Warm roof ventilation requirements

Warm roofs are typically designed with the insulation between and/or above rafters making the roof space habitable. 

Like a cold roof, warm roof ventilation is required at high and low level to remove moisture-laden air from the roof space and comply with Approved Document C and the British Standards BS 5250.

Warm roof ventilation specification

For a simple duo-pitch roof, BS 5250 specifies sufficient ventilation as being:
•25mm continuous opening at eaves level and a further continuous 5mm at ridge level.
•If ridge or eaves ventilation is not possible by means of a dry fix system then ventilation tiles can be installed at evenly calculated intervals to achieve the required airflow under BS 5250.

A minimum 50mm clear air path must be maintained between the insulation and the underside of the underlay to ensure a clear airflow from eaves to the ridge, alternatively if complete rafter fill, this can be achieved through the use of counter battens installed above or below the membrane.

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.