Rain-water tanks should be designed to suit their positions, and only a general description of their positions and construction can be given. The tank should be only a short distance from a house or building, so as to shorten the lengths of the drains leading to it, and should also be near the place where the pump necessary for raising the water can be fixed. The selected site should not be near any soil or sewage drains or any other place where there would be risk of the water becoming contaminated. After the excavation has been made to the desired size and depth, the bottom should be covered with Portland cement concrete from 9 in. to 18 in. in depth, according to the size of the tank and the nature of the soil in which it is being built. The walls should be from 12in. to 18in. thick, and made of concrete, or built with bricks in cement, and rendered inside to make it watertight. Some engineers puddle the outside with clay. The tank can be arched over or covered with rolled-iron joists about 18 in. to 24 in. apart, and tilled in between with Portland cement concrete. An access manhole should be made in a suitable position, and have a raised curb round it and a flat stone or hinged oak cover.

The overflow should be made of ordinary drain pipes, and be arranged to discharge into the open air in a field or other suitable place, but not into any soil or sewage drain. A trap is not required, but in some cases an iron grating on the outlet end is necessary for keeping out rats or other vermin.