Twenty years of spray guns taught me that a roof coating almost never fails in the middle of a sheet. It fails at the laps, along the ridge, around the screws, and in the first 300 mm above the spouting — every place where salt collects and water sits after the rest of the roof has dried. On a Devonport bungalow I looked at recently the paint was sound across the whole north face and lifting in ribbons along every horizontal lap. That roof had been washed. It had not been rinsed.
What salt actually does to a coating
Airborne chloride settles on everything within a few kilometres of the coast, and Auckland’s geography puts most of the city inside that band. Chloride is hygroscopic — it pulls moisture out of humid air — so a salted surface stays damp far longer than a clean one. Paint applied over that film loses adhesion from underneath, and where the substrate is bare or lightly rusted, the salt drives corrosion under the new coating rather than stopping it.
This is why preparation is 70 per cent of a roof repaint and why the cheapest quote is almost always cheap in the same place. Not in the paint. In the wash.
Preparation, in the order it has to happen
- Treat the growth first. Moss and lichen have roots into the coating. A biocidal wash left to dwell for the stated time kills them; blasting them off wet leaves the anchor points behind and they return within two seasons.
- Wash, then rinse with clean water. Detergent lifts the salt into solution; only a fresh-water rinse removes it. Skipping the rinse leaves a salt-rich film that is invisible when dry.
- Deal with the rust properly. Surface rust gets wire-brushed or abraded back to sound metal and spot-primed. Perforated sheets and rusted-through laps are a replacement job, not a painting job, and no coating system bridges a hole honestly.
- Check the fixings. Old lead-head nails on long-run steel work loose and let water under the sheet. Replacing them with screws and neoprene washers before painting is cheap; doing it afterwards damages the new coating.
Primer and system choice
On weathered coated steel with sound factory coating remaining, a waterborne or solvent-borne roof primer suited to that substrate goes on first, with an etch or metal primer used on exposed bare steel and galvanised areas. On concrete tile, the priority is a sealer that binds the chalky surface — an unsealed tile drinks the topcoat and you get a patchy, powdery finish within a year. Two topcoats is the standard for a reason: the first fills, the second builds film thickness, and film thickness is what buys years.
Expect a roof coating system on a well-prepared Auckland roof to give 8 to 15 years before it needs recoating, less on a hard coastal exposure and less again on the windward face. Anyone promising twenty-plus on a marine site is quoting the tin, not the postcode. Costs for a full wash, prep and two-coat system commonly sit somewhere around $40 to $75 per square metre of roof depending on pitch, access and how much rust remediation is involved.
Timing the job
Coatings want a surface temperature above roughly 10°C, relative humidity below about 85 per cent, and no dew forming within the drying window. In Auckland that rules out most of June and July for anything except a settled run of days, makes mid-winter mornings unusable, and pushes the best windows into October–December and February–April. Late afternoon application in autumn is the classic mistake: the film is still soft when the dew lands and you get a bloomed, uneven finish that never fully hardens.
Rain within the recoat window is the other one. A shower two hours after the topcoat on a roof that has not flashed off will wash pigment down into the spouting, and that is a repaint rather than a touch-up. This is the main reason experienced roof painters working around Auckland will walk off a job at midday when the forecast turns, and why a quote that promises a fixed two-day window in September should be read carefully.
Frequently asked questions
Can a roof be painted over existing peeling paint? No. Anything lifting has to come off back to a sound edge, and if more than a modest proportion of the roof is failing, the whole coating needs removing. Painting over failure just transfers it to the new film.
Does roof painting stop rust? It slows it where the steel is still sound and it does nothing where corrosion has already gone through. Coating is protection, not repair.
How often should a coastal Auckland roof be washed? A fresh-water rinse once a year, ideally at the end of summer, measurably extends coating life. It is the highest-value maintenance available on a metal roof and almost nobody does it.