Ventilation ensures that the dew point does not occur on the underside of the zinc. The air gap located beneath the support allows the evacuation of humidity caused by internal condensation. The following rules apply with low to moderate hygrometry levels.
Roof ventilation rules
Air gap is necessary to allow the evacuation of humidity. In addition, if water infiltration occurs, moisture can be absorbed by the wood support and evacuated through the air gap. This air gap should have a minimum thickness of 40 mm. If the slope exceeds 12 metres, this minimum should be 60 mm. To ensure the proper airflow in the ventilation space, an air inlet should be set at the eaves and an outlet at the ridge.
Cladding ventilation rules
In general, building facades can be divided into two main categories :
- Curtain walls
- Rain screen walls
A curtain wall is a sealed cladding, the building is completely isolated from the exterior facade by a sealed airtight cavity. Normally there is no ventilation happens behind curtain wall.
A rain-screen wall is an open cladding which allow the ingress of some water. The cavity behind the cladding is drained and the ventilation at the back promotes rapid evaporation or removal of any water ingress. For example : a masonry cavity wall incorporating a water barrier, flashing and frequent, narrow width openings (weep holes) is this type of rainscreen wall.
For most zinc cladding, the air gap must be 20 mm minimum, to ensure air inlet at the bottom and air outlet at the top.