Signs your roof space needs more airflow
Mould or black spotting on ceilings that comes back after you clean it, especially in bathrooms and rooms that get little sun.
Droplets or rust streaks on the underside of a metal roof, or insulation that's damp and musty. Condensation dripping onto a ceiling is often mistaken for a leak, and our guide to condensation or a leak helps tell them apart.
A roof space that's stifling on summer afternoons and keeps upstairs rooms warm well into the night. CSR Bradford, which makes roof vents, says an unventilated roof space can reach 70 degrees C in summer.
How roof ventilation actually works
Air needs a way in low and a way out high. Intake usually comes through vents in the eaves; exhaust leaves near the ridge through whirlybirds, ridge vents or a fan. Bradford's rule of thumb is one natural roof vent for every 90 square metres of roof, with four eave vents for each one.
Exhaust vents with no intake don't work well. They end up pulling air out of the house through downlights and gaps in the ceiling, which can mean pulling out the air you've paid to cool or heat.
Check where your bathroom and kitchen exhaust fans go, too. The building code requires them to discharge to outdoor air, so one that just blows into the roof space is feeding the moisture problem vents are meant to fix.
The main types of roof vent
| Type | How it works | Suits | |
|---|---|---|---|
| Whirlybird | Wind spins a turbine that draws air out | Most homes; no power needed. See whirlybirds | |
| Static or box vent | A fixed, covered opening that lets hot air rise out | Cheap, silent and works with no wind | |
| Solar or powered fan | A fan driven by a solar panel or mains power pushes air out | Hot roof spaces where the wind is unreliable | |
| Ridge and eave vents | Continuous vents along the ridge and under the eaves | New roofs and re-roofs, where they're easiest to add |
Older house? Old vents can contain asbestos
ASSEA lists asbestos cement exhaust ventilators, vent bases and ventilating ridges among older roofing products. If a vent or the roof around it might be asbestos cement, don't let anyone cut or remove it until it's been tested.
Ventilation and insulation work as a pair
Insulation slows heat moving through the ceiling. Ventilation clears the hot or wet air sitting above it. Your Home warns that over-sealed roof spaces raise the risk of condensation, and in the coldest climate zones (6, 7 and 8) NCC 2022 requires the roof space above the insulation to be ventilated to outside air.
If you're doing one, look at the other. Our pages on roof insulation and sarking explain the layers under your roof.
Questions to ask a roofer
- How many vents does my roof need, and how did you work that out?
- Where will the replacement air come in?
- What base or flashing will you use for my tiles or sheets?
- Will passive vents do the job here, or is a powered fan worth it?
- If we're in a bushfire-prone area, do the vents need ember screens?
What you can check, and what a roofer does
You can
- Feel the heat from the manhole on a hot afternoon, without crawling through the roof space
- Find out where your bathroom and kitchen fans are ducted
- Make sure eave vents haven't been painted over or blocked
- Photograph mould patterns so the roofer can see where moisture gathers
Leave to a roofer
- Cutting and flashing vents into tiles or sheets
- Adding eave or ridge vents
- Ducting exhaust fans out through the roof
- Mains wiring for powered fans, by a licensed electrician
If it were our roof space
We'd deal with the moisture first: duct the bathroom and kitchen fans outside and dry clothes outdoors where you can. Then we'd add passive vents, a couple of whirlybirds with some eave intake, before paying for powered fans. If mould keeps coming back after that, we'd have a roofer look for a leak or another source of water.
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.