How to ventilate the cavity under the roof tiles properly

A timber batten that rots prematurely under a tiled roof is almost never a problem with the batten itself: it's a problem of insufficient ventilation in the air cavity beneath.

What the ventilation vent does

The roof ventilation vent lets the air cavity under the tile ventilate, stopping condensation building up on the batten and the underside of the roof. Without that ventilation, condensation moisture stays trapped under the tile continuously, even on roofs where the external waterproofing is perfect.

The usual mistake: concentrating the vents at a single point

Space several vents along the whole length of the eaves instead of concentrating them at one point. What you are after is a continuous airflow under the whole roof, not just ventilating the area closest to the vents — a few vents together leave the rest of the roof slope unventilated, still accumulating damp just as if there were none.

Why treated batten alone isn't enough

Always use autoclave-treated timber for the batten — untreated, the condensation moisture under the tile rots it much sooner than expected. But timber treatment doesn't replace ventilation: they are two complementary measures, not alternatives. A treated batten in a badly ventilated cavity still degrades over time, only more slowly.

Frequently asked questions

Why does a batten rot if the roof has no visible leak? Because of condensation trapped in the air cavity under the tile, not rain leaking in. It's a problem of insufficient ventilation, not waterproofing.

Is it enough to put a few ventilation vents at one end of the roof? No: they must be spread along the whole eaves to get a continuous airflow under the whole roof, not only in the area near the vents.

Does autoclave-treated batten still need ventilation? Yes: the treatment slows degradation from damp, but doesn't replace ventilation — both work together, they're not alternatives.

Conclusion

Ventilating the cavity under the tile well, with vents spread along the whole eaves, is what stops condensation rotting the batten over time, even with perfect external waterproofing. It's an item that can't be seen once the roof is finished, but it determines how long the timber structure beneath will last.