Every penetration through a roof is a deliberate hole in a surface designed to keep water out. Whirlybirds, eave vents, ridge vents and plumbing vents are all holes, and they matter for two separate reasons: they are the main defence against roof-space condensation, and they are the first thing that has to be managed before a wash.
What roof ventilation does
A Sydney roof space in summer reaches 50 to 60 degrees and higher under dark tiles. In winter it is cold, and moisture rising from the house — showers, cooking, drying washing, and in many houses a bathroom fan ducted straight into the cavity — condenses on the underside of the sarking.
Ventilation addresses both. Hot air out in summer reduces the load on the ceiling and the air conditioning. Air movement in winter carries moisture out before it condenses.
That second function is the one that matters most and is least understood. Roof-space condensation is a major cause of ceiling mould, and the symptom usually appears in a bathroom — see mould on a bathroom ceiling.
Intake matters as much as exhaust
The commonest mistake, and it is worth stating clearly.
A whirlybird is an exhaust. For it to move air, air has to be able to get in. That intake is usually the eave vents, and if those are painted over, blocked by insulation batts pushed too far out, or were never fitted, the whirlybird spins and achieves very little.
Rule of thumb: roughly equal intake and exhaust area, with intake low at the eaves and exhaust high at the ridge.
Two whirlybirds with open eave vents outperform four with blocked ones.
How many
Depends on roof area and climate, but as a general guide, one standard 300mm whirlybird per 50 square metres of roof is a common starting point, which puts a typical Sydney house at two to four.
Position them near the ridge, on the side that gets least wind-driven rain, spread along the roof rather than clustered.
Alternatives worth knowing: ridge vents run along the ridge line and are less visible and more effective per metre; solar-powered extractors move far more air than a passive whirlybird and work when there is no wind, which is when the roof is hottest.
Why they matter before a wash
This is the part relevant to cleaning.
Any roof vent is an opening. Directing water at one — from a pressure washer, or even a soft wash lance at the wrong angle — puts water straight into the roof cavity, onto the insulation and the ceiling plaster.
The damage shows up days later as a ceiling stain, and by then it is not obviously connected to the wash. It is the same principle as weep holes and subfloor vents: openings that exist for a reason and must never be targeted.
On our jobs the vents are identified before work starts, and in some cases covered temporarily. Whirlybirds in particular have an open throat under the cowl that water will find if you spray upward into it.
Worth noting that this is one more reason a roof should never be pressure washed at all — covered here.
Maintenance
Whirlybirds have bearings, and they fail. A unit that has stopped spinning, or squeals, is doing nothing. Most last ten to fifteen years and then want replacing rather than repairing.
They also accumulate debris, and the flashing where they meet the roof is a common leak point as the sealant ages.
While anyone is on the roof for a clean, this is the obvious time to check that vents are turning, flashings are sound, and eave vents are clear. It takes a minute and it is the kind of thing a photo report should cover.
Catchment roofs
If the roof feeds a rainwater tank, vents are a contamination pathway in reverse — debris and bird activity around a vent ends up in the catchment. Worth checking screens are intact, and worth including in the inspection covered in cleaning a rainwater tank.
Birds and possums
Unscreened vents are entry points. Gable and eave vents in particular are a common way possums get into a roof space — see possums in the roof.
Screening them is cheap and solves a problem that is expensive once it has started.
If you are being quoted for ventilation
Ask whether the intake is being addressed, not just the exhaust. A quote for three whirlybirds with no mention of eave vents is a quote for three ornaments.
And if the actual problem is moisture rather than heat, check where the bathroom exhaust fan discharges before spending anything on the roof. That one is free to look at and fixes the issue surprisingly often.
For a roof clean that includes a condition check on vents, flashings and penetrations, see roof cleaning.
