If you are building your own garden pond and have decided to use a pond liner, you have two choices – a flat sheet or a box welded pond liner made to the exact dimensions of your pond. To some extent the shape and depth profile of your pond will determine which is the most appropriate.
Flat 2D pond liners
Garden ponds which are principally ornamental but may contain a few gold fish usually have gently sloping sides and an organic shape – maybe fitting in with the available space in your garden. These are ideal for a flexible pond liner supplied as a flat sheet of PVC, Butyl or Epalyn (EPDM) to approximately the right dimensions. The pond itself is easy to prepare – the hole can be dug to any shape that pleases the eye with deep areas and shallow ledges and needs no further construction work other than the careful removal of sharp stones or roots and a protective underlay, which should be laid in overlapping strips. The underlay is a tough geotextile usually made from polypropylene and designed to protect the liner and also to allow the escape of naturally formed gases.
A flat liner is installed by spreading it out over the top of the pond and allowed to sink into the hole with an even border all the way round. The materials, especially EPDM and butyls are flexible enough to follow uneven contours and can be pleated and stretched slightly to fit most shallow ponds with only slight wrinkling that is not normally visible once the pond is established. Leaving the liner in the sun for a while will make it more flexible and easier to handle. Around the edge, the pond liner should extend over any areas where the water can reach and be anchored either by weight or by folding into a back-filled anchor trench.
Box welded liner
If your pond is to be more formal and geometrical in shape with the intention of stocking larger fish, such as Koi carp, tench or golden orfe, then a box welded pond liner (or tailored-to-fit liner) in fabricated from sheets of Epalyn or butyl, is the ideal solution. Koi fish require large volumes of water and, because they produce a lot of waste, they need a pond which has smooth, near vertical sides that won’t trap the waste and will allow it to move naturally to a bottom drain.
This type of pond will normally have sides constructed from concrete blocks to prevent soil being eroded by ground water and building up behind the liner causing it to stretch and weaken.
A flat pond liner can be used but trying to fit it into a pond which has vertical sides and a flat bottom will require folds and pleats and these will cause a build-up of waste matter and create problems for bacteria and fish further down the line.
Box welded pond liners, on the other hand, are made to order with seams bonded together using high pressure and temperature to create a join which is stronger than the material itself. Because they are made to fit the shape of your pond, a smooth, flat finish is possible, ideal for Koi and other larger fish.
Rather than relying on your plans, which may change during construction, it is best to place your order for the liner once the hole for your pond has been constructed and you can take accurate dimensions. Make sure you allow a sufficiently large lip or flange around the edge to cover proposed planting and whatever system you are going to use to anchor the liner securely.
As with the traditional pond, the hole should be lined with a protective geotextile underlay, ensuring as even a finish as possible. The tailored-to-fit liner is then placed in the pool in the correct orientation, and, with the installer standing in the middle, the sides can be lifted gently into place and then adjusted to remove any wrinkles or creases. Water can be added during this process to bed in the liner, taking care not to stretch the liner material.