Installing a pre-formed pond liner

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Last updated 30th May 2014

1. Advantage of pre-formed pond liners
2. Types of pre-formed pond liners
3. Plant shelves
4. Size of pre-formed pond liners
5. A pre-formed pond liner suitable for fish
6. Preparation for installing a pre-formed pond liner
7. Siting a pre-formed pond liner, and marking out
8. Digging and preparing the hole for a pre-formed pond liner
9. Positioning a pre-formed pond liner in the ground
10. Backfilling around a pre-formed pond liner in the ground
11. Positioning a pre-formed pond liner for a raised pond
12. Adding waterfalls and services to pre-formed pond liners
13. Edging a pre-formed pond liner

1. Advantage of pre-formed pond liners

The great thing about pre-formed pond shells is that you can see the final shape of the pond in advance. This solves the problem of designing an outline, although you still have to decide which shape to buy.

However, be aware of the deceptive size of these liners as they look so much larger when on display than once installed. So be sure of the size you require before you go shopping, and take your tape measure with you. Also think about how you’ll get the liner home – you may have to pay extra for delivery if it won’t fit in the car or on the roof-rack.

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2. Types of pre-formed pond liners

Various materials are used by different manufacturers. There’s no need to worry too much about this though – just find out the length of guarantee, which is a good indicator of durability and resistance to ultraviolet (UV) light (ie the part of daylight that causes plastics to deteriorate).

Pale coloured liners are more susceptible to UV degradation, so choose black in preference to shades of blue or stone.

Cost depends on materials, but also shapes and sizes. In any case, there may not be that much choice if you’re looking for a specific shape and depth.

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3. Plant shelves in pre-formed pond liners

Most pre-formed pond liners include a shelf for pots of marginal plants. This is because some plants prefer to be only just submerged rather than being at the bottom of a pond.

There are designs which incorporate a marsh margin; ideal for creating a pond with a natural-looking edge to it instead of paving. The marsh margin consists of a narrow channel around the edge of the liner. The margin retains soil or pebbles and stays wet because the retaining ridge between the pool and the channel is not continuous.

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4. Size of pre-formed pond liners

A pond that is at least 46cm (18in) deep with a minimum surface area of 37sq metres (40sq ft) will not freeze completely in winter, unless you live in a particularly cold area, where you should have a depth of around 60cm (24in) or even 76cm (30in) to be on the safe side.

Small volumes of water, especially shallow ones, not only freeze easily, but heat up and cool down very quickly. This puts fish under stress and also encourages green water due to algal growth. The trick is to create a pond where changes occur slowly so that natural processes keep the water clear and the fish happy.

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5. A pre-formed pond liner suitable for fish

The surface area should be large enough for plenty of oxygen to dissolve in the water, which will keep fish happy. Of course, if you’re keen on stocking Koi, and big ones at that, you’ll have to design the pond with them in mind. So take advice first before installing a pond which may turn out to be unsuitable.

It’s also important to have some method of moving water around during still summer days to increase the oxygen content of the water. The sound of moving water is pleasant too, and adds to the pleasures of a pond.

One simple and attractive way to move the water around is with a fountain. But do check the spread of the fountain, and make sure that the pond liner is large enough to catch the returning water, otherwise the pond level will drop dramatically. Although topping up a pond is easy, making sure that the water quality is suitable is not so simple since tap water contains various impurities which can be toxic to fish and encourage algal growth.

An alternative to a fountain is a waterfall. These are straightforward to install with pre-formed liners, but it’s best to incorporate them at the same time as the main pond liner. Most liner manufacturers make waterfall sections to match their range.

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6. Preparation for installing a pre-formed pond liner

Although pre-formed liners save you some design decisions, they are hard work to install. This is because it’s essential to position them exactly level. If you install a liner on the slant, the water level will be a permanent reminder of your mistake, with water full to the brim on one side of the pond and exposing the liner on the other. This is very difficult to disguise and it’s far better to ensure that the liner is correctly placed to start with.

It’s also preferable to have none of the liner exposed to sunlight – so keep the water level topped up and shade the top with edging stones or overhanging plants.

Essential tools/materials before you start:

  • Liner
  • Sand or sieved soil for backfilling.
  • Builder’s spirit level plus a straight piece of wood that is slightly longer than the length of the liner.
  • Depth gauge – use a cane or stick marked with paint or tape.
  • Wheelbarrow to move soil and sand.
  • Spade.
  • Rake.
  • For a raised pond – materials to hide the liner sides.

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7. Siting a pre-formed pond liner, and marking out

It’s important to position the pond correctly the first time, unless you enjoy doing everything twice.

To make sure that the site is right, place the liner in the chosen spot. If it’s to be a sunken pond, then mark its outline with a hosepipe or line and move the pond shell out of the way. View the liner or the outline from all angles – even from an upstairs window. If it doesn’t look right, try again until it does.

