Quality Creditations

British Standards
BS 5250 2002 Code of Practice for Control of Condensation in Buildings.

Roofs: General Points

1. Moisture should be extracted at source to reduce the risk of water vapour transfer from
occupied areas to the roof

2. To achieve adequate cross ventilation, ventilation openings should, where possible, be placed
on the longer sides of a typical rectangular roof

3. If ventilation openings are sited at intervals, they should be of equivalent area to the
continuous openings recommended, and avoid stagnant air pockets due to inadequate through-flows.

Opening sizes are minimum provisions, with certain constructions there are advantages
in increasing these in order to ensure air movement through the roof void.

4. Ensure that ventilation openings cannot be blocked by dust, airborne debris, paint or frost, and
that the ingress of rain, snow, birds and large insects is prevented. A nominal mesh size of
4mm is recommended, which would achieve this and avoid excessive airflow resistance.
Consideration should be given to the use of proprietary ventilators where these will avoid
problems, or be more practicable in use. The ventilation openings should provide a continuous
weatherproof path from the roof void to the outside without impairing the waterproof function
of the structure, eg. the roof underlay, vapour control layers or the roof covering.

5. The minimum free airspaces defined should be maintained throughout and particular attention
should be paid to potential restrictions at changes in roof slope, changes in constructional
details and at junctions with walls.

6. Where the roof construction includes wet processes, or where wetting occurs from rain during
construction, it should be ensured that moisture is not trapped between the waterproof roof
covering and a vapour control layer positioned at a lower level. If necessary, provision should
be made for venting any trapped water vapour.

7. For pitched roofs where ventilation at high level (eg. at or near the ridge) is recommended, this
is in addition to any low level requirement. High level ventilation should never be used on its
own as the suction effect created will increase water vapour transfer into the roof void.

8. Thermal bridging should be minimised particularly at the external wall / ceiling junction.

9. Gaps in the ceiling should be minimised and service openings should be sealed.

10. Where the insulation is at ceiling level and where the eaves ventilation airway is provided
between the insulation and the underlay, they should be separated from one another by at
least 25mm.

The Building Regulations
Approved Document part F ‘Ventilation’ (revised 1995) states that:

‘Adequate provision shall be made to prevent excessive condensation in a roof or in a roof void
above an insulated ceiling’.

It also draws attention to the BRE report ‘Thermal Insulation: Avoiding Risks’. This has been
prepared to give guidance to Approved Document part L - ‘Conservation of Fuel and Power’.

In particular it stresses the importance of:
a) Ventilating the roof space in accordance with BS 5250.
b) Insulating ventilation ducts in the roof space.
c) Avoiding cold bridging by carrying loft insulation over the wall plate and butting against the
wall insulation.
d) Avoiding freezing of water in the tanks and pipes by insulating pipe airflows and the top and
sides of water tanks.

NOTE
THE BUILDING REGULATIONS REQUIREMENTS FOR SPECIFIC SITUATIONS ARE
DETAILED OVERLEAF AS AN ALTERNATIVE, THESE REQUIREMENTS CAN BE MET
BY FOLLOWING THE RELEVANT RECOMMENDATIONS OF BS 5250 (ALSO SHOWN).