Pond construction and repair

There are a number of ways of creating a pond. Most involve excavating a hole of the required shape: what is different is the type of impervious material used to line the hole. These include flexible waterproof liners, semi-solid preformed ponds and concrete.

Constructing a pond with a flexible liner

Quick facts

Suitable for Sites in full sun
Timing Best constructed spring or autumn
Difficulty Moderate or difficult

Suitable for...

Ponds should ideally be sited in full sunlight and away from overhanging trees to avoid excessive shade and leaves fouling the water.

The appropriate shape, size and material for a pond depends mainly on the size and style of the garden, the preferred method of construction, and cost. Flexible liners allow maximum freedom of shape and size.

In a formal garden a square, rectangular or circular pond sits comfortably. For less formal gardens, an irregular-shaped pond looks more natural.

Where fish and water plants need to be accommodated aim for a minimum surface area of 38 sq ft (3.5 sq m) to ensure the water is kept well aerated and clear. Smaller ponds will require an artificial filtration system.

The depth of water depends on the kind of plants and should be not less than 75cm (30in) to allow maximum planting range.

Water less than 15in (38cm) deep is liable to overheat in summer causing deoxygenation for fish, algal blooms and freezing in winter. A small but deeper pool is often most practicable but with at least one shallowly sloping edge for wildlife. All ponds should have slightly sloping sides at about 20 degrees to make them less vulnerable to winter ice pressure.

Types of pools

Butyl: flexible

Flexible liners allow maximum freedom of shape and size and are particularly suited to creating a natural look. Butyl is a rubber material, heavier and more expensive than PVC but easy to install and with a life of 25-50 years or more.  Can be successfully patched if accidentally pierced. Available in black only, the most suitable and natural colour for a pool liner.

Preformed: rigid

Cast in various shapes and sizes from reinforced plastic or fibreglass these are rigid and lightweight. Often they include planting ledges for marginal plants. They are relatively easy to install but there is only a limited range of shapes and sizes. They are best suited to small features. Fibreglass is very durable. The exposed edges of ordinary plastic models can become brittle and crack with age. Choose darker internal colouring, as it is more natural in appearance and more aesthetically pleasing.

Concrete: rigid

The most durable of pool lining materials when soundly constructed and particularly effective for pools in formal settings. The drawbacks are that construction can be expensive and time-consuming. If cracks develop, concrete pools can be difficult to repair. Concrete must be treated or allowed to season before introducing fish or plants.

Clay: malleable

Lining pools with clay is an acceptable approach where there is underlying clay. The drawbacks are the difficulties in obtaining a sufficiently plastic clay uncontaminated with sand or stones and the degree of skill needed in applying and working the clay until it is completely free of air pockets and forms a uniformly deep cover (of not less than 15cm (6in).

Construction

Assess site for presence of buried utilities. Seek advice on electricity supply and cabling from a specialist water centre to power pond pumps if required.

The easiest way to mark out any proposed site is to lay rope or hosepipe to form the outline. Cut around the marker with a spade. Avoid extreme shapes and precise geometrical shapes unless the surroundings are formal.

Position marker pegs at equal intervals around the proposed pond. Set the first peg at the ideal level for the pond edge. Using a spirit level align the remaining pegs.

Excavate a hole to about 30cm (1ft) sloping the sides outwards by 20 degrees and level the bottom. Mark a marginal planting shelf about 30cm (1ft) wide and then dig out the central area to the desired depth.

If adding edging stones or bricks to the finished pond remove sufficient soil to a width of 30cm (1ft) all around the pond, so that the stones can be firmly bedded in.

Flexible liners

To calculate the size of liner required, measure the maximum length and width of the marked-out area. To each measurement add twice the depth, then allow an overlap of at least 15cm (6in) all round so that the liner can be held firmly by paving or tucked under turf. 

Commercially available underfelts should be used beneath liners. Fibreglass roll as used for loft insulation can also be used. On particularly stony soil a 1in (2.5cm) layer of damp sand can be put down first

Draw the liner over the hole and hold it in position with bricks. Let water from a hose gradually weigh down the liner into the hole, smoothly and with a minimum of creasing or wrinkling. To prevent stretching lift the bricks occasionally to allow the liner to move under the weight of water and mould itself into the contours of the pond. Fold in creases evenly. Cut off any excess liner, leaving a 15cm (6in) flap all around.

Vertical pool walls can be built, using bricks or concrete blocks, with butyl then used as an overall cover for walls and earthen pool bottom. To construct the walls lay footings of concrete with a smooth finish 15cm (6in) deep in the shape required. Then lay bricks or blocks in the normal way until the required height is reached.

