In March 2023 I was sent to a house in Devonport, about 300 metres from Cheltenham Beach, to look at a repaint that had lifted after eighteen months. The product was one I had supplied. I could peel the topcoat off in strips 200 mm wide with my fingers. Underneath was the chalked surface, and on the chalk a faint white crust: salt. The painters had blasted the roof and primed it the same day, in a nor’easter. They had done everything on the label except read the line that said the substrate must be washed free of salt and tested before coating.
Why salt kills a repaint
Sea air carries chloride, and chloride settles on every surface downwind of the sea. On a North Shore roof within about 500 metres of the water, NZS 3604 calls this the sea spray zone, and the salt deposition is high enough that the surface never dries clean. Chloride does two things to a coating: it sits between the paint and the steel and prevents bonding, so adhesion fails from day one; and it is hygroscopic, pulling moisture through the film so the steel corrodes while the surface still looks new. Every acrylic roof coating on the New Zealand market fails on a salty substrate, because none of them is being asked to stick to steel. They are being asked to stick to dissolved salt.
What a proper wash-down looks like
The wash-down is a separate job from the repaint and should be priced as one. The sequence I specify:
- Fresh-water rinse at low pressure, 60 to 100 bar, from ridge to eave, to lift loose salt and chalk without driving it into the laps.
- A roof wash with a mould and chalk cleaner, left for 10 to 20 minutes, then scrubbed at the laps and fixings where salt concentrates.
- High-pressure rinse at 150 to 200 bar, working down the sheets, with attention to the underside of the laps and the drip edge into the spouting.
- A salt test: a swab and conductivity check, or a chloride strip from a marine supply shop. The reading must be below the coating maker’s limit, around 50 milligrams per square metre for most roof acrylics.
- Priming within 24 hours, never after a night of onshore wind. At Devonport that means watching the forecast and sometimes waiting a week.
If the test fails, the roof gets rinsed again. Some roofs at Takapuna took three rinses, and at Castor Bay the salt came back overnight.
Cut edges and screws
Salt works hardest at the edges. Every cut edge, screw head and lap is a place where the factory coating has been cut or pierced. On the Devonport roof the drip edge had 15 to 20 mm of red rust the full length, hidden under the peeling topcoat. This edge needs wire-brushing to bright metal, a rust-converting or zinc-rich primer, then a brush-grade edge sealer at about 300 microns before any topcoat. That runs roughly NZ$10 to NZ$18 per lineal metre and is the line item most often missing from a coastal repaint quote. Galvanised screws corrode fast in sea air; I expect a screw replacement allowance of NZ$2 to NZ$4 per screw for stainless or Class 4 replacements.
Costs and how long it lasts
For a single-storey North Shore house with 150 to 180 square metres of long-run roof, a coastal repaint costs NZ$6,500 to NZ$10,500 in 2024 pricing. The wash-down and salt testing is NZ$700 to NZ$1,400 of that, the edge and screw work another NZ$800 to NZ$1,800, and the coating itself the balance. That is 10 to 20 percent more than the same roof in Hamilton, and the alternative is what I saw at Devonport: NZ$7,000 of paint on the ground after eighteen months. Expect a coastal repaint to last 8 to 12 years, against 12 to 15 inland. Most warranties require an annual fresh-water wash.
Owners on the Shore find that a North Shore painter who has done a dozen repaints within sight of the sea already prices the wash-down and salt test without being asked, and treats the forecast as part of the job. The Devonport roof was redone that winter, washed twice, tested, and primed on a still morning. It has not moved since.
Frequently asked questions
How close to the sea does a roof need a salt wash-down before painting? Within about 500 metres of the coast, the NZS 3604 sea spray zone, always. Between 500 metres and about 2 kilometres, wash and test anyway on the North Shore, because the prevailing nor’easter carries salt well inland.
How much does a coastal roof repaint cost on the North Shore? NZ$6,500 to NZ$10,500 for a single-storey house with 150 to 180 square metres of long-run steel, including wash-down, salt testing, edge treatment and two coats. That is 10 to 20 percent above an inland repaint.
How long does roof paint last near the sea? 8 to 12 years for a quality acrylic system applied to a properly washed and tested substrate, compared with 12 to 15 years inland. Skipping the salt wash can bring that down to under two years.