In July 2024 I stood in a hallway in Kamo with a homeowner certain her roof leaked. There were three brown rings on the ceiling, one of them 400 mm across, and a bucket on the carpet. It had not rained for four days. The roof was dark grey Colorsteel, about fifteen years old, and when I got into the roof space at 7.30 in the morning the underside of every sheet was beaded with water. Nothing was coming through the roof. It was coming out of the house.
Why I knew it was not a leak
Leaks follow rain and follow a path. Water comes in at a screw, flashing or lap, runs down the underlay or purlin and drops somewhere you can trace back uphill. Condensation does not behave like that. It appears on cold mornings regardless of rain, is spread across the whole underside of the roof rather than concentrated at one point, and is worst over the rooms that produce the most moisture: bathroom, kitchen, laundry. In that Kamo house the ceiling stains were directly under the purlins above the bathroom and main bedroom, and they were faintly ringed, the mark of water dried and rewetted dozens of times, not the single dark bloom of a real leak.
Perhaps one in six of the leaks I am asked about in Northland between May and September turns out to be condensation. It is the most misdiagnosed roof problem in the region.
Why Northland roofs sweat
Three things have to line up. First, moist air: Whangarei winter mornings regularly sit at 85 to 95 percent relative humidity, and a family of four adds another 10 to 15 litres of vapour daily through cooking, showers and unvented dryers. Second, a cold surface: on a clear still night a steel roof radiates heat and can drop 2 to 4 degrees below air temperature, condensing moisture on the steel when it falls below the dew point. Third, a path for house air to reach the steel: gaps around downlights, the hatch, pipe penetrations, and bathroom fans ducted into the roof space rather than outside.
Roofs without underlay, or with old torn bitumen paper, are far worse, because underlay catches the drips and lets them evaporate rather than fall on the ceiling. Dark roofs run slightly colder overnight and sweat a little earlier.
The three things I check in the roof space
- Where the extractor fans go. In the Kamo house the bathroom fan had a flexible duct that stopped 300 mm short of anything and simply blew steam into the roof. That was the whole problem. Ducting it through the soffit with a proper outlet cost about NZ$450.
- The underlay. Look for it sagging between purlins, torn at the laps, or missing over the last two sheets where somebody did a repair. If there is no underlay at all, every drip lands on the insulation or the ceiling.
- Ventilation. A roof space needs air moving through it. Blocked soffit vents, insulation pushed hard against the eaves and a sealed ridge mean the moist air has nowhere to go.
What fixes it and what does not
A coating will not fix condensation. Repainting changes nothing on the underside of the steel where the problem lives, and I have watched owners spend NZ$6,000 to NZ$9,000 on a repaint that made no difference to the ceiling stains.
What does work, roughly in order of cost:
- Ducting every extractor fan to the outside, NZ$300 to NZ$700 per fan.
- Sealing the ceiling penetrations and the hatch, NZ$200 to NZ$600.
- Adding eave and ridge ventilation, NZ$800 to NZ$2,500 depending on roof length.
- Topping up ceiling insulation so the ceiling stays warm and the moisture stays in the rooms where it can be vented, NZ$1,500 to NZ$3,500 for a typical three-bedroom house.
- Replacing torn or missing underlay, which realistically only happens when sheets come off for other reasons, since the underlay sits beneath the steel.
Anti-condensation fleece bonded to the underside of the sheet is available on new long-run steel and works well, but it is a specification for a new roof, not a retrofit.
The fan ducting and the ceiling sealing are within reach of a careful homeowner. Underlay, ventilation cut into the roof and anything that means lifting sheets is roofer’s work, and in the north that means a Whangarei roofing contractor who has seen enough sweating roofs to look inside before quoting anything on the outside. The Kamo owner had the fan ducted and the hatch sealed in an afternoon, and the beading on the steel was gone within a fortnight.
Frequently asked questions
How can I tell condensation from a roof leak? Condensation shows up on cold clear mornings regardless of rain, is spread across the whole underside of the roof, and stains the ceiling in faint repeated rings over the wettest rooms. A leak follows rain, comes from one point, and can be traced uphill to a screw, lap or flashing.
Does a dark roof cause more condensation? Slightly, in winter. A dark steel roof radiates heat faster on a clear night and its surface drops further below the dew point, so it sweats earlier and for longer. The difference is small compared with an extractor fan venting into the roof space.
Will repainting the roof stop condensation? No. Condensation forms on the underside of the steel, where the coating does not reach. Fix the moisture source, the ventilation and the underlay first, and repaint only for the reasons a roof normally needs it.