Flexible liners made of PVC, polyethylene or butyl rubber can be used for pond construction.

PVC is available in single or double thickness, and in various colours. It is stretchy and easy to fit, but is not resistant to ultraviolet light, and eventually deteriorates.

Triple thickness low density polyethylene (LDPE) is more durable than PVC, but less durable than butyl rubber.

Butyl rubber is the most durable, and suppliers guarantee a life of at least 10 years with proper care. Although more expensive than PVC or LDPE, it is a worthwhile investment for most pond projects. Normal specification is black, 0.75mm thickness. Butyl is slightly elastic.

Various other reinforced geomembrane liners are manufactured for reservoirs, large ornamental lakes, dirty water lagoons and other industrial and environmental applications, and are normally installed by specialist contractors. See here for information on suppliers of flexible linings.

Advantages

  1. All flexible linings are cheap compared with other types of pond lining.
  2. Flexible linings conform to minor undulations and will not fail if the underlying soil settles or cracks.
  3. Flexible linings are quick to install, requiring no special skills. Provided the project is properly designed and supervised, a reasonable sized pond can be constructed by volunteers in a day.
  4. Flexible linings are widely available commercially, and are easy to transport and store.

Disadvantages

  1. All flexible linings are vulnerable to accidental or malicious damage. Polyethythene and PVC linings puncture easily. Butyl is fairly puncture resistant, but is easily cut with a knife. Maintenance is difficult, as wading in or using tools to remove plants or silt can damage the lining. Dogs and grazing stock may damage linings.
  2. Flexible linings must be handled carefully during installation. A layer of sand and old carpets or other material is needed to prevent stones in the soil puncturing the lining from below.
  3. Larger ponds require on-site joining of the sheets, and this may need to be done by specialist contractors.
  4. Planting shelves must be carefully constructed before the lining is laid down, and designed so that any substrate laid over the liner holds securely without slipping. Avoid gradients steeper than 1:3. A general gradient of 1:6 is suitable.
  5. Problems can occur under the liner. Strong growing weeds such as couch can push up through the liner, and rats and other small animals may burrow through and hole a liner. A geotextile layer beneath the liner will help prevent this. Take specialist advice if the presence of methane is a possibility (see below).

Method for laying butyl lining

Flexible liners should be of sufficient size to allow for a 300mm overlap around the edge of the pond. The minimum size of liner should be as follows:

Length of pond = A

Width of pond = B

Depth of pond = C

Size of liner = (A + 2C + 0.6) x (B + 2C + 0.6) m2

Laying a butyl liner is easier on still, dry days. If there is a hard frost all day, postpone the work, as the liner may be damaged if it’s laid down on frosty ground.

  1. Mark out and excavate the pond, carefully checking levels. Levelling is very important to avoid areas of exposed liner above the water line, which are difficult to conceal. Allow a 300mm margin around the perimeter for the pond liner to be overlapped with turf or other edging. Remove and stack the turf from this margin. Some pond designers recommend digging a shallow trench around the perimeter of the main excavation, to anchor the lining and avoid any risk of it becoming exposed at the surface. This may cause a problem on disturbed sites which may contain methane from buried industrial or domestic rubbish, or from farm waste, as the methane is then unable to escape from under the liner and causes it to bubble up. Take advice as necessary on the past use of the land and the likelihood of methane being present.
  2. Remove all stones, roots or debris in the soil to avoid damage to the lining. Tamp the soil surface to an even finish.
  3. The slopes should not exceed 1:6 gradient. Excavate marginal shelves to the dimension required. An average of 250mm width and 250mm depth is suitable for ponds of about 60-100 square metres.
  4. Spread a 75mm layer of builders’ sand evenly over the excavated surface. This acts as a buffer between the soil and the liner, and lessens the chance of damage.
  5. Over the sand, lay a geotextile or layer of old carpets, to act as further protection to the liner.
    Pond made with flexible liner
  6. Carefully unfold the liner away from the pond to check for faults, and to allow static electricity to disperse. Crease it up again leaving the edges free, and with sufficient helpers to avoid disturbing the sand layer, lift the liner into place. As long as the liner is placed correctly, with sufficient overlap at the edges, it does not have to be moulded exactly to the contours of the excavation, as the water will do this. Temporarily weigh down the edges to secure the liner in place, but take care not to use anything that will damage the liner.
  7. Most suppliers of butyl liners advise the use of a protective geotextile layer over the butyl. This provides some protection against stones or other objects damaging the liner, especially if underwater features such as rounded stones or marginal shelves are being added. The geotextile layer also helps anchor the soil layer. Trim the geotextile to below the planned water line, or it will act as a wick, drawing the water out of the pond. Once the butyl and geotextile are laid, minimise trampling on the lining. Wear soft shoes and work carefully to avoid damaging it. The soil can be added before or after filling with water, depending on the size and layout of the pond.
  8. Fill slowly with water. Avoid disturbing the soil layer by running the hose over any pebble beach or stony feature, or by using a sheet of polythene temporarily anchored over the soil. Once full, cut away the surplus liner, leaving an overlap of at least 300mm.
  9. Replace the turves around the edge to anchor and hide the liner. Paving, boardwalks or other features can be added later, but ensure the construction does not damage the lining.
  10. Shovel  sieved  soil  into  the  water  if  soil  was  not spread earlier. Allow to settle for a few days before planting.

For more information on planting and initial maintenance see Chapter 8 – The planting and management of new ponds.

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