In January 2019 I was asked to look at a house in Tikipunga on the northern edge of Whangarei, where the owners were convinced the roof was leaking. A brown stain was spreading across the hallway ceiling and no rain had fallen for nine days. I went into the roof space at 2 pm with an infrared thermometer and read 61 degrees on the underside of the sheeting. The roof was a 2011 long-run in a colour close to Ebony, painted over the original Zincalume by a previous owner. Nothing was leaking. The stain was condensation from the night before, driven by a roof that ran hot all afternoon and dumped heat into a poorly ventilated ceiling cavity as the air cooled after dark.
What colour does to sheet temperature
The number that matters is total solar reflectance, usually written TSR on a colour chart. A white or off-white roof reflects 70 to 80 per cent of the sun’s energy. Mid-greys and greens sit around 30 to 45 per cent. The charcoals and blacks that have been fashionable on new builds for fifteen years reflect 5 to 12 per cent. On a clear Northland day in February, with the air at 27 degrees, I have measured a light roof at 42 degrees and a dark one on the neighbouring house at 68. That 26-degree gap is not a rounding error. It goes straight into the roof space and, on a poorly insulated ceiling, straight into the rooms below.
Whangarei makes this worse because the humidity is high. When a dark roof heats the cavity to 55 degrees and the night air drops to 16, vapour condenses on the first cold surface it meets, which is the underside of the steel. Owners see a wet ceiling and ring a roofer about a leak that does not exist.
Coating life on dark colours
Paint chemistry does not care about fashion. Every 10 degrees of surface temperature roughly doubles the rate of the reactions that break a coating down, so a roof running 25 degrees hotter is ageing its paint several times faster than the pale roof next door. In practice I see chalking begin on dark north-facing pitches in Northland at eight to ten years, where the same paint in a light colour on the same roof would go fifteen. Thermal movement adds to it. A 6-metre sheet of steel expands about 7 mm between a cold dawn and a hot afternoon; on a dark roof the swing is greater, and the fixings work in their holes a little more each day. Loose screws and cracked sealing washers follow.
The cost shows up in the repaint cycle. A wash and two-coat acrylic repaint on a 180-square-metre single-storey roof runs NZ$5,500 to NZ$9,000 in Northland at 2026 prices. Doing it every nine years instead of every fifteen is the real price of a black roof.
Options that keep a dark look
- Heat-reflective pigments. Several NZ coating ranges now use infrared-reflecting pigments that give a dark visual colour with a TSR of 25 to 35 per cent rather than 8. They cost 15 to 25 per cent more per litre and are worth it on any pitch facing north or west.
- Go one step lighter. A mid-grey instead of a charcoal drops peak sheet temperature by 10 to 12 degrees on the days I have measured, and most people cannot tell from the street.
- Fix the roof space, not just the colour. Ceiling insulation to at least R3.3, and eave or ridge ventilation so the cavity can breathe, does more for a hot bedroom than any paint.
- Dry-film thickness. On a dark repaint I want 80 to 100 microns total dry film, not the 50 to 60 that a thin two-coat job leaves. Thicker film buys years.
Checking before you choose
Ask for the TSR figure of the exact colour on the exact product, not the brand’s general claim. Ask whether the primer is compatible with the existing surface; a dark topcoat over a badly bonded primer fails faster because the heat works on the weakest layer. Go into the roof space on a warm afternoon with a NZ$40 infrared thermometer before you sign anything. If the underside is already over 50 degrees, a darker colour is going to make the house harder to live in.
The house in Tikipunga was repainted the following winter in a heat-reflective dark grey, with ridge vents cut in and the ceiling insulation topped up. The work was done by a Whangarei painting contractor over four dry days in July, and the owners told me the following summer that the hallway ceiling stayed dry and the bedrooms were noticeably cooler by evening. The steel underside read 47 degrees when I checked it the next February. Same house, same colour family, fourteen degrees less heat going into the ceiling.
Frequently asked questions
Do dark roofs really make a house hotter in Whangarei? Yes. A black or charcoal roof can run 20 to 26 degrees hotter than a white one on a clear summer day, and most of that heat goes into the roof space and the rooms below unless insulation and ventilation are good.
How much sooner does a dark roof need repainting? Expect the first chalking on north and west pitches at eight to ten years rather than twelve to fifteen. Heat-reflective pigments close most of that gap.
Can I keep a dark colour and reduce the heat? Yes. Choose a heat-reflective version of the colour, build the coating up to 80 to 100 microns dry film, and add ridge or eave ventilation to the roof cavity.