The hail came through Bishopdale on a Sunday afternoon in late November, ten minutes of marble-sized stones ahead of a nor’west front, and my mother rang from Harewood Road to say her roof sounded like a drum kit. By the time I got there the lawn was white and the long-run roof on her 1968 house was pocked from ridge to spouting. Her first question was whether she needed a new roof. The honest answer took two hours on the roof with chalk and a torch: most hail damage on painted steel is cosmetic, but the part that is not cosmetic is easy to miss.
What hail actually does to a steel sheet
A 20 millimetre hailstone falls at around 70 kilometres an hour and hits with enough energy to dent 0.40 millimetre steel between the ribs. The pan of the sheet flexes, the dent stays, and the paint over the dent is stretched. On a factory Colorsteel finish that stretch is usually inside what the coating can take. On a roof that has been repainted with a brittle or aged acrylic, the paint fractures in a ring around the dent, and that ring is a path for water to the steel.
The dents themselves do not leak. A sheet dented in the pan still sheds water to the spouting because the dent is a shallow dish, not a hole. The problems are where the hail hit something other than a flat pan:
- Ribs and crests. A crease across a rib weakens the sheet and can crack the paint through to the steel.
- Laps and flashings. A dent right at a side lap can open the lap slightly and let capillary water creep in.
- Screw washers. Aged neoprene washers split under direct hits, and each split washer is a small leak that will show up in the ceiling months later.
- Translucent sheets. Old polycarbonate or fibreglass skylight panels shatter or craze. These are the only things that fail outright.
Cosmetic or structural: the test I use
I wait for low sun, early morning or late afternoon, and walk the roof along the purlin lines looking back into the light. Dents show as shadows. I mark every one with chalk, then count them in a one metre square in three places. On my mother’s roof that gave 8 to 12 dents per square metre, fairly even across both faces.
Then I check each chalk mark with a fingernail and a torch. If the paint is intact over the dent, it is cosmetic. If the paint has cracked, flaked or shows bare metal, that spot needs treating before autumn or it will rust. If the sheet has creased across a rib, or a lap has opened, that sheet needs a roofer to look at it, and possibly replacing. Of roughly 1,500 dents on her roof, about 40 had cracked paint and 3 sat on ribs. Nothing had creased. The roof was cosmetic with a maintenance job attached.
Why Canterbury gets it
Christchurch hail is a warm-season habit. Hot nor’westers build tall convective cells over the plains, and when a cold front pushes under them the cells drop hail in narrow bands a few kilometres wide. The climate record kept by NIWA shows Canterbury averaging a handful of hail days a year, concentrated from October to March, with damaging events in the late-spring window when the ground is hot and the upper air still cold. Insurance for hail is a private policy matter, not natural hazard cover. Photograph the roof and the hail on the lawn the same day; the claim is easier with a date on it.
What the repair costs
A roof inspection after hail, with a written report and photographs for an insurer, runs NZ$250 to NZ$500. Spot-treating cracked paint over dents, which means sanding, priming and touching in, is NZ$400 to NZ$1,200 depending on the count and the access. Replacing split screw washers is NZ$3 to NZ$6 a screw. Where the paint has fractured widely, a full wash, prime and two-coat repaint on a 150 square metre roof is NZ$4,500 to NZ$8,000, and the insurer will often meet that when the coating failure is documented.
Cracked paint over a dent is a corrosion problem waiting a year or two. Steel does not care whether the coating was opened by salt or by ice, a process I watched run its course on a twenty-year-old Whanganui roof. On my mother’s roof we sanded and primed the forty bad spots, replaced 60 washers, and left the other 1,460 dents alone. Two winters later there is no rust.
Frequently asked questions
Does hail damage mean a steel roof needs replacing? Rarely. Dents in the pan of a 0.40 millimetre sheet with the paint intact are cosmetic and do not leak. Replacement is only warranted where sheets have creased across ribs, laps have opened or the steel is already thin from rust.
How can I tell if hail dents have damaged the paint? Check each dent in low sunlight with a fingernail and torch. Intact paint is cosmetic; paint that has cracked in a ring around the dent, flaked or shows metal needs sanding and priming within a few months to stop rust.
Is hail damage to a roof covered by insurance in New Zealand? Usually by the house policy with a private insurer, not by natural hazard cover. Photograph the roof and the hailstones on the day, get a written inspection for NZ$250 to NZ$500, and claim for coating repair or repaint where the paint has fractured.