If you are building a geometric, formal pond, you may want to consider using concrete to form the structure. Concrete can be used to create a pond of almost any size; however, installation tends to be more complex than using a flexible or rigid liner.

It is best to build concrete pools with sides that slope at an angle of 30-45° to minimise the risk of cracking during periods of severe frost. If the walls are angled, any ice can expand upwards and outwards, reducing the pressure on the walls. However, you will need to bear in mind that building the pool with such a slope will reduce the amount of deep water in comparison with a vertically walled pond. This may be of importance if you intend to stock fish.

Preparing the Ground

Level the ground as much as you can before you start to work; check this with a plank and spirit level.

Digging the Hole

Mark the outline of the pool using string and pegs, and then make a second line about 15 cm (6 in) further out to show the area to be excavated. This will allow for the volume occupied by the cement. Dig out the hole to about 38 cm (15 in) to create a shelf for marginals. Mark a line so that the shelves are approximately 23-30 cm (9-12 in) deep (PLUS an extra 15 cm (6 in) for the concrete) then continue digging to the required depth; about 45-65 cm (18-26 in) plus the concrete layer is usually quite adequate.

Lining the Hole

Spread a 500 gauge plastic sheeting so that it covers the entire hole, and then pour 5 cm (2 in) of concrete over the base. Lay a sheet of galvanised wire reinforcing mesh over this layer to provide additional strength and cover with another 5 cm (2 in) of concrete. When this layer of concrete has hardened slightly, roughen the surface of a 15 cm (6 in) strip around the perimeter of the base. This will provide a key when you add the concrete for the sides.

When the base is dry (about 48 hours later), fix a solid wall of wooden shuttering around the edges of the pond, about 10 cm (4 in) in from the sides. You will need to add a brace across each corner to hold the structure firm. Soak the shuttering with water and then push a piece of galvanised wire reinforcing mesh in between the pond walls and the shuttering sides. Pour in the concrete up to the layer of the marginal shelf.

Keep the shuttering in place until the sides have set, and then repeat the process to form the base of the marginal shelves and then the remainder of the walls.

Forming the Edge

Form an area for the edging by removing a strip of soil around the perimeter of the pond 8 cm (3 in) deep. Fill this with hardcore to provide a base for the edging stones.

Sealing the Concrete

To prevent lime from the concrete from leaching out and harming fish and plants, the pond must be sealed with several layers of a plastic sealant. This is usually available in a clear translucent finish or may be bought in colours such as black, green or blue.

Filling the Pond

When the sealant is thoroughly dry, fill the pond and check the pH of the water with a testing kit. The water should be at pH 7 before you start to introduce any plants. If the pH is higher than this (i.e. alkaline) leave the water to stand for a few weeks and check again; it should gradually become more neutral. You should wait at least 2 weeks after you have added the plants before you begin to introduce any fish.