There are many uses for flexible pond liner in water gardening, whether for repairing an old cracked concrete pond or turning a brick lined pit into an impromptu pond. But their primary use, and what they do best, is to allow the building of informal ponds and water features like streams and fountains.

There are different pond liner materials to choose from, depending on how much you want to spend and how long you want the material to last. The larger the area you need to cover, the thicker the material you should get. Different colors are available, but black is the best choice for blending in to the environment.

- High-density polyethylene liner is your lowest cost option, and also the lowest quality. Only use it in double thicknesses for ponds; since it cracks upon long-term exposure to UV sunlight, exposed areas should be hidden by plants or soil.

- A slightly better option is low-density polyethylene, which is more robust and flexible. Higher up the list is quality polyvinyl chloride (PVC), which should be 32 mils thickness; these typically come with a guarantee. Make sure you do not buy swimming pool PVC liners, they contain biocides and herbicides that are toxic to aquatic plants and fish.

- The most durable and robust of all flexible liner materials are ethylene propylene diene Monomer rubber (EPDM) and butyl rubber. Since these are more elastic than PVC, you can install them with less folds and wrinkles. Additionally, they have better resistance to UV damage and retain their flexibility in cold weather. Both EPDM and butyl liners are usually sold with a warranty of 20 years and come in thicknesses up to 60 mils, though 45mils is the standard liner grade.

Underlay is available for pond liners, and just as with carpet, pond liner underlay will help give your liner a longer life, protect it from damage and add cushioning. It is matting of polyester that is sold in rolls. Homemade pond liner underlay can be fashioned from insulation felt, discarded wall to wall synthetic carpeting, or even old newspapers, although the latter will not last too long.

How to Calculate Pond Liner Size

Take measurements of the place you want the pond, or the pond you have excavated, using a flexible tape measure. Write down the measurements of length, depth and width at their deepest, widest and longest point. Then do the following steps to calculate how much liner you need to purchase.

1. Take the maximum length, width and depth of the pond
2. Add the maximum length to the twice the depth
3. Add the maximum length to twice it’s depth
4. Add an additional 18 inches to the length and width (for extra in case of mistakes)

For example, if your pond will measure 100 inches long, 50 inches at it’s widest, and 20 inches at its deepest point, then (100 + 40 +18) x ( 50 + 40 +18) = 158” x 108” is the size liner needed. Do not splice together sheets of smaller size, as this will create leaks and you will need to drain the pond and re-line it with one large sheet anyway.