Effective cold roof ventilation

An image of a 10mm eaves vent system, available from Marley.

A cold roof is the most common type of roof construction, often taking the form of an uninhabited storage space. The minimum requirements for cold roof ventilation are set out in BS 5250 (Code of practice for Control of Condensation in Buildings) which takes into consideration span, pitch, roof area, and insulation type and location. The wider the span, the more free airflow is required within the roof space.

In cold roofs, insulation is laid at ceiling joist level, leaving the roof void colder in relative terms than the accommodation below. If moist air from the living area is allowed to condense within the roof space, it can eventually lead to structural damage, and/or damage to contents if stored in the space.

Sufficient cold roof ventilation

To comply with Approved Document C and BS 5250, crossflow ventilation should be provided at both the eaves and / or ridge level. This should ensure that effective through-ventilation of the whole roof is achieved, and assists with pulling moisture laden air out of the building and limiting the risks from condensation.

According to BS 5250 - and based on several variables - sufficient ventilation should be:

  • 25mm along the length of the eaves for pitches of 10° - 15°
  • 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.

These ventilation requirements vary based on the circumstances of the building and the interpretation of these requirements by BS 5250. However in most circumstances, the ventilation requirements outlined in BS 5250 can be accommodated by using dry fix and ventilation accessories such as eaves ventilation systems and dry ridge and hip systems.

Visit our British Standards for roofing pages to learn more for your next project. 

Category: FAQs Roofing Technical