Not many people create clay-lined ponds these days but if you're considering it, read our clay pond link.

For the range of sheet and shell liners, visit a water gardening centre or flick through a catalogue to see the range available. There are many pre-formed shapes made but it is easy to misjudge their size. Arm yourself with measurements before buying, otherwise you'll come home with something far too small.

Pre-formed shapes work out more expensive than

liner. These liners are made of a flexible, slightly stretchy material, so the weight of the water moulds it to the sides of the hole. These liners aren't so stretchy that you can skimp on the amount you buy though. You must have enough material to drape over the sides, base and edge of the hole. The length of guarantee is a good indicator of quality - don't be tempted to use ordinary polythene or PVC sheeting because it is not made for this job and will fail within a few years.

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Price varies according to the grade of liner, but the cheaper ones are more susceptible to damage by sunlight and puncturing. Stones in the soil puncture liners, as do tree roots and some grass roots, such as couch. The more the liner is stretched, the more susceptible it is to puncturing.

A protective layer between the liner and the soil is essential. Sand plus a layer of old carpet, cardboard or newspaper are adequate to cushion a pre-formed liner, but a purpose-made underlay sheet is easier to handle and offers better protection for a sheet liner.