water storage tank

[Andrew Vevers]

I want to install a rain water storage tank in the loft of my garage. It would get rain off the main roof of the house and be used for watering the garden and maybe flushing the toilet.

How can I calculate what size of tank the loft can support. It's got conventional prefabricated roof trusses and I'm looking to store about 1 cubic metre of water (1 tonne), though not all in the same tank.My aim would be to put several tanks in to spread the load and make it possible to actually get them up there

[Bob Hill]

Refer to BS 5628: Part 3: 1998. Assuming that the roof trusses are standard gang-nailed trusses, the rule of thumb for supporting tanks is as follows:

For tanks upto 230 litre capacity (standard domestic water storage tanks) the the tank should be supported on a minimum 18mm plywood panel supported on a minimum of three 100x50mm cross bearers (i.e. parallel to the trusses) in turn supported on a minimum of 2 no. 100x50mm bearers (at right-angles to the truss) spread over a minimum of 3 trusses. Spreaders should be placed close to the nodes of the trusses.

For tanks upto 300 litres, spread the load over four trusses. For tanks more than this, or multiple tanks, consult a structural engineer. It is YOUR responsibility to ensure the installation is safe. If in doubt, consult an engineer.

Bob H