How to Build a Custom Concrete Rain Water Retention Tank

Map out the site for your water retention tank. This site may be on the nearest hill to take advantage of gravity to pipe water down to your residence. Ideally, you want the site to be close to the house for easy access to the water, but away from any sources of contamination, such as animal containment or chemical storage. You also want it to be away from trees that can introduce litter and wildlife waste into the unit.

Excavate the hole for the water tank. Digging the hole can be done with a backhoe that you can rent or contract with a local company to do this work for you. You will have to determine the size of your water retention tank that fits your needs and the space on your property. A 6,500 gallon tank, for instance, would be a 12-by-12-by-6-foot hole.

Construct the floor of the tank by making a frame with 2-by-4 lumber. Secure these into place with 2-by-4 stakes at 2-foot intervals from the inside of the frame. This framework wood at the bottom will be sacrificed, that is, it will stay in place at the bottom of the tank after the concrete is poured and will become part of the permanent unit.

Wire lengths of rebar together in a grid of 1-foot sections and lay at the bottom of the hole to reinforce the floor.

Lay down concrete reinforcing mesh at the bottom of the tank.

Pour the concrete for the floor to a 4-inch depth.

Build the forms for the walls by constructing a framework much as you would for a wall, that is, with 2-by-4 lumber set at intervals of 16 inches. First, lay out the bottom wood plate, connecting the horizontal lengths together with nails.

Attach the vertical lengths of wood, cut to length, with nails. Ensure that the pieces of wood are perfectly straight by checking with a plumb line.

Add the wood top plate along the top edge of the hole, connecting it to the vertical two-by-fours. Installing diagonal pieces of lumber to support the vertical pieces will help to withstand the pressure of the concrete when it is poured.

Cover the wooden wall framework with ¾-inch plywood panels nailed into place securely.

Wire together the rebar onto a 1-foot grid and insert into the forms before pouring for additional reinforcement of the walls.

Pour the concrete into the wall forms. Pour in a circular pattern around the formed walls to avoid putting too much pressure on any part of the walls at one time. Allow the concrete to dry thoroughly.

Remove the inner wood forms. This lumber can be used for other projects. The exterior lumber of the form that is closest to the dirt is sacrificed and left in place.