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 (Management of moisture in buildings – Code of practice) sets out the minimum ventilation requirements of a cold roof taking into consideration span, pitch and roof area. The wider the span, the more ventilation required.

Where insulation is at ceiling level and the void is therefore uninhabited and ‘cold’ and an HR (non-breathable) underlay is used , 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 and for lean-to or mono-pitch roofs.
If an LR (breather-type) underlay is used, the level of ventilation required is determined by whether the ceiling is well sealed or normal (see BS 5250 for definition of well- sealed and normal ceilings) and whether the roof covering is air permeable or air impermeable.

If a building has a well-sealed ceiling, and the roof covering is air permeable, then fortuitous ventilation at the eaves via normal eaves construction (3mm) is all that is required, provided the requirements of UKAS Third Party certification are fully met. Alternatively, 5mm ridge (or high level) ventilation can be used.

If the roof has a normal ceiling, the roof void should be ventilated at the eaves, and where the roof covering is air impermeable, the batten space should also be ventilated.
Larger buildings other than dwellings require 5mm eaves ventilation with a well- sealed ceiling or 10mm eaves ventilation with a normal ceiling in addition to 5mm ridge ventilation.

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