How to line a garden pond

Video by the RSPB. This guide draws on "Building a Pond with a Liner" from the Royal Society for the Protection of Birds, the UK’s largest nature conservation charity. Their demonstration covers the key stages clearly, with particular attention to laying underlay before the liner goes in — a step many first-timers skip and later regret. Worth watching before you hire the digger.

1. Plan the shape and dig the hole

Mark out the pond outline with a hosepipe or spray paint. Aim for a gentle, irregular shape — round ponds look fine, but a natural kidney or kidney-with-bay shape suits British gardens better. Avoid perfectly symmetrical geometric outlines unless you are going for a very formal style.

Dig in stages: a shallow shelf at around 23–30 cm for marginal plants, then a deeper central zone of at least 60 cm. That deeper area matters for fish to overwinter and for the pond to stay stable through a hot summer.

2. Check the edges are level

Lay a long spirit level or a plank with a level on top across the pond rim. Go around the whole perimeter. Any high or low points will show as a gap under the water surface when the pond is filled — and they look terrible. Shave high spots down, build up low spots with compacted soil.

This step is easy to rush and expensive to fix later. Take the time now.

3. Remove all sharp stones and roots

Get down into the hole and run your hands over every surface. Remove any flint, stone, or root stub you find. Even a small sharp edge can puncture a liner over time as the weight of water presses down on it.

Once cleared, tamp the base firm. Loose, crumbly soil shifts under the liner and creates unsupported hollows.

4. Lay a bed of sand and underlay

Spread a 5 cm layer of sharp sand across the base and shelves. On the sloping sides, use purpose-made pond underlay — a thick, fibrous matting sold by the roll. It cushions the liner against anything you missed in step three and adds real peace of mind.

That said, sand can work on the sides too if you dampen it first so it sticks. The underlay is better, though. It is not expensive and lasts the life of the pond.

5. Calculate and cut the liner

To work out how much liner you need, use this formula: length = pond length + (2 × maximum depth) + 60 cm overlap. Do the same for width. Add a generous margin — you can trim excess later, but you cannot add more liner once it is in.

Butyl rubber is the most durable option and the one most pond specialists recommend. EPDM is a close second and usually cheaper. Avoid thin polythene sheeting; it will not last.

6. Drape the liner into the hole

On a warm, sunny day if possible — liner is easier to handle when it is not cold and stiff. Unfold it over the hole and let it drape naturally into the contours. Do not pull it tight at this stage. Leave plenty of slack, particularly on the shelves and at the base corners.

Anchor the edges temporarily with smooth boulders or paving slabs. No sharp rocks against the liner at any point.

7. Fill with water and smooth out folds

Start filling slowly. As the water rises it will pull the liner down and into shape. Go round the pond regularly and work out large folds by hand, folding them neatly against the contours. Small wrinkles are inevitable and will not cause leaks — large unsupported folds can weaken over time, so smooth them while the liner is still adjustable.

Fill right to the brim.

8. Trim the edges and anchor the liner

Leave a 30–45 cm overlap of liner around the edge and trim off the rest. Bury the overlap in a shallow trench around the perimeter, or pin it under edging stones, paving, or turf. The goal is to keep the liner protected from ultraviolet light, which degrades it over time.

Bedding stone or paving into mortar on top of the liner edge gives the neatest finish and helps lock everything in place. Allow 24 hours before planting.

When to call a handyman

Call Richard if the excavation is large or awkward, the ground is very heavy clay or full of flint, or the pond needs a pump and filtration circuit wiring up. Lining is satisfying work once the prep is done — but getting the digging level and the edges right first time is where most DIY ponds go wrong.