As the name implies, pond liners are the specially made linings for ponds that create watertight foundations to hold water. Pond liners are typically either preformed shells or flexible liners. Pond liners are necessary for an informal or formal pond. Flexible pond liners are more popular, given their design adaptability, ease of installation and economic value, according to pond guide author Kathleen Fisher. Pond liners can age and start to develop leaks that can no longer be fixed by patching. When it is imperative to replace pond liners, the new liner can easily be placed over the old.
Flexible pond liners come in a variety of materials and differ in cost in accordance with their quality. Rubber liners are more durable, stretchable and resistant to ultraviolet rays compared to the PVC liners. In general, the thicker the liner, the longer it will last. The top quality rubber liners have a lifespan of 20 to 30 years while a lower grade PVC liner is good for five to seven years. Flexible pond liners are delicate and can be punctured by gravel, rocks, hard tree roots, or similar sharp objects. Hence the ground needs to be cushioned before the pond hole is lined with a special underlay much like the one used under carpets. This is a tough flexible material made especially for pond liners and is sold under different names. It is also possible to underlay the pond with two to three inches of newspaper, fibreglass insulation, or sand. Sand and newspapers are, however, not recommended, since sand doesn't pack well into the sides and newspapers deteriorate over time. So it is best to buy the underlay designed for pond liners. Many pond liners now come with this underlay already bonded onto the liner, according to Fisher's 2005 book "Complete Guide to Water Gardens, Ponds, and Fountains."
Over time it is not unusual for pond liners to get holes and spring leaks. The usual remedy for this is to patch the holes. Plastic and rubber liners can be patched with readily available patching kits. However, if liners get really old then repairing rapidly appearing cracks and leaks can prove to be tedious. The best option is to line the pond with a new liner. First transfer plants or any fish into a large bucket and drain out all the water. Then simply place the new liner on top of the old one.