Flexible pond liner or preformed pond. Which should I choose?

A pond liner (rigid or flexible) is the unsung hero of a successful pond. A pond’s ability to hold water is only as good as the material used to construct it and rather ironically, the more reliable the pond liner the less notice we are likely to give it.

A pond liner is the equivalent to a ship’s hull or the under seal of a car, keeping water at bay in its unglamorous role which we can easily take for granted. It is bought to do a job and not to be seen, and having been laid, we are not likely to see it again until it is cleaned out or renewed after a lifetime of loyal service.

Preformed or pond liner?

There are 2 off-the-shelf- solutions for creating a pond.

a. The preformed pond (semi-rigid plastic or glass-fibre ponds) b. The flexible liner (a flexible water-proof sheet)

a. Preformed Rigid Ponds

Preformed ponds offer an instant solution to creating a pond. Simply measure your desired plot, assess its shape and choose a pond that fits and offers you the depth and shelving you require.

The range of preformed ponds is astounding and you may only find it possible to select a pond from a catalogue as even the largest aquatic retailers find it a challenge stocking all of the designs available. Check the time it will take to order, and that the shop will deliver. A 500 gallon pond is not something that will easily fit into the boot of a car!

Glass fibre ponds have the greatest life expectancy of all other preformed ponds and will usually come in a range of colours such as black, green, grey and even blue.

Installing a preformed pond is a relatively simple DIY project and could be completed in a weekend. There are two precautions to take when installing a preformed pond to ensure that it will last well beyond its 20year life expectancy.

1. Level Installation.

Every effort must be taken to provide a level bed on which to lay the pond. This will reduce any stressing of the glass-fibre shell and ensure that the pond fills to the brim around its entire perimeter. Nothing detracts more from a pond’s natural appearance than a 2″ vertical face of green fibreglass pond appearing above the water-line where on the opposite side of the pond, the water is brimming up to the edge. As a pond is only going to be laid once every 20 years, it makes sense to get it as close to perfection as possible first time.

2. Back-fill the pond for support.

When installing a pre-formed pond, the excavation should be greater than the pond’s volume to allow for levelling and minor repositioning. If the pond were to be completely filled with water while sitting in a void, the weight of water on the unsupported sides will stress the structure, perhaps even causing it to crack.

Once the pond is bedded down on a level layer of sand, it should be partially filled with water to keep it stable and then back-filled with loose spoil until all sides are well supported. The pond can then be filled to the brim to confirm that it is level.

When choosing a preformed pool, find one that is as deep as possible (min 18″). Be prepared to experience the pond keeper’s optical illusion where the size of the pond appears to halve as soon as it is set into the ground. To avoid being disappointed, choose as large a preformed pond as possible as most first time pond keepers regularly wish they had chosen a larger pond soon after installing their first pond.

b. The Flexible liner. Flexible material and flexible pond design

This is by far the most popular option for would-be pond keepers when installing a pond. It is also the relailers’ preferred choice as the same roll of liner will satisfy would-be pondkeepers wishing to construct a formal or informal pond, large or small, deep or shallow. With a flexible liner, not only are you the civil engineer, but also the pond’s designer. A pond liner puts you in direct control of the pond, from concept to finished reality. You can dictate the shape, design, depth and volume and even change your mind halfway through if you wish (as long as you buy the liner once you have finally reached your desired pond design). A flexible liner is particularly useful if you are planning to build a waterfall as the piece of liner can be extended and laid beneath the stone waterfall construction. This guarantees that any water that may leak through the waterfall structure will return back to the pond.

Which liner material?

The two most common types of liner to choose from a PVC and butyl. The PVC is a UV stabilised material which means that it remains supple and flexible, even after years of exposure to the sun’s destructive UV rays. PVC liners are generally thinner that butyl liners, being about 0.5mm thick so additional care and expense must be taken to protect against punctures and stresses during their installation. A combination of a bed of soft sand and a layer of underlay may be necessary.

Butyl is a flexible rubber liner which is usually 1mm thick and offers greater flexibility than the PVC liner. It is also much heavier to work with and more expensive than even the premium PVC liners which offer a similar lifetime guarantee. Liners are relatively straight forward to install but problems can arise with more intricate pond shapes. Liners are best suited to large ponds with sweeping and curved corners so that folds and creases can be kept to a minimum. Even so, folds are unavoidable when installing a liner, but area particular problem in smaller ponds. Dirt and debris can also collect in folds which may even cause hand nets and pond vac to snag the liner.

Care should also be taken when digging and sculpting the shape and shelves of the pond. Unlike the preformed ponds, liners take their shape directly from the shape constructed in the ground. Shelves must be carved carefully from the solid compacted earth as they are difficult to reconstruct from loose earth. Once the pond has been excavated as desired it should be given a 1-2″ deep covering of sand and lined with a pond underlay. This will reduce the likelihood of stones and roots from piercing the liner.

The liner should be stretched flat across the hole and filled with a hose. It will stretch under the weight of water, making it fit snugly into the pond’s contours. Placing the liner out in the sun prior to laying makes a neater job as it stretches better into the corners, leading to fewer folds in the finished pond.

Calculating the area of liner required.

The size of liner required (which can be bought pre-packaged or off-the-roll) can be calculated by measuring the maximum length, width and depth of the pond.

To the width and length, add double the depth and an extra foot for the overlap.

For example: A pond’s dimensions are: 12′x8′x3′

The liner required is: (12′+3′+3′+1′) x(8′+3′+3′+1′)

Which works out at: 19′ x 15′

Punctures and Repairs

Compared to the preformed fibreglass ponds, flexible liners are far more likely to suffer a puncture. A hole may only be very small (and difficult to see) but its effect on the pond level can be dramatic. The pond will drain down and stop at the level of the puncture which can be located by inspecting the ‘tide mark’ around the pond’s perimeter. Fortunately, punctures are a rare occurrence and aquatic suppliers will carry repair kits which are used in a similar way to bicycle or dinghy repair kits.

A preformed pond should be chosen wisely as it will be the significant factor in the pond’s size, design, cost and lifetime. However, once the pond has been filled and carefully finished off with plants and edging, there should be no evidence of which type of liner was used, with either providing you with at least 20 years of pond life.

Kill blanketweed and string algae.