How do I ventilate a warm roof?

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 batten cavity.  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 can be used at high or low level to achieve the required airflow.
A minimum 50mm clear air path must be maintained between the insulation and the underlay to ensure a clear airflow from the eaves to the ridge and can be achieved through the use of counter battens.
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 the British Standards BS 5250.