Pond Liners: How to Choose, Size and Install One

The liner is the one part of a pond you really can't afford to get wrong. Once the edging, plants and fish are in, replacing it means draining and stripping the whole pond. Choose a good one, size it properly and lay it carefully, and you won't have to think about it again for decades.

This guide covers the main types of liner, how to work out the size you need, and how to install one without leaks.

Flexible or preformed?

There are three ways to line a pond:

• Flexible liners are sheets of rubber or plastic that you lay into a hole you've dug. They fit any shape and size, and they're what most people use.
• Preformed liners are rigid shells, usually made of plastic or fiberglass. You dig a hole to match the shell. They're quick and simple for small ponds, but you're limited to the shapes on sale.
• Concrete is poured and shaped on site. It's very durable but expensive, and it's usually a job for a professional.

Flexible liners also look the most natural. Dark rubber disappears once the pond is full, which makes the water look deeper than it is.

Compare liner materials

EPDM rubber is the standard choice in the US. It's soft and stretchy, so it folds neatly into curves and shelves, and it stays flexible in freezing weather and strong sun. Look for 45-mil thickness.

RPE is lighter than EPDM and very puncture-resistant for its thickness, which makes big sheets easier to handle. It's stiffer, though, so tight curves leave more visible folds.

PVC is the cheapest option. It's fine for a small pond or a short-term project, but it usually cracks and stiffens years before rubber does.

Preformed liners

A preformed shell is the fastest way to get a small pond in the ground, and the planting shelves come built in. The catch is that the shell must be fully supported. Dig the hole to match its shape, including the shelves, and bed it on sand. A shell resting on a few high points will flex under the weight of the water and crack.

Concrete ponds

Concrete suits formal ponds and very large builds, but it needs steel reinforcement and footings, and it can crack where the ground freezes hard. New concrete also leaches lime into the water, which raises the pH to levels that can harm fish. It must be cured and sealed with a pond-safe sealant before fish go in.

How to work out the size

A flexible liner has to cover the pond's floor and both sides, with some spare at the edges to tuck under the stones. Measure the pond at its longest, widest and deepest points, then use:

Liner length = pond length + (2 × deepest depth) + (2 × overlap)

Liner width = pond width + (2 × deepest depth) + (2 × overlap)

Allow about 1 ft of overlap on each side. That's enough to anchor the liner under your edging.

For example, a pond 10 ft long, 8 ft wide and 2 ft deep needs a liner of:

• Length: 10 + 4 + 2 = 16 ft
• Width: 8 + 4 + 2 = 14 ft

Liners are sold in standard sizes, so you'd buy the next size up, such as 20 × 15 ft. Planting shelves don't change the sum, because you always measure to the deepest point.

How deep should your pond be?

Depth affects both the liner size and whether fish can live in the pond year-round.

• Plants only: 18 inches is usually enough.
• Goldfish: at least 2 ft. In areas with hard winters, aim for around 3 ft so there's unfrozen water at the bottom.
• Koi: at least 3 ft, and ideally 4 ft or more. Koi grow large and need the space and stable temperature that deeper water gives.

Planting shelves around the edge are typically 8 to 12 inches deep and about a foot wide. See our koi pond and fish pond guides for more on planning a pond for fish.

How to install a flexible liner

1. Call 811 first. It's the free national call-before-you-dig number. Call a few days before you start, and your local utilities will mark any buried lines.
2. Mark out and dig. Outline the shape with a hose or rope, then dig in stages: shelves first, then the deeper middle. Slope the sides slightly rather than cutting them straight down.
3. Level the rim. Use a long, straight board and a spirit level all the way around. Water always finds the low spot, and an uneven rim leaves liner showing on one side and overflowing on the other.
4. Clear the hole. Remove stones, roots and anything sharp. Spread an inch or two of sand on the floor if the soil is stony.
5. Lay underlayment. Cover the whole hole with pond underlayment, a thick felt-like fabric that protects the liner from punctures.
6. Lay the liner. Unfold it into the hole, ideally on a warm day when it's soft and easy to shape. Work in socks or bare feet, not shoes.
7. Fill slowly. Run water in and smooth out the folds as the weight of the water settles the liner into place. Make a few large, neat pleats rather than lots of small creases.
8. Edge, then trim. Once the pond is full, fold the overlap under your edging stones. Only trim the spare liner after the pond has been full for a day or two and everything has settled.

Look after your liner

• Keep it covered. Sunlight is what ages a liner fastest. Hide any exposed edges with stones, plants or gravel.
• Watch the waterline. If the level drops faster than evaporation explains, check the edges for low spots before assuming there's a hole.
• Clean gently. Use a net or a pond vacuum, not a stiff brush or a sharp tool, when you clear sludge from the bottom.
• Patch small holes. Clean and dry the area, then use a patch kit made for your liner material.