How to go about building a cistern

Q: I want to build a cistern, perhaps 8 feet in diameter and 12 feet deep, to hold soft rainwater from my roof. There are no regulations in this county governing cisterns. How should I build it? Would a 5-inch-thick concrete floor be adequate? Would a round wall of bricks laid in waterproof mortar suffice? Would it make a difference if the cistern were square instead of round? We want to use the water for nondrinking purposes in order to reduce public water usage and save money. George Wesley Buchanan Wheaton, Md. A: Bully for you with the cistern idea! Get in touch with a local civil or structural engineer for a below-ground cistern design which shows the thickness of the concrete floor and reinforcement, reinforced footing, thickness and material for the walls, interior waterproofing, and cap.

A round cistern is likely more economical to build than a square one.

The floor might be 5-inch-thick concrete, but the footings under the perimeter walls would be thicker.

Compacted backfilling behind the masonry walls is critical for their structural integrity against the thrust of the stored water. Smooth waterproofing of the interior surface is obviously important. The design engineer should specify the material and method of application.

Ask the engineer to also specify the needed cistern pump and plumbing.

In olden days folks who had cisterns and wells planted trout in them to keep the water pure. Are there any trout in your neighborhood looking for a new home?

''If you allow the first part of the rain to wash the roof and drain to waste , the water that goes into the cistern will be less polluted,'' according to the Farmers Bulletin No. 2237, US Department of Agriculture, Water Supply Sources for the Farmstead and Rural Home (Page 18).

''It still should be treated, however,'' the bulletin goes on.

''Cisterns should be disinfected in accordance with Public Health Service recommendations and your local health officer recommendations.

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