Building a Cistern

This article is based on a 16ft diameter, 4 ft tall tank, holding 6,000 gallons. However, you can alter the dimensions to fit your needs.

Round tank:

π (3.14) x radius x radius x height x 7.5 = gallons

(e.g. 3.14 x 8ft x 8ft x 4ft x 7.5 = 6028.8 gallons)

Square tank:

length x width x height x 7.5 = gallons

(e.g. a 18ft square that is 4ft tall will hold 9,720 gallons)

Liner Dimensions:

You want to make your liner a little larger than the tank’s dimensions, so that it has some slack. Also make it 1ft taller than your tank’s walls.

Even though a square tank is more efficient with space and thus your liner, we would unequivocally recommend going with a circular design. We have done both and the round one is far stronger and requires less work. Any money you might save on the liner for a square tank is negated by the extra strength you will have to add to the frame. If you decide to go square, bury the bottom 1/3 of the tank.

1. Mark out the area where you wish to build your tank, and level it. You can dig down or fill in, though a combination of the two is often the least labor intensive.
2. Put a layer of sand, about 6″ deep over the whole area and compact it well.
3. Place a rebar or post in the center of the area and attach a string to it.
4. Tie the other end of the string to a stick or piece of metal, so that the distance between the stick and the center post equals the radius of your tank, in this case 8 ft.
5. Keeping the string taut and the stick upright, mark the sand in a circle around the central post.
6. Center bricks over this line all the way around the circumference, leveling them with each other.
7. Fill your circle with sand, then compact it well, so that the sand is an inch or two below the top of the bricks.
8. Fill that inch or so with finely screened sand and compact again.

1. Drill 1/8″ holes every 6″ on a 4ft long strip of metal (there will be 7 holes in total). This will be the jig that you use to drill the sheet metal, so that all your holes line up exactly.
2. Clamp the jig onto one 4ft side of sheet metal and then drill through the jig’s holes into the metal. Make sure that the edges of the metal jig line up well, so that you will get a precise position on each sheet of metal. Repeat this process until all five pieces of sheet metal have holes on both 4ft sides.
3. Using a 3/8″ drill bit, drill out all these holes so that your 3/8″ bolts pass through easily.
4. Stand up two pieces of sheet metal, so that their bottom edge is centered on the ring of bricks, and the holes on the sides line up with each other.
5. Bolt them together, making sure the bolt heads are on the inside of the circle and the threads on the outside. Start from the bottom bolt to the top.
6. Continue bolting together the sheet metal until you have completed the circle.
7. Drive in small pieces of rebar at the base of the walls periodically. These rebar should go in the cracks between the bricks. This will make sure the structure doesn’t shift and fall off the bricks.
8. Paint the metal walls, inside and out, with a quality metal primer and finish paint.

There are three sets of plumbing to consider: the intake (which can go into the tank roof and will be addressed in the roof section of these instructions), the outlet and the overflow.

Outlet: You can make this any size you wish. The larger it is, the faster water will come out, but the more expensive your pipes will be. Generally, pipes to a house are either 1/2″ or 3/4″. You want the outlet to be as low to the floor as you can get, and in a location that is convenient to wherever you want to use the water. On our tanks, we use 1 1/2″ outlet sizes, to be able to hook multiple tanks together.

Overflow: The size of the overflow depends on the source of water that will be filling the cistern. For a rain catchment system, your overflow wants to be able to match or exceed the size of pipes going from your gutters to the tank. At least 3″ is recommended, although more if the catchment area is very large and you experience heavy, fast rains. The overflow point will want to go as close to the top of the walls as you can make it. Think about where you want the overflow to go – whether to another cistern, an orchard, or somewhere else that can safely handle the excess water – and place it accordingly.

Please note, that in this system the overflow point and the overflow pipe are 2 different things. Because we have a self cleaning overflow design, the overflow point is actually higher than the pipe hole in the metal wall.

1. To position the overflow hole (where the overflow pipe goes through the wall), hold a 3″ T against the side of the wall. The T should be on it’s side, so that the long part is vertical. The bottom of the small side part is the overflow level. Make this about 4 inches below the top of the tank. This is the overflow point. Mark this spot with a marker.
2. Cut a 4″ piece of 3″ PVC. On one end of this piece, put a 3″ elbow. On the other end, place another elbow, but turn it 90 degrees compared to the first elbow.
3. Line the top elbow up so the bottom of its side hole is lined up with your overflow point. The bottom elbow should be facing the wall. Draw a circle around the elbow. This marks your overflow hole.
4. Now, position the bulkhead fitting over this hole, and draw a line around it so that the line is slightly bigger than the threads on the fitting. Cut out this hole.
5. For the outlet hole, hold the 1 1/2″ bulkhead in place, about 1″ from the bottom of the tank. Draw a line around the threads of the bulkhead. This is the outlet hole.
6. Drill out each hole with a metal hole saw. If you don’t have a hole saw that fits the size perfectly, use a jigsaw with a metal blade. Drill a 3/8″ hole on the inside edge of your line, and then use this hole to start cutting along the line.

1. Fold five strips of vinyl tarp, 4.5ft long and a couple of inches wide.
2. Line these strips over each set of bolts, to protect the liner from any rough edges of the bolts. Tuck the bottom of each strip under the sheet metal walls and fold the top over the top of the sheet metal and attach it temporarily to the highest bolt.
3. Also put a strip of tarp on the top edge of the walls.
4. Place another piece of vinyl tarp on the floor of your circle, so that your liner will be protected from any little rocks in the sand.
5. Get out your liner somewhere outside of the tank and fold it in a way that will be easy to unfold in place. This is best done by unfolding completely, then folding the sides towards the center, then rolling the whole thing up from one side to the next. Lift the liner inside the cistern walls.
6. Position the liner on one side of the tank, then unroll it. Position the bottom of the liner on the floor, so that it meets the walls evenly, then lift the sides of the liner up against and over the metal walls. There should be about a foot of extra liner on the outside of the walls, and once it is all in place, it will hold itself. Make sure to leave a bit of slack on the sides.

