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3.37.2.13 Dormer construction
Dormers shall be adequately constructed. Issues to be taken into account include:
- structure
- ventilation
- insulation
- control of condensation
- proprietary dormers.
Site formed dormer roofs:
- should be constructed in accordance with the design
- cheek studs should be supported by either a double rafter or a double floor joist
- trimming members should be large enough to support additional loads from the main roof members, dormer framing and cladding
- which are unusually large or of bespoke design should have an appropriate support designed by a suitable qualified person or a structural engineer in accordance with Technical Requirement R5
- should have a suitable lintel provided over the opening
- lintels should be structurally independent from the window frame
- timbers should be preservative treated or have adequate natural durability in accordance with Table 1 of Chapter 3.3 ‘Timber preservation (natural solid timber)’
- should have roofs braced in accordance with this chapter, where roof trusses are used
- should have roof coverings in accordance with this chapter or Chapter 7.1 ‘Flat roofs, terraces and balconies’.
The external walls of a site formed dormer:
- should be sheathed and protected by a suitable breather membrane
- have a drained and ventilated cavity behind claddings, in accordance with Clause 6.2.10.
Lead, used to clad dormer cheeks, should be detailed in accordance with the Lead Sheet Training Academy ‘The complete manual’ and be:
- a minimum Code 4 (blue) or 5 (red) in a sheltered to moderate exposure zone or
- code 5 (red) or 6 (black) in a severe exposure zone.
Ventilation#
Pitched roofs shall be adequately ventilated to limit the risk of interstitial condensation. For cold roofs, ventilation should be provided from eaves to eaves or at ridge level, in accordance with Table 8.
Table 8: Dormer pitched roof ventilation requirements
Notes
1. High level vent 5000 mm2/m based on the longest horizontal dimension should be provided where the roof pitch exceeds 35°.
2. A normal ceiling typically has an air permeability of 300 mm2/m2 (0.3%). A well-sealed ceiling conforms to Clause 12.4.2 of BS 5250 and BS 9250 and typically has an air permeability of not more than 30 mm2/m2 (<0.03%).
3. Alternatively, a high-level vent 5 000 mm2/m based on the longest horizontal dimension of roof can be provided.
Where air impermeable outer weatherproof coverings are used, please refer to Clause 7.2.15 for guidance on roof ventilation strategies.
Flat roof dormers should:
- have falls to the front or sides
- be either of warm deck or cold deck construction (in accordance with BS 5250)
- be ventilated where of cold deck construction in accordance with Clause 7.1.11
- be constructed in accordance with Chapter 7.1 ‘Flat roofs, terraces and balconies’.
Insulation#
Dormer roofs and cheeks should be insulated to meet the requirements of the Building Regulations.
Control of condensation#
To limit interstitial condensation, site formed dormers should incorporate an air and vapour control layer to the walls.
Air and vapour control layers should also be provided to flat roofs, in accordance with Chapter 7.1 and are recommended to be applied to the warm side of the insulation within pitched roofs.
Further guidance on air and vapour control layers can be found in Clause 7.2.15.
Proprietary dormers#
Propriety dormers (eg glass reinforced plastic - GRP) should hold a satisfactory assessment by an appropriate independent technical approval’s authority acceptable to NHBC.
Proprietary dormers should meet the Technical Requirements and:
- be suitably durable
- limit the risk of interstitial condensation occurring to the walls and roof
- be installed in accordance with certification requirements.
Walls and roofs of proprietary dormers should typically incorporate an air and vapour control layer. The air and vapour control layer specification should be in accordance with manufacturers requirements.
Last updated: 2nd January 2024