How to Build an Underground Cistern


In dry, drought-like conditions, finding ground water for everyday use can be virtually impossible. An underground cistern can store rainwater or hauled water to solve this problem. A cistern is a reservoir that can be made of metal, plastic, fiberglass, reinforced concrete or concrete blocks. The cistern acts as an underground rain barrel. The water is dipped or pumped out of the cistern to water flowers, vegetable gardens or trees. The water can also be used for washing clothes, cars or other items that can put a drain on fresh drinking water resources.

Things You'll Need

  • Measuring tape
  • Shovel
  • Rake
  • Level
  • Trowel
  • Chalk
  • Heavy-duty plastic sheet
  • Cement blocks
  • Cement mortar
  • Gravel
  • Sand
  • Water
  • Waterproof epoxy caulking
  • Waterproof sealant
  • Paint brush
  • Gloves
  • Decide how big your cistern will be and what it will be made of. Concrete blocks are more economical but are difficult to keep watertight, which is why sealing all the seams and edges with waterproof epoxy is so important.

  • Measure the area for your cistern. Building the cistern against an existing concrete wall like a foundation or a retaining wall will mean that you only need to build three walls instead of four. The area chosen should be sloped to drain surface water away from the cistern.

  • Dig the hole. Square the sides of the hole with the shovel to make a perfect square. Use a level to make sure the sides are level. Flatten the floor of the hole with a rake so that it is even.

  • Lay the plastic liner in the hole, flat on the floor and square to the sides.

  • Place one row of concrete blocks in the hole to form three sides of your cistern, with the foundation wall as the fourth side. Cement all the blocks and the foundation together with mortar. Make chalk marks on the blocks at least four inches high from the bottom. The floor and the walls should be four to six inches thick.

  • Pour concrete into the hole to form the floor of your cistern. Spread it around evenly to all four corners with a trowel, up to the chalk marks on the cement blocks. Leave the concrete to cure according to the label on the bag of concrete.

  • Build your cistern’s wall by continuing to place the concrete blocks side by side and on top of each other, cementing the blocks together with mortar. Make sure the top of your cistern is level. Allow the mortar to cure.

  • Seal all the seams of your cistern with waterproof epoxy caulking. Allow the caulking to cure and dry according to the directions on the label.

  • Seal the entire surface of your cistern with waterproof sealant. Follow the directions on the label for curing time and any additional coats.

  • Build or find a cover for your cistern. It must be heavy enough not to be blown away by the wind or to be removed by small children. A lock on the cover is essential.

Tips & Warnings

  • Rain water that is collected in a cistern is not to be used for drinking water.
  • The concrete mixture for the floor should be comprised of five gallons of water, 2 1/4 cubic feet of sand and three cubic feet of gravel for each bag of cement.
  • If you live in a cold climate where the ground freezes in the winter, empty the cistern of water before the ground freezes, otherwise the cistern will potentially crack when the water freezes.
  • Always contact the municipality where you live to ask to have someone come to your site and mark where there are underground utility lines to avoid rupturing a pipe or cable.

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