Fitting a pond liner

This page is about getting a flexible liner into a dug hole without damaging it. Working out how big a sheet to buy is a separate job, and the sum is on the liner size page. If you are still deciding between a sheet and a rigid shell, read preformed pond or liner first.

The liner is the one part of the pond you do not want to replace. Most of the effort goes into the surface underneath it and into the edge around it.

Prepare the hole

Start with the hole you dug, following how to dig a garden pond. Go over every surface with your hands, not just your eyes. Pull out stones, flints, roots, bits of brick and anything else that pokes out. A small stone under the weight of a full pond keeps pressing on the same spot for years.

Check the rim is level all the way round with a straight board and a spirit level. Water finds a level of its own. If one side is lower, the liner shows on the high side and the pond spills on the low one. Fix the soil now, because fixing it once the liner is in means draining the pond.

On stony ground, the RHS step-by-step guide for a rain-fed wildlife pond suggests lining the hole, and any dip the liner will extend into, with sand to protect against punctures. Some people use a layer of soft sand on the base and shelves before the underlay as well. Sand does not stay on a steep side, which is one more reason to keep the sides sloping.

Underlay

Underlay goes between the soil and the liner. Its job is to stop stones and roots wearing through the sheet. It is not waterproof, and it never replaces the liner. The liner size page has more on that.

Lay it over the whole hole, including the shelves and the area where the liner edge will be buried or covered. Where you need more than one piece, overlap the pieces rather than butting them together, so there is no gap for a stone to find. Smooth it into the corners and down the shelves so it lies against the soil rather than bridging a hollow.

Buy a product sold as pond underlay. Carpet offcuts and builders' fabrics are sometimes used, but rot, sharp backing and unknown chemicals are all reasons to stick to something made for the job. Follow the liner maker's instructions on what they expect underneath. Some liner guarantees mention underlay.

Laying the liner

Choose a calm, mild day. A warm sheet is more flexible than a cold one, and a big sheet catches the wind like a sail. You will want at least one helper for anything bigger than a small pond.

• Unroll the liner beside the hole, on a clear patch of lawn or on a tarpaulin, not on gravel or a patio with grit on it.
• Take shoes off, or wear soft-soled ones, if anyone has to step onto the liner.
• Lift the sheet over the hole and lower it in, centred, so there is roughly the same overlap all round.
• Let it sag into the deepest part rather than dragging it. Pulling a liner across underlay can ruck the underlay and stretch the sheet.
• Leave slack. The RHS rain-fed pond guide says to lay out the liner leaving a bit of slack for when it is filled. Water pushes the sheet into every corner. A liner pulled tight before filling is stretched once it is full.

Hold the edges down loosely with smooth bricks or bags of sand, not sharp stones. They need to let the sheet slide inwards as the water settles it.

Folds and creases

A flat sheet in a curved hole always folds somewhere. Work the creases into a few larger, neat pleats rather than lots of small wrinkles, and lay them flat against the side. Fold toward the deepest point where you can. Pleats are cosmetic, not a leak, as long as the sheet itself is not cut.

Fill slowly

Start filling with a little water in the bottom. As the weight builds, ease the folds and adjust the edge weights so the liner moves down into the shelves without being pulled. Keep checking the edge as the level rises.

Rainwater is the default for a wildlife pond. The RHS rain-fed guide says to let the pond fill naturally with rainwater, and the Freshwater Habitats Trust says most tapwater is full of nutrients and suggests waiting for rain or using water butts. How to build a wildlife pond covers that choice. If the pond stands part-filled for weeks, protect the edge and keep children out of it. A half-full hole is still a pond.

Trimming and hiding the edge

Do not trim until the pond is full and the liner has had time to settle. Then cut the surplus back, leaving a generous margin past the water's edge. You cannot add sheet back once it is gone.

The edge can be finished in several ways:

• Tuck it into a trench around the rim and backfill it. The RHS rain-fed guide says loose edges can be tucked into the soil with a spade.
• Hold it under stones, paving or a timber edge. Keep the liner turned up behind the edging so the water cannot wick out under it. Plankley covers paving and edging around the garden.
• On a wildlife beach, cover it with gravel and stones sloping into the water.

Keep any sunlight off exposed liner where you can. Stones and plants at the edge hide it and protect it. Keep turf from running right up to the water, because grass at the edge draws water out.

Common ways a new liner gets damaged

• A stone or root left in the base or a side.
• A sharp edging stone dropped onto the sheet.
• A garden fork or a spade used to plant the margins.
• Boots on the liner while the pond is being planted.
• Trimming before the pond is full and finding the edge is now short.

If the level drops after the pond has settled, the leaking pond guide explains how to tell evaporation from a puncture.

Do I need underlay if the soil is soft?

The liner page treats underlay as part of the job, not an option. Even soft soil hides stones and roots. Use underlay unless the liner maker says otherwise, and add sand on stony ground.

When should I trim the liner?

After the pond is full and the sheet has settled. Trim too early and the liner can pull inwards and leave the edge short.

Can I walk on the liner while I plant?

Only if you have to, in soft shoes, and never with grit on your soles. Planting baskets on a shelf go in more safely from the bank.