For sunken ponds, use the outline as a guide to mark out a new working outline further out, at least 15cm (6in), but preferably more all round. You must dig this far out so that there is space to backfill around the liner once it’s in the hole.

Using the cane or stick as a depth gauge, mark off the depth of any shelves and the depth of the base in relation to the surface, taking into account an extra 5cm (2in) depth of sand or sieved soil. If you’re planning to finish the edge with flat stones, take their thickness into account too so that you can place them flush with the surrounding ground

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8. Digging and preparing the hole for a pre-formed pond liner

If you’re digging out a hole in the lawn, then skim off the turf first. This rots down into good garden compost if you stack it in a compact heap.

Dig out the hole carefully, making sure that you don’t excavate too deeply around the edge where the shelf will rest. Use the marked-off stick to measure how far down to dig.

Once you have excavated the hole, check the base for stones and remove them. If left in the hole, they could damage the liner, because the volume of water exerts a tremendous pressure on the base.

Dampen the sand and evenly spread a 5cm (2in) layer across the base of the hole.

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9. Positioning a pre-formed pond liner in the ground

Place the liner in the hole and move it around to bed it into the sand. Never stand in the liner – you could easily damage it.

Check that the lip of the liner is in the right place in relation to the ground to suit your edging plans. Place the length of wood across the liner and check it with a spirit level. Do this across different parts of the liner. If the liner isn’t level, remove it and rake the sand out of the high spots and into the low spots. Repeat until you get this right.

When you think the liner is perfectly positioned, start to add water. Only put in a small amount to start with, to a depth of around 10cm (4in), to weigh down the liner and settle it firmly onto the sandy base. You may like to cross your fingers at this stage, because if the liner settles unevenly, you’ll have to empty out the water, remove the liner, and level the sandy base again.

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10. Backfilling around a pre-formed pond liner in the ground

Once the liner is level in its hole, begin to backfill between the liner and the hole’s sides, using some of the waste soil (having first removed any sharp stones). If your ground is stony, it’s simpler to backfill with sand instead. But don’t be tempted to use old turves, because these will gradually rot away to leave less than solid support for the liner.

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11. Positioning a pre-formed pond liner for a raised pond

The same principles as for a sunken pond (described in sections 9 and 10) apply for a raised pond. However, it’s much easier to make a perfectly level surface when working at ground level instead of in a hole.

As you build up the sides with brick courses or other materials, add backfill and gradually add water to the liner at the same time, continuing to check levels throughout the job (as described in section 10).

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12. Adding waterfalls and services to pre-formed pond liners

Fountain¬†units are straightforward to install. Choose sections in a finish that will blend in with any rockery or planting that you’re planning.

Install the units in the same way as the pond liner, making sure that the units sits at the correct angle for water to flow properly, while retaining a pool when the pump is switched off. Also make sure that both sides are level with each other.

Now is also the time to incorporate any hosepipes or electrical wiring. Pipes and wires are best disguised beneath the edging material, so it’s worth sleeving in a plastic or metal pipe to prevent accidental damage. Another advantage of sleeving is the ease of positioning or removing the hose or cable at any time later without disturbing the edging.

Backfill slowly, tamping down the loose material so that it forms a firm support. But don’t tamp down so much that you distort the liner. Work methodically around the liner and bring the water level up as you go, keeping it level with the backfill.

Push the backfill under the ledges with your hands to remove any air spaces. Once the ledges are properly supported, add more water to cover the ledge and check the liner’s position again, just to be sure.

Continue to backfill and add water until both the hole and the liner are full.

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13. Edging a pre-formed pond liner

To achieve an established look, finish off the edges so that the pool blends in with its surroundings. Choose materials appropriate to the setting and pay attention to colour – a warm buff may be more suitable than cold grey or glaring white.

If paving the edge of a sunken liner, leave the pond a week or so to settle into its final position. Then make up a mortar mix of 5:1 sand: cement (plus plasticiser), place a layer of this mix around the pond and bed the stones into place, ensuring that they are level. Smooth the mix into the joints and clean off all excess mortar before it has time to dry. Be careful not to drop mortar into the water though, otherwise you’ll have to empty the pond and re-fill it.

A natural look is also possible with sunken ponds, by merging the edge with existing vegetation. You could turf right to the water’s edge, or plant up a border around the edge. Plants on the marginal shelf or marsh margin provide continuity, making it difficult to tell where the garden finishes and the pond begins.

A complete circle of plants blocks your view of the water’s surface though, so it’s a good idea to use large cobbles instead at some points. Pile them up at the pond’s edge and arrange them over the ground to create a beach. This has the practical result of giving access to wildlife, including creatures like hedgehogs which tend to fall into pools and find it impossible to climb out again unless there’s a ramp of some sort.

However, this natural approach is not always practical or appropriate. Raised ponds don’t look natural anyway, so finish the edge with coping stones or flat pieces of stone, which also make a good seat.

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