Formerly the attic tank, which supplied the house with water under tank pressure, was of large size, holding several hundred gallons. To-day, however, much smaller tanks are used for this purpose. They are supplied with a ball cock, thus allowing water to enter the tank at the same rate that it is drawn out.

The storage tank, although it may be used for the same purpose and in the same way as the common attic tank, is generally used as an auxiliary to the pressure system of supply, and may be of any size, from a capacity of a few hundred gallons to many thousands. These tanks should be of wood or iron, or of wood lined with heavy tinned sheet copper.

The best materials for wooden tanks are cypress, white and yellow pine, cypress being the most satisfactory.

The storage tank should be supported on heavy iron beams which will not sag under the immense weight of the tank and its contents.

In many cases the storage tank must be placed above the point that the pressure supply can reach. Its supply must then be pumped into it. In high buildings it often happens that during the day time, when the mains are being heavily drawn on, the street pressure is not sufficient to force water into the tank, but during the night it is sufficient. A supply can thus be stored at night for use during the day time on those floors not reached by the city pressure.

Tanks should always be covered in order to keep out dust, foul gases, and odors.