How do I ventilate a cold roof?

A cold roof is the most common roof construction often in the form of an uninhabited storage space. Insulation is laid at ceiling joist level, leaving the roof space relatively colder than the accommodation below. If moisture laden air from the living area is allowed to condense in the roof space, it can eventually cause structural damage, or damage to any contents stored within.

BS5250 (Control of Condensation in Buildings) sets out the minimum ventilation requirements of a cold roof taking into consideration span, pitch and roof area. The wider the span, the more free airflow required.

Where insulation is at ceiling level and the void is therefore uninhabited and ‘cold’ BS 5250 specifies sufficient ventilation as being:

25mm along the length of the eaves for pitches of 15° or less 10mm along the length of the eaves for pitches of more than 15°

Additional continuous 5mm ventilation at high level for roofs where pitch exceeds 35°, or for roofs of any pitch with a span of more than 10m for lean-to or mono-pitch roofs.