Brief description

A waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater.

Use / Function

• Primary use: Storing rainwater for household use (drinking, washing).
• Secondary uses: Emergency water storage, fire fighting reserve.
• Scale: Household (tank) to castle/fortress (underground vault).

Operating principle

• Catchment: Rain falls on a roof or catchment area.
• Conveyance: Gutters and downspouts direct water to the cistern.
• Filtration: A “first flush” diverter or filter removes debris before entering the tank.
• Storage: The tank holds water in a dark, cool environment to prevent algae growth.

How to create it

1. Excavation: Dig a hole for underground cisterns (keeps water cool).
2. Lining: Build walls with brick, stone, or concrete.
3. Waterproofing: Apply a layer of waterproof mortar (cement + sand + waterproofing agent) or a plastic liner.
4. Covering: Seal the top to prevent evaporation, mosquito breeding, and contamination.
5. Access: Install a hatch for cleaning and a pump/bucket system for extraction.

Materials needed

• Structure: Concrete (ferrocement is excellent), Brick, Plastic (modern tanks).
• Waterproofing: Hydraulic mortar, bitumen (outside only), or liners.
• Plumbing: Gutters and pipes.

Variants and improvements

• Ferrocement Tank: Thin shell of mortar reinforced with chicken wire. Very strong and cheap.
• Rain Barrel: Simple collection from a downspout into a Barrel.
• Impluvium: Roman style pool in a courtyard to catch rain.

Limits and risks

• Contamination: Bird droppings on the roof can pollute water. First-flush diverters are essential.
• Mosquitoes: Standing water breeds disease vectors if not sealed.
• Algae: Sunlight promotes algae; cisterns must be opaque/covered.
• Cracking: Leaks can drain the entire supply and undermine foundations.

Related inventions

• Gutter: Collection mechanism.
• Water Filter: Essential post-treatment.
• Water Supply: Alternative to centralized supply.
• Concrete: Common material.