Signs that point each way
| Condensation | Roof leak | |
|---|---|---|
| When it shows up | After cold, clear, still nights, mostly in the colder months, rain or not | During or soon after rain, especially heavy or wind-driven rain |
| What it looks like | Lots of small spots, a grey haze or black mould, often in corners | One stain or a few, usually with brown tide-mark rings |
| Which rooms | Bathrooms, laundries, kitchens and bedrooms where moist air builds up | Rooms under chimneys, skylights, valleys or spouting |
| In the ceiling space | Droplets or damp across the underside of the roofing or underlay | A wet trail on timber leading down from one spot |
| Gets worse when | Long showers, washing dried inside, unflued gas heaters running, windows shut | The rain gets heavier or the wind swings round |
How condensation ends up on your ceiling
Showers, cooking, drying washing inside and even breathing put water vapour into the air. Unflued gas heaters add more. Warm, moist air rises into the ceiling space through gaps around downlights, the hatch and extractor fans, and when it meets the cold underside of the roof, it turns back into water.
Iron and longrun roofs show it most, because metal cools quickly on a clear night. Water beads on the underside, drips onto the insulation or ceiling and looks a lot like a leak, often in several places at once.
A common culprit is a bathroom or kitchen fan that vents into the ceiling space instead of outside. All that steam gets dumped right where it will condense.
The Building Code's external moisture clause, E2, expects concealed spaces to be built so outside moisture doesn't build up and cause condensation, mould or decay. Many older homes have a ceiling space with little ventilation and no underlay, which gives moisture nowhere to go.
Where in New Zealand it shows up most
Humid places and frosty places both get it, for different reasons. NIWA says relative humidity is high in all seasons across Auckland, with a mean 9 am reading of 82% at Auckland Airport. At Henderson in west Auckland it averages 86% and climbs to 92% in June and July.
Inland, it's the cold nights. Hamilton's Ruakura station averages 21 air frosts a year and Taupō 35.2, mostly between May and September. On those clear, frosty nights a metal roof gets colder than the air beneath it, and moisture in the ceiling space collects on it.
Run a two-week test
Keep a morning log
Each day, note whether it rained overnight, whether it was a cold, clear night, and whether new marks appeared.
Photograph the marks
Same spot, same angle, every few days. Marks that grow after rain point to the roof. Marks that grow after frosty nights point to condensation.
Look in on a cold morning
Before the sun warms the roof, look from the ceiling hatch with a torch. Damp across the whole underside of the roofing or underlay suggests condensation. A single wet trail suggests a leak.
Follow the fan ducts
Check where the bathroom, kitchen and laundry fans end up. If a duct stops in the ceiling space, that moist air is being parked where it condenses.
Change habits for a week
Run fans longer, open a window during and after showers, dry washing outside or in a vented dryer, and give the unflued gas heater a rest. If new marks stop, you've got your answer.
Looking in the ceiling space safely
Stay on the ladder at the hatch rather than crawling across the joists, because the ceiling lining won't carry you. Keep well clear of wiring. Old insulation and dust can irritate skin and lungs, so look rather than touch. WorkSafe says homes built before 2000 may contain asbestos and that you can't confirm it by eye, so leave any old sheet material or crumbly lagging alone. And if a ceiling is sagging, stay out of that room.
What fixes condensation
Get the moisture out where it starts. Extractor fans in bathrooms, kitchens and laundries, ducted to the outside rather than into the ceiling space. Vent the clothes dryer outside too.
Let the ceiling space breathe. Roof vents and soffit vents let damp air out and drier air in. Our page on roof ventilation compares the options.
The right layer under the roofing. Underlay beneath iron and longrun catches drips and helps manage moisture. E2/AS1 says all profiled metal longrun and metal tile roofing must have underlay, so a re-roof is the usual time to add it where an older roof has none. Some COLORSTEEL roofing, sold as DRIDEX, has an absorbent fleece on the underside that soaks up condensation and releases it later; NZ Steel says it replaces separate underlay in many applications, and its warranty excludes inadequate ventilation. Our roof underlay page and the guide to insulation and underlay explain the choices.
Insulation without gaps, and not jammed against the roof. Patchy insulation leaves cold spots where moisture collects. Kingspan Thermakraft notes a mandatory 25 mm minimum air gap between insulation and roof underlay, so insulation stuffed hard up under the roofing can cause its own damp. MBIE says retrofitting insulation in a ceiling space doesn't need a building consent.
When it's both
Sometimes there's a small leak and a damp ceiling space at the same time. If your log shows marks after rain and after frosty nights, get the roof checked first, then sort out the ventilation. A roof repair does nothing for condensation, and a vent does nothing for a hole.
What you can try, and what needs a professional
You can
- Running extractor fans during and after showers and cooking
- Drying washing outside, or venting the dryer outside
- Keeping a two-week log of weather and new marks
- Checking from the hatch where fan ducts end
- Swapping an unflued gas heater for a drier form of heat
Leave to a roofer
- Finding and fixing a real roof leak
- Fitting roof or soffit vents
- Adding or replacing underlay, usually during a re-roof
- Ducting fans out through the roof or soffit, often with an electrician
- A roof inspection if you still can't tell which it is
What we'd try first
Before paying anyone, we'd keep the log for a fortnight. Marks that track frosty mornings say condensation, so the first spend would go on habits and fans: longer extraction, ducts that end outdoors, washing dried outside, and a different heater if an unflued gas one is in use. Only if marks kept appearing after rain would we bring in a roofer, and we'd ask them to show us the entry point before agreeing to a repair.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.