Do fibre cement slates need a ventilated counter batten?
Now that many fibre cement roof installations incorporate vapour permeable underlays, is a ventilated counter batten still needed to meet ventilation requirements? In short the answer is yes, a ventilated counter batten is required.
Fibre cement slates are close fitting and do not allow much airflow through the laps and joints. It is therefore necessary to provide a 50mm ventilated batten cavity above the underlay and below the slates (comprising a 25mm deep counterbatten and 25mm deep tiling batten). This cavity should be ventilated equal to 25mm at the eaves and 5mm at the ridge.
This recommendation should be followed for all types of roof construction where fibre cement slates and vapour permeable underlay are specified to minimise the risk of harmful condensation or moisture build-up in the batten space.
If it is not practical to install a counterbatten, a non vapour permeable underlay should be used with traditional low and high level ventilation (below the underlay and above the insulation) in accordance with BS 5250 ‘Control of Condensation in buildings’.
You can find out more about pitched roof specification considerations in our guide.