Signs that point each way
| Condensation | Roof leak | |
|---|---|---|
| When it appears | Cold, clear mornings, mostly in the cooler months, rain or no rain | During or after rain, especially heavy or windy rain |
| What it looks like | Lots of small spots or a grey haze, often with mould | One stain or a few, with brown tide-mark rings |
| Which rooms | Bathrooms, laundries, kitchens and bedrooms where moist air collects | Top-floor rooms under valleys, chimneys, skylights or gutters |
| In the roof space | Damp on the underside of the sheets or sarking over a wide area | A wet trail on the timber leading down from one spot |
| Gets worse when | Long showers, clothes drying inside, heaters on with windows shut | The rain gets heavier or the wind swings to one side |
How condensation ends up on your ceiling
Showers, cooking, drying clothes and even breathing put water vapour into the air. Warm, moist air rises into the roof space through gaps around downlights, the manhole and exhaust fans. When it meets the cold underside of the roof, it turns back into water.
Metal roofs show it most, because the sheets cool quickly on clear nights. Water beads under the sheets, drips onto the insulation or ceiling and looks a lot like a leak. Lysaght says reflective foil laminate draped between the supports under metal roofing acts as a vapour barrier.
Well-sealed homes can make it worse. Your Home says over-sealed roof spaces raise the risk, and that a well-ventilated roof space carries excess water vapour away and helps avoid mould.
A common culprit is an exhaust fan that blows into the roof space instead of outside. The NCC requires exhaust from kitchens, bathrooms, toilets and laundries to discharge to outdoor air, either directly or through a duct.
A simple two-week test
Keep a daily log
Each morning, note whether it rained overnight, whether it was a cold night, and whether any new marks have appeared.
Photograph the marks every few days
Same spot, same angle. Marks that grow after rain point to the roof. Marks that grow after cold, clear nights point to condensation.
Look in the roof space on a cold morning
Early, before the sun warms the roof, look from the manhole with a torch. Moisture all over the underside of the sheets or foil suggests condensation. A single wet trail suggests a leak.
Find out where the fans go
Check where the bathroom and laundry fans' ducts end. If they stop in the roof space, that moist air is being dumped right where it condenses.
Change your habits for a week
Run fans longer, crack a window during and after showers, and dry clothes outside or with a vented dryer. If the new marks stop, you've found your answer.
Looking in the roof space safely
Stay at the manhole rather than crawling across the ceiling, which won't take your weight. Keep away from wiring, and from foil near cables, because Your Home warns foil insulation is electrically conductive. If the house was built before 1990 and you can see loose, fluffy fill between the joists, don't touch it: some older homes used loose-fill asbestos as ceiling insulation. And if a ceiling is sagging or bulging, keep out of the room.
What fixes condensation
Get moist air out where it starts. Exhaust fans in bathrooms, laundries and kitchens, ducted outside. For new work, the NCC sets minimum flow rates of 25 L/s for bathroom and toilet fans and 40 L/s for kitchen and laundry fans.
Let the roof space breathe. Roof vents or whirlybirds, with eave vents to let fresh air in, carry damp air out. CSR Bradford says humidity-responsive powered roof vents can reduce condensation risk in winter. Our page on roof ventilation compares the types.
The right layer under the roof. CSR Bradford recommends non-permeable sarking in hot, humid climates and vapour permeable sarking in mild-summer, very cold winter climates. Under metal, foil laminate or a blanket against the sheets does the same job. Our sarking page explains the options.
Newer homes in cold climates. In climate zones 6, 7 and 8, NCC 2022 requires a roof space at least 20 mm high above the insulation, or vapour permeable sarking, ventilated to the outside. Older homes weren't built to that rule.
When it's a bit of both
Sometimes it's a small leak and a damp roof space at the same time. If your log shows marks after rain and after dry, cold nights, get the roof checked first, then look at ventilation. Fixing the leak won't stop condensation, and vents won't stop a leak.
What you can try, and what needs a pro
You can
- Running exhaust fans during and after showers and while cooking
- Drying clothes outside, or venting the dryer outside
- Keeping a two-week log of weather and new marks
- Checking from the manhole where the fan ducts end
Leave to a roofer
- Finding and fixing a real roof leak
- Fitting roof vents or whirlybirds
- Adding foil, a blanket or sarking under the roof
- Ducting fans out through the roof or eaves, often with an electrician
If it were our ceiling
We'd run the two-week log before spending a cent. If the marks follow cold mornings rather than rain, we'd start with the cheap fixes: fans on longer, ducts taken outside, clothes dried outdoors. If that doesn't do it, a roofer can look at ventilation and what's under the roof. We'd only pay for leak repairs once someone shows us where water is actually getting in.
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.