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All About Flexible Pond Liners
EPDM and PVC pond liners are by far the most widely available. PVC stands for polyvinyl chloride. While that doesn't sound like something you'd want to expose your fish too, PVC is quite safe for all aquatics. EPDM stands for ethylene-propylene rubber. What's the difference between EPDM and PVC? Well, PVC pond liners are more flexible than EPDM ones, but EPDM liners can handle temperatures down to 40 below, whereas PVC liners shouldn't be exposed to much more than 20 below.
Both kinds of pond liner have very good seal capabilities (so if you need to “glue” two bid sheets together, either PVC or EPDM will do fine), but EPDM tends to have better UV resistance, and is less likely to tear. Even if you do use an EPDM liner, you'll still want to limit the sun exposure. Finally, PVC liners do have a reputation for having a shorter lifespan than EPDM liners, but there are no real-world tests that prove that.
Given its slightly extra strength and ability to withstand long-term exposure to sunlight, EPDM liners are ultimately a better choice. But if all you can find is a PVC liner, go ahead and use it with confidence. The best-selling brand of EPDM liners is Firestone, which offers a 20 year limited warranty for their 45 mil and 60 mil liners. The difference between 45 and 60 mil is thickness - 60 is thicker than 45. Both weights make an excellent liner. If you can afford 60 mil, pay the extra for the added durability. Once your pond is filled and stocked with koi, the idea of having to drain it and search for one tiny hole to fix a leak will make you grateful you bought the 60 mil liner.
Some pond centers also sell butyl liners, which are comparable to EPDM liners in strength, flexibility and UV resistance. Many long-term pond builders say butyl liners considerably outlast even EPDM liners. They do cost more, however, and are harder to find. If you're satisfied enough with a 20 year life for your liner, EPDM is probably the best compromise of quality and cost.
If you really want your pond to last a lifetime or longer, you'll need to upgrade to a full-fledged concrete pond complete with re-enforced rebar). The cost for a concrete pond is at least three times or more what a flexible liner will be, but if you are going to be keeping $10,000 worth of koi in the pond, it's worthwhile to simply hire a professional koi pond builder and build a first class pond.
If you can't afford a concrete koi pond, but still want a flexible pond liner to last as long as possible, buy pond underlayment. The store you buy the pond liner from will also sell underlayment. If the extra cost of the underlayment is prohibitive (it will be about a quarter the cost of the liner), you can also use heavy grade roofing felt, or even carpet or any other kind of padding. Just be sure the underlayment won't disintegrate in 20 years, and - ideally - that it is non-toxic. Just in case a leak does happen, it would be a shame to poison your fish with the underlayment.
Leaks? They will happen, and its a matter of not if they will happen, but when they will hapen. Do yourself a big favor and buy one of the sealer kits available before you need it. That way, when the leak does occur, you won't lose precious time (and water) going out and getting the sealer kit... Or worse - waiting for it to arrive in the mail.
