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Ponds with flexible liners Flexible liners made from PVC or polyethylene sheeting are ideal for small and mediumsized ponds, allowing you to choose whatever shape and size you want. The sheeting is inexpensive and reasonably durable, and is easily repaired.

 Marking out the pond If you have made a rough sketch of the proposed outline of the pond, use it as your guide when you mark out the shape of the pond on the ground.

1. Mark out the pond area with a hosepipe or length of rope, trying different shapes and sizes until you come up with something you like.

2. Use a spade to mark the edge of the pond clearly, following the hosepipe or rope.

3. Remove the hosepipe and cut a second line beyond the pond outline to mark the outer rim of the edging. The distance from your original line should be the width of a slab less 50mm, so that the slabs overlap the pond edge by 50mm – this not only hides the liner but also protects it from direct sunlight.

Excavating the pond

The best way to ensure the stability of a paved edging is to dig a trench round the pond before you excavate it. A concrete pad can then be laid in the trench once the liner is in place. This secures the liner and provides a base on which to rest the paving slabs.

1. Dig a trench between the two guidelines using a spade and a pickaxe if necessary. The depth should be 75mm for the concrete pad, plus the thickness of the edging slabs, plus a mortar joint of 15mm. Roughly level the trench as you go.

2. Cut a series of 50mm x 50mm pegs, about 300mm long, and sharpen them to a point at one end. Drive one into the centre of the trench with its top at the finished water level. This will be lower than ground level by the depth of the edging (with a 25mm-thick slab and 15mm mortar joint, the tops of the pegs should be 40mm below ground level). Mark the first peg so you can recognise it to level all the other pegs from. Drive in pegs not more than a metre apart around the trench and set them with a long spirit level or laser level. The ground surface need not be wholly level but the pegs must be.

3. Start to dig out the pond to the level of the marginal planting shelf, using the inside line as your guide. The shelf should be about 230mm below water level and 250mm wide. For stability, the sides of the pond should not be vertical but should slope slightly inwards at an angle of 20° (or more in soft crumbly ground).

 4. Dig out the central part of the pond to the required depth, sloping the pond sides between the marginal shelf and the pond bottom at 20°. Make sure there are no stones sticking out of the ground. Hammer a stake into the centre of your pond and use a spirit level and a straightedge to set it level with your first peg (or use a laser level). Use the stake to double check that all the other pegs are level, and make slight alterations if necessary.

 5. When all the pegs are at the right level, mix a fairly stiff mortar using three parts sand to one part cement, and lay a line of mortar around the rim of the pond. The top of the mortar line should be level with the pegs. Make sure its edges are rounded off and the surface is smooth. Use the central stake to check the level again all the way around the rim of the pond. Leave the mortar to dry for 24 hours.

6. Using a plastering trowel, cover the excavated pond with a 25mm layer of damp sand. If the ground is particularly stony or there are sharp corners, use pond underlay or lengths of old carpet on top of the sand to protect the liner.

Calculating the liner size

When calculating the liner size, always allow for an overlap on the edge of the pond so that it can be secured either underneath a paved edge or buried in the ground if the grass runs up to the edge of the pond. Allow an overlap of 150mm all around, more if you plan to bury it. • To calculate the length of the liner you need to buy, measure the maximum length (including overlaps) and add twice the maximum depth • To calculate the width of the liner, measure the maximum width (including overlaps) and add twice the maximum depth