A warm roof, for example a ‘room in the roof’ or a loft conversion will require ventilation at high and low level to remove moisture laden air from the roof batten cavity. It is also recommended that the ceiling is ‘well -sealed’ with an associated air and vapour control layer (AVCL) For a simple duo-pitch roof BS 5250 specifies sufficient ventilation as being:
25mm at eaves on each slope and 5mm at ridge. If ridge or eaves ventilation is not possible, ventilation tiles spaced at the required distance can be used at high or low level to achieve the equivalent airflow.
A minimum 50mm clear air path must be maintained between the top of the insulation and the underlay to ensure a clear airflow from the eaves to the ridge which can be achieved through the use of counter battens of suitable depth fitted vertically beneath the battens to coincide with the rafters.
Marley provides a range of Universal ventilation products and accessories that can be used to achieve the required airflow for a healthy roof space and can be used to comply with BS 5250.