Dormer Construction and Storey & a Half Construction
It is important to ensure that dormer roofs are constructed in such a way so as to ensure adequate height to accommodate rooms while still adhering to conventional roof conditions. Where a satisfactory structure cannot be achieved by timber construction alone the use of steel may be incorporated i.e RSJ's (Rolled Steel Joints) and UB's (Universal Beams).
Diagram D47 - Dormer and storey and a half roofs
The following diagrams detail two common cut dormer roof design options.
Diagram D48 - Dormer roof - Option A
Option A uses timber purlins instead of steel purlins. Due to the fact that timber has far less strength in tension compared to steel, the purlins will require vertical support from below, usually in the form of a load-bearing partition. The fact that the purlin is being supported by a load-bearing partition means that the floor joists or precast concrete floor will carry part of the roof load. The load to be carried by joists or the precast concrete floor must be designed appropriately.
Diagram D49 - Dormer roof - Option B
Option B design uses steel purlins to carry the load. Due to the structural characteristics of the steel beams, they are capable of carrying loads over longer spans, meaning the partition walls are not load-bearing, thus the floor joists or precast concrete floor do not support any roof load.
The following diagrams detail two common cut storey-and-a-half roof design options.
Diagram D50 - Storey and a half roofs
Option 1 uses steel purlins to carry the load. Due to the structural characteristics of the steel beams, they are capable of carrying loads over longer spans, meaning the partition walls are not load-bearing, thus the floor joists or precast concrete floor do not support any roof load.
Option 2 uses timber purlins instead of steel purlins. Due to the fact that timber has far less strength in tension compared to steel, the purlins will require vertical support from below, usually in the form of a load-bearing partition. The fact that the purlin is being supported by a load-bearing partition means that the floor joists or precast concrete floor will carry part of the roof load. The load to be carried by joists or the precast concrete floor must be designed appropriately. Additionally, to compensate for the lack of triangulation of the roof, additional structural tie-backs can be included.
The purpose of the additional ties in storey-and-a-half roofs is to compensate for the lack of triangulation in the roof. These ties must be securely nailed to both the rafter and the floor joist as illustrated. Ties should be at least 150 mm deep, installed ideally at 45 degrees to the horizontal, and the number of ties depends on many factors, including roof configuration and loading.
Diagram D51 - Additional tie back storey and a half
Providing daylight to rooms
Daylight is generally provided to dormer roofs through roof lights or windows. It is recommended that the area of such roof lights or windows be approximately 10% of the floor area of the room they are lighting. However, there are no specific requirements for daylight in the Building Regulations.
Typical dormer window construction with double joists designed and incorporated to carry the dormer cheek studs has been detailed in the diagram below.
Diagram D52 - Typical dormer window construction
It is important that ventilation is adequately provided to prevent condensation. Also, a vapour control layer should be provided on the warm side of the insulation.
Ensure these windows are fully framed, sheathed, and jointed before roof finishes are laid for the dormer roof or the main roof. Cutting of the main roof members will result in additional loads. It is important that the trimming members around the dormers are appropriately sized. No loads should be transmitted through the window frame by the dormer window structure. A lintel that is suitably designed should be incorporated to span over the window frame.
At junctions where flat roofs meet pitched roofs and in situations where valleys or gutters occur, the waterproof membrane should extend 150 mm under the tiling with the roof underlay overlapping it. For clarity in the following diagram, the insulation has been left out.
Diagram D53 - Junction of flat roof and pitched roof
The gable wall should be strapped to the roof structure with 30 mm x 5mm galvanised straps at 2 m maximum centres to help stabilise the gable wall. As illustrated below, the straps must be tied to at least two rafters. The floor joists are to be strapped to the external wall. It is vital that timber bridging is located directly below the strap.
Diagram D54 - Lateral restraint strap locations
To ensure adequate triangulation is achieved, effort should be made to ensure the joists and rafters run in the same direction. If the layout of the joists does not allow this, check with the engineer to ensure that there is sufficient tying back to the rafters in place as illustrated opposite. The design engineer should be suitably and possess appropriate professional indemnity insurance.
The position of the water cistern is important and should be given careful consideration. As the cistern is usually located in the triangular portion above the ceiling joists, the roof should be designed such that there is sufficient space for an appropriately sized cistern to fit. If possible, locate the cistern on an appropriate platform above a load-bearing partition/wall.
Fire resistance of ceiling/floor under dormer
It is necessary to provide modified half-hour fire resistance to the ceiling below the triangular voids as well as full half-hour resistance to the floor of the dormer accommodation. For dormer construction, in a three-storey configuration, fire resistance to all floors must be a full half-hour.
Diagram D55 - Below dormer window roof detail
Detail A – Joists and rafters running in line, the rafter is tied to the appropriately designed joist.
Diagram D56 - Below dormer window - Detail A
Alternative detail “A” – A situation where the joists runs parallel to the wall plate and perpendicular to the rafters. The partition should bear on timber joists or a steel member designed to take the load. Note the inclusion of the additional ties.