Preformed Pond Liners versus Flexible or Sheet Pond Liners
There are two primary kinds of pond liners: preformed and sheet or flexible liners. I'll list the pros and cons of each, but generally if you want a very small, shallow pond, a preformed liner is easiest. If you want a larger pond that's deeper (and is much better for your fish, and essential if you want to keep koi properly), you'll need to go with a flexible sheet liner.
Pros of a Preformed Liner:
- No creativity required
- Easier to level
- If you want a round pond, you'll have perfect sides right out of the box
- Available at most garden or home improvement centers
Cons of a Performed Liner:
- Usually 18 inches deep or less (not really deep enough to overwinter even goldfish)
- Less than 200 gallon capacity
- Limited selection; may not work with the surrounding landscape the way you want
- More than twice as expensive than a flexible liner that would cover the same area
- Some pond owners say preformed pond liners tend to break or leak more often than flexible liners
- The hole for the liner needs to fit it exactly. Water is heavy, and if you have an unsupported gap between the liner and the hole, there's a real chance the liner will break much sooner than it should.
Pros of a Flexible Liner - Can create any width, depth and length of pond you want - square round or kidney shaped
- Half the price of a preformed liner of the same size (a pond liner 15 feet square will cost anywhere from $80 to $160)
- Easier to dig a hole for (the liner will fill in exactly where the hole goes)
Cons of a Flexible Liner
- All that freedom of design may force you to take a bit more time to plan how you want your pond to look
- Weight: a 20x20 foot pond liner will weigh over 140 lbs and require two adults to move it. One strong person might be able to lay the liner out in the hole, but having two people for the job is much easier
If you still prefer to use a preformed liner, but can't find one you like at the garden center, try to find a ranch or farm supply store. Water troughs, feed tanks and horse waterers make great preformed ponds. Many of them are two feet deep, too, which is better for your fish - it will help them stay cool in the summer, and will be deep enough to overwinter hardy goldfish.
When you're installing the pond liner, be absolutely sure that the edges are level. If it's a very small pond, you can use a 4 foot carpenter's level. If it's a larger pond, get some strong string and a string level. When doing the leveling work, test the pond's levelness from at least three different points around the edges. It may seem like a hassle, but having an uneven pond looks terrible, and you'll end up spending more time trying to fix it later than you would have spent getting it right the first time.
Finally, do consider using pond underlayment - a protective barrier between the pond liner and the rocks and dirt it sits on. Underlayment can be bought from a pond or garden supply store, or you can use roofing felt, or even old carpet. If money's tight, go with the roofing felt; it will cost half as much. If not, buying bona-fide pond underlayment may work just a bit better.