Ventilation to negate the effects of condensation in roofs is essential to comply with Building Regulations, Building Standards and British Standards. Ventilation openings should be provided on the longer sides of rectangular roofs and designed to prevent the ingress of rain, snow, birds and large insects. Minimum mesh/grille sizes should not be smaller than 4mm to avoid excessive airflow resistance. Particular attention should be paid to potential restrictions at changes in roof slope or changes in constructional details and at junctions with walls.
The Hambleside Danelaw range of ventilation products is manufactured to enable compliance with all current technical requirements and standards.
Building regulations – the requirements
Approved Document C2 requires that roofs be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial condensation. This requirement will be met if the roof is designed and constructed in accordance with Annex H of BS 5250 ‘Code of practice for control of condensation in buildings’ and BS EN ISO13788; ‘Hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods.’ Further guidance is given in BRE Report BR262 ‘Thermal insulation: avoiding risks’.
To avoid excessive moisture transfer into roof voids, gaps and penetrations for pipes and electrical wiring should be filled and sealed, particularly in areas of high humidity such as kitchens and bathrooms and an effective draught seal should be provided to loft hatches to reduce the inflow of warm air and moisture. Vapour control layers can reduce the amount of vapour entering roof voids but cannot be relied on as an alternative to ventilation. A complete barrier to moisture is needed for this.
Scottish Technical Handbooks section 3.15 requires that dwellings shall be so constructed as to protect the building and its users, so far as may be reasonably practicable, from harmful effects caused by surface and interstitial condensation. Both these requirements are deemed to be satisfied by following the guidance given in BS 5250.
BS 5534; Slating and Tiling for Pitched Roofs and Vertical Cladding – Code of Practice recommends that roof ventilation be provided in accordance with BS 5250.
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