Rain-water is fit for human consumption only after boiling or filtering through a filter fine enough to stop the tiny bacteria which may swarm in it. Even after treatment it may be unsafe if it has been stored in lead-lined tanks or passed through lead pipes, as it is able to dissolve lead. Rain-water should be stored in iron or concrete tanks and conducted through iron pipes.
Where a house has to depend mainly on a well for its water supply, stored rainwater will be a very useful addition in time of drought, for washing and cleaning purposes, and for watering the .-garden.
A house with a roof area of 900 square feet will have from 10,000 to 15,000 gallons of Tain fall on it during a year – an average of 28 to 42 gallons per day. Underground storage is the best; and the most suitable material for the construction of tanks is reinforced concrete.
A tank to hoid 5,000 gallons would be 10 feet square inside and 8 feet deep. Sides and bottom should be 8 inches thick; and reinforced with circumferential steel rods inch thick, running round the sides in the centre of the concrete, their spacing increasing from 6 inches at the bottom to 12 inches at the top. Crossing the rings should -be f-inch vertical rods, spaced 30 inches apart. These last should extend across the bottom.
To allow for doming over the top, the hole will need to be 3 or 4 feet deeper than the vertical parts – unless it be decided to have the neck above ground level and bank it round with the excavated earth; which would save a good deal of labour.
Its horizontal dimensions will have to be 16 inches in excess of 10 feet each way, to allow space for the 8-inch wall all round. This assumes that the ground is firm enougr to vender lining unnecessary.
The concrete should consist of 4 parts stone passed through a £-inch mesh screen, 2 parts sand, and 1 part Portland cement, mixed rather wet, and well pugged against the outside of the wooden form used.
The form is built up of vertical boards backed by horizontal rings of timber, well shored; and Is supported 8 inches above the bottom of the excavation by detachable legs which must be with drawn as soon as the concrete used for covering the bottom has set.
When the sides have been completed, the form is floored over at the top, and well supported by struts reaching to the bottom. A hole 2 feet square at the centre has a wooden trunk built round it, 3 or 4 feet long. Consolidated earth covered with sand is piled up to form a pyramid with its apex near the top of the trunk.
A 4-inch layer of concrete, with reinforcing rods buried in it, is spread over the sand from the top of the walls upwards- When this has set for a fortnight, a flooring board is removed to let the earth and sand fall into the tank, from which it is removed in buckets through the opening. The form and flooring can then be dismantled and sent up.
During the cementing of the dome openings must be left for the inlet and pumping pipes, and short ventilating pipes should be fixed at each comes, with bent-over ends facing in different directions, and covered with gauze to exclude insects.
The tank will be more easily cleaned if its bottom has a small fall towards one side; and its watertightness will be assured if the concrete is washed over with neat cement, mixed with water to the consistency of cream.