How to build a pond

Summary: Learn how to install a flexible pond liner, calculate the size of a pond liner, dig a pond, fit a pond liner and fit a rigid pond liner.

There are two types of pond liner. A flexible pond liner will mould to the shape of the pond you have dug, allowing greater freedom in your pond design. A rigid liner is made from reinforced plastic and requires the hole to be dug to a specified size and shape.

Installing a flexible pond liner

Flexible liners are usually made from PVC or butyl rubber. When it comes to choosing a liner, remember that the cheaper the material, the shorter it lasts. As a guide, choose one with a long guarantee. High quality liners should last for 20 years.

Calculating liner size

To find a liner to fit your pond, you will need to work out the size of the pond. To do this, there are two calculations you need to perform.

To find the liner length:

Length = maximum overall pond length + (2 x maximum depth) + 300mm overlap around the edges

To find the liner width:

Width = maximum overall pond width + (2 x maximum depth) + 300mm overlap

The above formula is suitable for all ponds regardless of shape or size.

Digging the pond

When you are ready to start work, the first thing you need to do is plan the shape of the pond.

Mark out the shape of your pond by using a length of rope or a hosepipe. Take your time inspecting your shape from different angles to ensure it is want you want. It is a good idea to view it from a bedroom window to get a birds eye view.
When you are satisfied with the shape, mark it out by pouring sand around the perimeter and then remove the hosepipe.
Start digging out the pond crater from just inside the sand mark, keeping the sides of the pond sloped at a 20° angle to stop soil collapsing into the hole.
Dig a series of steps descending to the bottom of the pond. Each step should be 300mm (1ft) deep and 250-300mm (10-12in) wide.
Outside of the sand line, remove a strip of turf 450mm (1.5ft) wide and 50mm (2in) deep all the way around for the liner overlap.
Using a spirit level placed on a plank, check that the edges of the pond are level all the way round. Use the spoil from the hole to build up any low areas.
Check that the underwater steps are also level, as any unevenness will be apparent when the water is added.
Remove any protruding stones or other debris that could damage the liner.
Compact the sides of the pond with your hands so that it does not collapse later on.
Fit a protective underlay the same size as the liner into the hole. This is made from a woven material which is rot resistant and extremely difficult for sharp objects and roots to penetrate.

Dig a series of steps descending to the bottom of the pond. Each step should be 300mm deep and 250-300mm wide.

Installing the liner

Cut the pond liner to the required size, and lay it centrally over the pond, being careful not to tear the liner on sharp objects. This part of the job is easier with two people.
Weight down the corners of the liner with bricks.
Using a hosepipe start to fill the pond with water. The weight of the water will push the liner into the recesses of the hole.
As the pond fills up, remove the bricks securing the liner
Stop filling the pond when the water reaches just below the rim.
Trim the liner to leave an overlap 150mm (6in) wide all the way around
Bury 50-100mm (2-4in) of the overlap under soil leaving the rest exposed.
Finally, position pebbles or paving stones around the edge of the pond to conceal the liner.

After weighing down the corners of the liner with bricks, start filling the pond with water.

Rigid pond liners

Rigid pond liners are sold in a variety of shapes. Because the hole you dig must be an accurate match for the liner, the installation will be easier if you choose a symmetrically shaped liner.

To mark out the shape of a symmetrical liner, turn the liner upside down and mark around it using sand.
To mark out the shape of an asymmetrical liner, place it on the ground the correct way up. Use canes to keep the liner stable and to indicate its outline. Do not be tempted to turn it upside-down to make it easier to mark out, as this will produce a mirror image of the pond.
Remove the liner, and with a spade make cuts in the turf around your sand outline.
Liners are constructed with two or more shelves. Excavate only as deep as the shallow marginal shelf.
Using a spirit level placed on a plank, check that the edges of the shelf are level.
Place the liner in the hole, the right way up, and mark around the base (the deepest part of the pond) using sand.
Remove the liner and cut around the sand outline with a spade.
Dig a hole of sufficient depth to accommodate the deepest part of the liner, removing stones and debris.
If the ground is very stony, it is a good idea to dig the hole 50mm (2in) deeper to allow for a layer of sand to provide an even base for the liner.
Make sure the liner fits in the hole, removing more soil if necessary. Use a spirit level on the edges of the liner to check that it's level in all directions.
Start to slowly fill the liner with water. As it fills it will settle under the weight of the water. Again check that the liner is level and adjust its position if necessary before it fills too much.
As the liner fills up with water, backfill soil or sand into any gaps around the edges.
To create a neat finish paving slabs can be laid around the pond. First spread 100mm (4in) of hardcore or gravel, and compact it firmly.
Using a mortar mix (1 cement 4 sharp sand) bed each slab on top of the hardcore with the edges of the slabs slightly overhanging the pond. Tap each one into position with a mallet and frequently check that they are level.
When all the slabs have been laid, carefully fill the joints with mortar using a trowel.

As the liner fills with water, backfill soil or sand into any gaps around the edges.