1. From the outside, draw a circle through the outlet and overflow holes in the metal onto the liner.
2. Cut out each circle, but not all the way to what you marked. Make the liner hole about 1/8″ smaller on all sides.
3. You want the flat part of the bulkhead on the inside of the tank. Unscrew a bulkhead, but leave the grommet in place. From the inside of the tank, push the bulkhead through the liner hole, and then through the hole in the metal. Screw the bulkhead nut in place. Tighten it well and put a bead of silicon around it where it meets the liner.
4. For the overflow fittings, you don’t need PVC glue, especially for the fittings on the inside of the tank. Just push them together.
5. On the inside of the overflow bulkhead, screw a male thread to PVC adaptor. Cut a 4″ piece of 3″ pipe, and insert it into the adaptor. Now, take your elbow assembly from step 4, and push the bottom elbow on the pipe.
6. Cut a 4″ piece of 3″ PVC and insert it into the top elbow. On the other end of this pipe, push on the T, with the long part vertical.
7. Measure from the bottom of the T to the bottom of the tank. Subtract the height of an elbow from this measurement. Now, cut a piece of 3″ PVC to fit this measurement.
8. Put an elbow on one side of this pipe, and the other end should go up into the bottom of the T.
9. With the remaining 3″ PVC, lay it on the ground and make a line of 3/8″ holes for the entire length of PVC. These hole should be about 4″ apart. On the other side of the pipe from this line make a few 1/8″ holes, maybe 6 holes for the entire length of the pipe.
10. Place the 3″ cap on one end of this pipe, and the other end can insert into the elbow on the overflow assembly. Make the 3/8″ line of hole face the tank bottom. The pipe should lay on the ground, and try and make it go towards the outlet bulkhead as much as possible.
11. On the outside of the overflow’s bulkhead, screw a male thread to PVC adaptor. You can then attach PVC pipe to this to take the overflow’s water to where you want it to go.
12. On the outside of the outlet’s bulkhead, screw in a threaded nipple and then a valve. From the valve, you can attach pipes to take the water to wherever you want to use it.

1. Lay out two 20 ft and one 10 ft pieces of 2″angle iron. On one side of the angle iron, drill 3/8″ holes, one foot apart, about 3/8″ from the edge.
2. On the other side of the angle iron, cut the metal every 6 inches with a chop saw. This will allow the angle iron to bend easier.
3. Using a small strip of metal as a bridge, weld the five piece of angle iron together.
4. Paint the angle iron and metal strips with a high quality metal primer and finish paint.
5. Bend the continuous piece of angle iron to snugly fit the inside of the tank, so that the horizontal part is about an inch below the top of the walls. Mark where it joins itself, remove it and weld it together so that it is a circle.
6. Put the circle back inside the tank, so that the horizontal part is about an inch below the top of the walls, and clamp it in place.
7. Hold the 10ft x 2″ strips of metal on the outside of the tank walls, level with the angle iron. Through the holes of the angle iron, mark the strips of metal. Remove the strips and drill out the holes using a 3/8″ drill bit.
8. Bolt the strips to the angle iron, removing the clamps as you go. These two pieces of metal not only clamp the liner in place, but the angle iron also serves as a support for the roof.

This roof is designed to allow any rain that falls on it to enter the tank.

1. You need to make a ring of 3/4″ PVC that sits on the angle iron support. Within this ring, place twelve 3/4″ Ts alternatively at 37″ then at 63″. The Ts want to be angled up at about a 30 degree angle. None of this PVC needs to be glued together, which helps to make minor adjustments as you go along.
2. Cut 6 pieces of 17ft long 3/4″ PVC.
3. Cut 6 pieces of 17ft long 1/2″ rebar.
4. Feed the rebar into each piece of PVC and then place each piece of reinforced PVC into the Ts on the outer circle according to the diagram. As each piece goes into place, the structure will get increasingly strong.
5. Put a piece of vinyl tarp over your frame and tuck it under the PVC circle. Cut off any excess from the inside.
6. Do the same with a mesh like hail screen.
7. From the inside of the tank, screw the tarp and screen to the PVC circle, every 6″. Now you can screw the PVC circle into the angle iron.
8. Cut out a hole for your access door. It wants to be big enough for you to comfortably fit through. Add some extra PVC around the door opening for added support. You can make the door out of any material later, so long as it fits over the opening and is either removable or hinged.
9. Cut a hole out of the tarp and mesh where you want your inlet pipe to go. Place a PVC elbow (already attached to your inlet pipe if you wish) into the hole.

(NOTE: If you are using the tank for fish, use greenhouse plastic instead of the tarp and mesh, and you are done).

1. With large brushes on long handles, paint the whole roof with a layer of acrylic, cement and water. You want the consistency to be like sour cream. It will not completely cover all parts of the mesh.
2. Once the first coat is dry, you can apply the second coat. The second coat wants to be a mix of acrylic, cement and water with sifted sand added. Try and cover the mesh completely with this coat.
3. For the third and final coat, do not put sand in, just cement, acrylic and water.
4. Each coat dries fairly quickly. You can add more coats if you wish, but three are sufficient for this purpose. As it cures properly (over a week or two), it will get increasingly hard.
5. Paint the roof with a white, waterproof paint.
6. Sweep out the interior of the tank, and clean up any acrylic concrete that may have spilled inside. The tank can now be filled with water and used.