Cold water tanks should be properly supported on boarding laid over timber joists installed perpendicularly over ceiling joists.
This photo shows not only a poorly insulated and open tank which has led to a high volume of sediment and contamination to the water supply but the tank has been knocked by stored items within the roof space which has caused the tank to shift from its support. Added to that the tank is located above the bedroom. The cold-water tanks should not be located over habitable rooms in order to prevent the risk of collapse which will not only cause considerable water damage but also increase the risk of causing serious harm, injury or in extremely unfortunate situations, death.
After all, don't forget that a small 20 gallon water tank has nearly 100kg of water contained within it - and that is for a small house or flat. Larger tanks are needed for larger houses and thus the weight increases.
There are strict regulations for water tanks and builders and plumbers must ensure adherence to Byelaw 30 which is about protecting the stored water from contamination. See WRAS for more information.