Signs your ceiling space needs more airflow
Black spotting on ceilings that creeps back a few weeks after you clean it, most often in bathrooms, laundries and cold bedrooms.
Insulation that feels damp or smells musty, water beads or rust spots on the underside of iron roofing, or timber that's stained dark. Drips after cold, clear nights rather than rain point to condensation, and our condensation or leak guide shows how to tell the two apart.
In summer, a ceiling space that's stifling under a dark roof. NZ Steel's figures put Cloud, its palest COLORSTEEL colour, at a total solar reflectance of 69% and Ebony at 5%, so a dark roof soaks up far more of the sun's heat.
Start with the moisture, not the vents
A vent can only shift moisture that's already up there. Stopping it getting there is quicker, and usually does more.
Find out where your bathroom, kitchen and laundry fans actually finish. A duct that stops in the ceiling space, or a fan with no duct at all, parks steam right under the cold roof. Vent the clothes dryer outdoors as well, and fit a seal around the ceiling hatch.
Ducting a fan outside is usually a small job. Schedule 1 of the Building Act lets you make a hole of up to 300 millimetres across through an existing house, including the weatherproofing around it, for pipes, cables, ducts and similar, without a building consent. All building work still has to meet the Building Code, so that hole needs flashing or sealing by someone who knows your roof.
Ways to ventilate a ceiling space
| Option | How it works | Worth knowing | |
|---|---|---|---|
| Ducted extractor fans | Pull steam out of bathrooms, kitchens and laundries to the outdoors | Tackles moisture at the source | |
| Soffit or eaves vents | Openings in the soffit (the lining under the eaves) let drier air in low down | Older eaves linings can contain asbestos, so test before anyone cuts | |
| Roof vents and whirlybirds | Covered outlets or wind-spun turbines let warm, damp air out near the top | Every one is a hole through the roofing that must be flashed weathertight | |
| Ridge vents | A continuous vent along the ridge lets air out along its whole length | Easiest to add during a re-roof, when the ridge is off anyway | |
| Powered roof fans | A fan pushes air out of the ceiling space | Without a way for air to come in, it draws warm air up out of the house |
Underlay, insulation and the gaps between them
Plenty of older New Zealand roofs went on with no underlay (the building paper under the roofing), so condensation on the iron drips straight onto the insulation. E2/AS1 now says all profiled metal longrun and metal tile roofing must have an underlay, and a re-roof is the natural time to put one in.
Some COLORSTEEL comes with a fleece on the underside, sold as DRIDEX, that soaks up condensation and releases it later. Its warranty excludes inadequate ventilation, so it works alongside airflow rather than replacing it.
Insulation needs breathing room too. Kingspan Thermakraft notes a mandatory 25mm minimum air gap between insulation and roof underlay, so batts jammed hard against the roof are worth sorting out.
Skillion roofs, where the ceiling follows the roofline, have no ceiling space to vent. BRANZ's appraisal of Gerard's pressed metal tiles, for example, says a skillion roof needs a ventilated air gap of at least 25 mm under the tiles, which is a design question for re-roof time.
Where in New Zealand it bites hardest
Sheltered, foggy valleys see a lot of it. Wallaceville in Upper Hutt averages 43 fog days and 24.1 air frosts a year, with a mean 9 am relative humidity of 84.2%: damp air meeting a cold roof, night after night.
Geothermal areas add another layer. The metal roofing Code of Practice treats buildings within 50 m of a fumarole as a geothermal microclimate, where humidity and hydrogen sulphide together speed up corrosion, and counts much of Rotorua as geothermal anyway. A damp ceiling space there is hard on the roofing too.
Older home? Test before anyone cuts
WorkSafe says homes built before 2000 may contain asbestos and lists gables and eaves among the places it turns up, so have old eaves linings tested before soffit vents are cut into them. Health NZ warns the ceiling under a weathered asbestos-cement roof can hold high concentrations of asbestos dust. If you look into the ceiling space, do it from the hatch with a torch, keep off the plasterboard and stay clear of wiring.
Questions to ask a roofer about ventilation
- Where is the moisture in my ceiling space coming from, and can we stop it first?
- Where will fresh air come in, and where will damp air leave?
- How many vents does my roof need, and how did you work that out?
- What flashing will you use for my roofing, and who guarantees it against leaks?
- Is there underlay under my roof now, and should it go in at the next re-roof?
What you can check, and what needs a roofer
You can
- Run extractor fans during and after every shower, and for a while after cooking
- Check from the hatch where each fan duct really ends
- Dry washing outside, or in a dryer vented outdoors
- Photograph mould patterns and note which rooms and which weather bring them on
Leave to a roofer
- Cutting and flashing roof vents or whirlybirds
- Adding ridge vents or underlay, usually at re-roof time
- Ducting fans out through the roof or soffit, alongside an electrician
- Testing old eaves linings or cement sheeting before anything is cut
If it were our ceiling space
We'd chase the moisture first: fans ducted outdoors, the dryer vented outside and washing dried outdoors where possible. If the ceiling space was still damp after a winter of that, we'd have a roofer add intake low down and an outlet up high, and we'd ask about underlay before the next re-roof. We wouldn't buy a powered fan until passive vents had been given a fair go.
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.