The ‘tank’ in your loft or attic is in fact a cistern’. Cisterns are not sealed — though they should be covered — and so are subject to atmospheric pressure. Tanks are completely sealed — as with a hot water storage tank — and are not subject to atmospheric pressure.
Cold water cisterns have traditionally been made of galvanized mild steel and it is quite likely that you will find one like this in your loft. They are still available, but are not usually installed in new houses. Other materials used have been asbestos, cement, copper and glass fiber, but today the most common material is plastic, of which glass fibre reinforced polyester (GRP), polythene and polypropylene are the most common varieties.
The advantages plastics have over all other cistern materials are their lightness in weight, resistance to corrosion and flexibility. Galvanizes steel is heavy and liable to corrode, while asbestos and cement are not only heavy but can become porous and are prone to accidental damage. Don’t forget the capacity of a typical cistern is 227 liters (50 gallons), and this water alone weighs nearly 0.25 tonne (1/4 ton), so all cisterns must be fully supported on the joists. With rigid materials such as steel the cistern can rest across the joists, but with plastic and glass fibre a platform should be installed to support the whole area of the bottom, otherwise the material may develop local weaknesses.
Cisterns should be covered to prevent any contamination of the water. Where the underside of the roof is exposed dust and dirt are liable to fall in. The top and sides should also be insulated to minimize the risk of freezing. The bottom is left uncovered to allow rising warm air from rooms below to keep the water above freezing point, and so you shouldn’t insulate the roof space under the cistern.
Cisterns were often installed before the roof was put on and if you want to replace yours, perhaps because it’s made of steel and is corroding, you may not be able to get it through the trap door. While it is sometimes suggested that a cistern should be cut up to get it out this is in tact a very heavy and arduous job in such a confined space and it would be better to maneuver it to one side and leave it in the loft, installing a new cistern alongside. Modern plastic cisterns can literally be folded up so they can be passed through small loft hatches.