The roof in Woolston was twenty-two years old, a pale grey Zincalume corrugate on a 1960s brick bungalow two streets back from the Heathcote River, and from the footpath it looked fine. I was there in October 2022 because the owner wanted it painted. From the ladder the story changed. Along the bottom edge of every sheet ran a line of red rust 10 to 25 mm deep. The same line ran up every side lap. At the ridge the rust was 40 mm deep and the steel flexed under my thumb. Nothing on the flat of the sheet was wrong. Everything at an edge was.
Why the edge rusts first
Long-run roofing steel is coated in the mill as a flat coil. The zinc or zinc-aluminium layer covers both faces completely but cannot cover the edges, which do not exist until the sheet is cut. Every site cut, at a ridge, valley, penetration or lap, adds more bare steel.
Zinc and aluminium are sacrificial, corroding in preference to the steel and throwing protection about a millimetre or two from the coated face onto the cut. On a roof washed by rain and drying quickly, that is enough for twenty or thirty years. On an edge that stays wet, it is not. The drip edge holds water by surface tension; side laps hold it by capillary action. Once the sacrificial zone is used up, the steel core rusts, the paint lifts off the face beside it, and the line marches inward. BRANZ has written about this pattern for years, and their guidance on durability and coastal exposure is on branz.co.nz.
Where it is worst in Christchurch
Woolston is a bad place for steel roofs. It sits low near the estuary, in the path of the sea breeze from New Brighton and downwind of decades of industry. Salt, sulphur and damp air combine to make it the closest thing Christchurch has to a Wellington south coast for corrosion. The same pattern shows in Redcliffs and Sumner from salt, and in Bromley and Linwood from the estuary.
Cut edge corrosion also runs early anywhere rain does not reach. Sheets finishing under a wide soffit never get washed, so salt and grime sit on the edge indefinitely. Unwashed steel is a specific warning in every manufacturer’s warranty, and it is the reason most ask for an annual hose-down.
Judging how far gone it is
A flat screwdriver and firm push is the test. Rust that scrapes off to show grey metal is surface corrosion and the sheet can be saved. Rust that flakes in layers and leaves a pit is further along but treatable if the pit is under a third of the sheet thickness. Steel that flexes, cracks or lets light through is finished, and no coating will change that. On the Woolston roof the drip edge and side laps were savable; the ridge cuts were not, and the top 300 mm of six sheets had to be cut back.
Treating the edge before a repaint
This is the work that separates a repaint lasting twelve years from one that fails in three. Painting over a rusted edge seals moisture in and hides the problem until it bleeds through the topcoat. The sequence:
- Wire-brush or abrade the edge to bright metal, removing all loose rust. Powered flap wheels are quicker but tear the paint beside the edge, so hand work is often cleaner.
- Wash the dust off and let the edge dry completely.
- Apply a rust-converting or zinc-rich primer along the bare steel, brushed on, 50 to 75 microns.
- Apply a brush-grade edge sealer over the primer, wrapping the edge itself and about 20 mm onto each face, at 250 to 300 microns. This is the step that gets skipped.
- Topcoat the whole roof as normal, two coats at 40 to 50 microns dry each.
Priced separately, edge treatment runs NZ$9 to NZ$16 per lineal metre depending on how much rust has to come off. A typical single-storey house with 60 metres of drip edge and 80 metres of side laps comes to NZ$1,200 to NZ$2,300 on top of the repaint. Where the rust is coastal in origin, the salt has to be washed off before any of this starts, and the piece on salt wash-down before a coastal repaint covers that sequence in detail.
The Woolston roof was cut back at the ridge, edge-treated and repainted in early 2023 for NZ$9,800 all up. Without the edge work the painter had priced it at NZ$7,100, and it would have been rusting through the new coat by now.
Frequently asked questions
What is cut edge corrosion on a roof? Rust that starts at the cut ends of long-run steel sheets, where the mill coating cannot reach, and spreads inward along the drip edge, laps and site cuts. It shows as a red-brown line 10 to 40 mm wide along every overlap while the flat of the sheet stays sound.
Can cut edge corrosion be repaired without replacing the roof? Yes, if the steel is still solid. The edge is abraded to bright metal, primed, sealed with a brush-grade edge sealer at 250 to 300 microns and then overcoated, at NZ$9 to NZ$16 per lineal metre. Edges that flex, crack or let light through have to be cut back or replaced.
How long before a Zincalume roof shows cut edge rust? Around 20 to 30 years on a rain-washed roof inland, 12 to 18 years within a kilometre of the coast or near industry, and as little as 10 to 15 years on sheltered edges under soffits or carports that rain never reaches.