In trussed rafter roofs, BS 5268: Part 3: 2002 requires the building designer to advise the trussed rafter designer of 'the size and position of all water tanks and other ancillary equipment or loads to be supported on the trussed rafters'. This is clause 11.1 g. We can assume then that the trussed rafters will have been designed to carry water tanks and provided they are within prescribed limits and location within the truss, a standard tank support platform can be used.
Figure 8.19 shows a typical tank stand constructed to the guidelines laid down by BS 5268: Part 3, but for precise details of timber sizes reference should be made directly to the British Standard, or to the TRA technical literature, or to any of the trussed rafter plate manufacturers' technical brochures, or again to NHBC pitched roof clause 7.2. Depending on the tank size, it must be supported over three or four trussed rafters, always with bearers A as close as possible to the node points of the truss. A method of lowering the tank stand where restricted headroom is a problem is shown in the TRA Technical Handbook.
The question of restricted headroom must be considered when deciding upon the location of the tanks within the roof. The tank will normally be supported about the centre line of the roof truss, this generally being the largest clear void within a trussed
Typical tank platform for truss span \ up to 6 m and for a 230 litre \ v nominal tank. For a 300 litre
/s tank the load should be \ \ \spread over four trussed rafters.
Fig. 8.19 Water tank platform.
rafter roof. Low pitched roofs bring their own problems of restricted headroom, likewise the numerous timber members in a hip end roof generally prevent tanks being placed within the hip area. Not only must access to the tanks be considered, but also reasonable space must be left around the tank for initial installation and maintenance thereafter. Do not forget the additional thickness of tank insulation. NHBC also advises on this subject.
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