets up rather than pressing them down. On a roof pitched between 15 and 30 degrees the peak uplift sits in a band roughly 1 to 1.5 metres either side of the ridge line, with the ridge cap in the middle of it.
A nor’wester makes this worse in two ways. It is a hot, dry wind that has already been squeezed over the Southern Alps, so it arrives fast and turbulent rather than steady. And it comes from a consistent direction, so the same south-east faces on the same houses in Hoon Hay, Spreydon and Wigram get loaded every time.
What fails first
Ridge caps on 1960s and 1970s long-run roofs were usually fixed with lead-head nails through every second or third rib. Fifty years of thermal cycling works those nails out a millimetre or two a decade, and the timber under them has often dried and split. When suction arrives the cap lifts on the loose nails, the edge catches air, and it peels. Once the cap is off, the sheets below have nothing holding their upper edge and the next gust gets underneath.
On concrete tile roofs the sequence is similar. Mortar bedding under the ridge tiles cracks and lets go, the tiles lift, and the first two courses below are exposed to suction across their top edge. On older roofs those tiles are often not clipped at all.
Fixing schedules and wind zones
NZS 3604 puts most of Christchurch in the Medium or High wind zone, with design gust speeds of 37 and 44 metres per second, or about 133 and 158 km/h. The 2013 event exceeded the Medium figure and touched the High one. Roofs fixed to the 1960s schedule were never designed for that. NIWA holds the long gust records for Christchurch Airport, and they show gusts above 120 km/h come through most years.
The current schedule for long-run in a High zone puts screws through every rib at the ridge, the eaves and every sheet end, and every second rib in the body of the roof, with purlins pulled in to 900 mm or less near the edges. Ridge caps get screwed through every rib. The much harsher Wellington case is written up in Wind Zones and Fixing Schedules: Roofs in Wellington’s Very High Wind Areas, and the same logic scales down to Canterbury.
What it costs to get ahead of it
Refixing the ridge zone alone, meaning caps off, split purlins replaced, and caps and sheet tops refixed with 12-gauge screws and neoprene washers, prices at NZ$45 to NZ$80 per metre of ridge. A typical single-storey house has 18 to 25 metres of ridge, so NZ$900 to NZ$2,000. A full refix of the whole roof to a High zone schedule runs NZ$14 to NZ$24 per square metre, or NZ$2,500 to NZ$4,500 for 180 square metres, nearly always less than one insurance excess plus the ceiling repairs.
Replacing sheets after the fact costs NZ$180 to NZ$320 per sheet including labour, and ridge cap replacement is NZ$55 to NZ$90 per metre. Those 51 roofs in 2013 averaged NZ$3,800 in repairs each, plus ceilings. The check that gets ahead of all that is done from the ground with binoculars: look along the ridge for caps sitting proud, nail heads standing above the crest, or rust streaks bleeding down from a fixing, and look again after any nor’wester above 100 km/h.
Frequently asked questions
Why do roofs lose sheets at the ridge rather than the eaves? Because wind separating over the ridge creates the strongest suction there, in a band about 1 to 1.5 metres either side of the ridge line. Eaves fixings see pressure and some uplift, but the ridge sees the peak.
What wind zone is Christchurch in under NZS 3604? Most of Christchurch falls in the Medium or High zone, with design gust speeds of 37 and 44 metres per second. Exposed sites near the Port Hills or open plains can be higher and need a site-specific assessment.
How much does it cost to refix a roof against wind uplift? Refixing the ridge zone alone costs NZ$900 to NZ$2,000 for a typical house. A full refix to a High wind zone schedule runs NZ$2,500 to NZ$4,500 for 180 square metres of long-run.