Flexible UPVC Liners for Garden Ponds
These are basically lengths of waterproof sheeting, enabling you to build a pond of any shape or size. More planning is required for this type of pond, and calculating the amount of liner you need is not always easy.
This pond-lining material is ideal for informal schemes, since the sheeting will fit most shapes and contours, albeit with varying amounts of creasing. Many raised ponds, which at first appearance may seem to be constructed entirely of bricks or concrete, are actually lined inside.
What to look for
The best reliability comes with rubber sheeting known as butyl, but PVC and LDPE (low-density polyethylene) sheets are also to be recommended. Buy lengths with a ten or more year guarantee. Polythene is common at the cheaper end of the scale, but it lacks pliability and becomes brittle after prolonged exposure to sunlight. It is best to avoid these.
Finally there are ‘geotextile’ liners, which are rubber-based liners impregnated with sodium mentonite (clay). These are self-healing liners in that, should they sustain a minor puncture, the bentonite will plug the hole.
How to buy
Most garden centres and some large DIY stores offer a selection of flexible liners. Specialist aquatic and water garden retailers will offer a better selection and there are also a number of very good mail order companies specializing in water gardening products. Whereas these companies generally have a selection of liners in predetermined sizes, you will need to add the delivery charge to your costs.
Installing a flexible UPVC liner
Before you dig the hole for the pond, go around the outside of the proposed pond site, removing a thin layer of turf or topsoil, about 45cm (18in) wide, with a spade. This strip will eventually form the edge of the pond, onto which the flexible liner will overlap, and over which the pond edging — paving slabs, bricks, etc. — will be placed.
Then dig the rest of the hole. With this type of pond the sides can slope to almost any angle from 70 to 0 degrees (that is, a gently sloping pond or home-made ‘beach’). The shallower the slope, the better it is for wildlife— 20 degrees is best. Shape the sides of the hole and the shelves as you dig down, allowing enough width —30cm (12in) minimum —for standing planted aquatic baskets if these are required.
If youris very sandy or loose, it would be advisable to install a supporting back wall to the top shelf, to prevent damage to the edge once the pond is in use. This can be done by lining the back of the shelf with brick or building blocks, cementing them in place.
As excavation proceeds, keep checking with a spirit level to ensure that any shelves are horizontal. It is not so important for the bottom of the pond to be absolutely level, unless you want to place planted containers there.
Prior to laying the butyl or rubber liner, you should install ‘underlay’. This could be in the form of a layer of builder’s sand some 5cm (2in) thick, so the excavation should be deeper by this amount to accommodate it. Pre-made underlays consist of strips of old carpet, or proprietary underlay felt; with either of these, the thicknesses are so small that your excavation will not need to be adjusted to accommodate them.
Remove any sharp stones or boulders and position the underlay. If you are using sand, dampen it so that it stays in place. If you are using carpet or felt, press it down firmly. It is especially important to run this underlay over any sharp corners, such as that caused by a reinforced back wall of bricks or blocks.
Installing the liner is not difficult. The easiest way to do this is to mould the liner into the excavation, smoothing out as many creases as you can. Then run a hose into the deepest part, adjusting any folds that occur as it fills.
Once the pond is full of water, trim the edges of the liner, leaving sufficient width all around to form a good overlap.
Before you start with your flexible liner you should always make a good estimate of the amount you need . Fortunately, shortfalls can be remedied. Water pressure is usually sufficient to hold two overlapping sheets of liner in place. If the bottom of the pond is sloping, the section of liner on the higher level should overlap the one downstream of it. A thin layer of mortar can be used to ‘glue’ down the overlap.
Finally, cover the overlapping edge of the liner with the materials you have selected for your pond edging.
FLEXIBLE LINER: CALCULATING THE AMOUNT
When digging a hole for your pond, you can make it any shape you like, but accurately calculating the amount of liner required can be a nightmare. Use this simple formula for the best way to calculate the amount of liner required: Length of sheet required = pond length plus 2 x pond depth, plus an extra 60cm (2ft)
Width of sheet required = pond width plus 2 x pond depth, plus an extra 60cm (2ft)