In March 2023 I stood on a 1974 long-run roof in Mount Roskill, ran my palm along a sheet of what had once been Karaka green Colorsteel, and it came away white. The owner already had two quotes. One said wash and paint, three days, done. The other was nearly double and included a line she did not understand: “chalk removal to firm substrate.” I have priced roughly two thousand roofs and coated about four hundred of them, and I told her the second quote was the one to read, because the first would fail inside four years.
What chalking actually is
Chalking is the paint binder breaking down under ultraviolet light and leaving loose pigment on the surface. On Auckland roofs it shows fastest on north and west faces, on darker colours, and on any roof over about fifteen years old. The white residue is the top 5 to 10 microns of the old film turning to powder. Paint straight over it and the new coating bonds to dust, not to sound paint, and lets go in sheets after a couple of summers of thermal movement.
The quick test is black electrical tape pressed on a dry sheet and peeled off. A faint dusting is normal. A solid white band means a wash alone will not get you to a sound surface.
Auckland adds two problems most regions do not
The first is biological. Auckland’s humidity sits above 70 per cent for most of the year, so the chalk layer becomes a bed for mould and lichen, and both need a treatment step, not just water. A wash based on sodium hypochlorite or a quaternary ammonium compound goes on first, sits for 20 to 30 minutes, then gets rinsed. Skip it and the growth comes back through the new paint within a season.
The second is timing. Most Auckland roofs I have seen fail early were painted too late in the day. Dew forms once the sheet drops below the dew point, which in summer can happen by 4 pm, and an acrylic film that gets wet before it cures goes milky and loses adhesion. My specification reads: no coating after 2 pm between November and March, surface temperature between 10 and 35 degrees, relative humidity below 85 per cent.
The preparation sequence that holds up
- Treat with moss and mould wash and let it work.
- Water blast at 15 to 20 MPa, lance about 300 mm from the sheet, working down the pans, never across them. This is what removes chalk. A garden hose will not.
- Check the roof once it dries. Remaining chalk shows as pale streaks. Blast again.
- Grind every rust spot back to bright metal, including sheet ends at the spouting line and around every fixing.
- Spot prime bare steel with a rust-inhibiting primer, then a full coat of adhesion primer over the whole roof. On a badly chalked roof the full primer coat is not optional.
- Two coats of water-based acrylic roof coating, each at roughly 50 microns dry film thickness, giving 100 to 120 microns total over primer.
Preparation is around 60 per cent of the labour hours on a repaint, which is why the honest quote was double. The paint itself is usually only NZ$700 to NZ$1,200 for a 180 square metre roof; the rest is time on the roof.
Realistic costs and life expectancy
For a single-storey Auckland house with 160 to 200 square metres of walkable long-run, a prepared two-coat repaint prices between NZ$28 and NZ$45 per square metre, so NZ$5,000 to NZ$8,500. Add NZ$800 to NZ$2,000 for significant edge rust, and NZ$1,500 to NZ$3,000 if the pitch or height needs edge protection. Anything under NZ$20 per square metre has skipped preparation, and I have watched several of those peel within three summers.
A prepared and primed repaint should give 12 to 15 years, with a wash every second year. Painting over chalk gives 3 to 5.
Who should be doing this
Roof painting is not the same trade as house painting. The person on the roof needs to understand fixings, laps and edge corrosion, because half the value of the job is what gets fixed during preparation. When an owner asks me who to get, I say to ask for the primer product name and the dry film thickness before the price; whether you are looking at an Auckland roof painting crew or any other outfit, judge them on whether they can answer those two questions without looking it up.
Frequently asked questions
Can a heavily chalked roof be painted at all? Yes, provided the steel underneath is sound. Chalk has to be removed by water blasting at 15 to 20 MPa and the roof fully primed before topcoats go on.
How long does a Colorsteel roof repaint last in Auckland? A prepared and primed two-coat acrylic system lasts 12 to 15 years. A wash-and-paint job over unremoved chalk usually fails within 3 to 5 years.
What does it cost to repaint a chalked long-run roof? Expect NZ$28 to NZ$45 per square metre including preparation and primer, or NZ$5,000 to NZ$8,500 for a typical 160 to 200 square metre single-storey roof, before any edge protection.