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 waterproofing of the structure, eg. the roof underlay, vapour control layers or the roof cover.
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.
8. Thermal bridging should be avoided at the external wall / ceiling junction.
9. A well sealed ceiling should be provided
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.
11. BS5250:2002 recognises two types of underlay type LR, low vapour resistance and type HR, high vapour resistance.