Pools can be stepped into with reasonable care, and have stones or pebbles laid in them to create a beach or similar feature. They will also take breeze and concrete blocks wrapped in polythene or set on offcuts of butyl to hold up plant containers at a higher level instead of building concrete ledges within the pool.

Preformed rigid liners

Level the site and stand the mould the right way up, supported on bricks.

Mark out the contours with long canes pushed vertically into the ground and string around their base.

To dig a hole that matches the mould, first remove soil down to the level of the marginal shelf.

Place the pond liner in the prepared hole and press it down firmly onto the earth to leave a clear impression of the base.

Lift out the mould, then dig out the central, deeper area, allowing about 5cm (2in) extra depth for cushioning material.

Clear any stones, roots or debris from the hole. Tamp the soil firmly and line the excavation with pond underlay or a 5cm (2in) layer of damp sand.

Put the mould in place and check, using a spirit level, that it is level all the way round. Ensure the pond sits firmly on the bedding layer. Insert batons to hold it in place. Add about 10cm (4in) of water. With sand backfill around the sides to the same depth as the water ensuring there are no gaps and that the pond remains absolutely level.

Continue this process of adding water, backfilling and checking the level. Ensure the sand is well rammed beneath the shelf.

Concrete ponds

The most common way to construct a concrete pond is to use concrete walling blocks for the sides. These are skimmed with cement mixed with sharp sand and a fibrous reinforcing material.

Alternatively, a pond can be constructed from shuttering and poured concrete but shuttered ponds are prone to cracking.

Excavate the pool area, allowing an extra 15cm (6in) all round for the sides and 15cm (6in) extra at the base, to accommodate the thickness of the concrete. 

For larger pools concrete may need reinforcing and expert help is advisable. Plan construction so that the actual laying of the concrete floor and the pouring of the concrete into the wall shuttering is completed in one day, to ensure that the finished pool is waterproof.

In winter cover all concrete surfaces against frost for four days. In summer time water the concrete and shuttering. The setting of concrete is a chemical reaction and the slower the concrete sets, the harder it will be and the more resistant to cracking.

The supports can be removed two days after concreting, and the shuttering removed from four days afterwards.

Soften and round the sharp edges of the concrete with a concreting trowel.

Whatever method of construction is used, sweep out the pool to clear it of all bits of concrete and cement dust and paint the inside surfaces with a proprietary neutralising and waterproofing sealant to prevent lime leaching out into the water.

Pond edging

Informal ponds can be edged with turf, rocks or pebbles but paving may be more appropriate to the clean lines of a formal pond. Ensure slabs are securely mortared down on solid foundations and overhang the pond by 5cm (2in).

References

RHS Encyclopedia of Gardening Chapter 10 Water Gardening, Christopher Brickell (ed), Dorling Kindersley, ISBN 9781409383949 (2012)

For other books on water gardening and swimming ponds, visit the RHS Shop.

Marking out area for (flexible liner) pond. A garden hose is useful for getting natural curves.
Excavating the centre of the pond.
Checking depth.
Fitting a fabric underlay to prevent sharp stones puncturing the liner.
Loosely laying a butyl liner ensuring there is plenty of overhang.
Filling the new pond either from a hose or, better still, rain water.
Hiding the edge of the liner with turf, stones or pebbles.
    Marking out area for (flexible liner) pond. A garden hose is useful for getting natural curves. Excavating the centre of the pond. Checking depth. Fitting a fabric underlay to prevent sharp stones puncturing the liner. Loosely laying a butyl liner ensuring there is plenty of overhang. Filling the new pond either from a hose or, better still, rain water. Hiding the edge of the liner with turf, stones or pebbles.

    Repairs to leaking ponds

    Butyl liner pools

    Leaks in pools lined with butyl rubber can be repaired after draining, cleaning and drying, using butyl repair kits available from specialist aquatic suppliers.

    Preformed rigid liners

    Use a fibreglass repair kit available from specialist aquatic suppliers.

    Concrete pools

    Where the leak is slight, it will usually be at the level to which the water drops each time the pool is topped up, but there may also be others above this level. 

    First mark this level clearly then lower the water by a further 10–15cm (4–6in) and allow the sides to dry. Rub with a wire brush around the leak and the whole area at the same level and above it right round the pool. Brush off all dust until the whole area is free from debris, then paint a wash of cement and water mixed to a paste onto the moistened surface of the original concrete and allow it to dry. Use a proprietary neutralising or sealing product to cover the same area, plus 10cm (4in) or more below it to seal the cracks.

    Alternatively, use a chisel to widen the crack slightly, brush clean and fill with mortar or a proprietary sealing compound. Once dry treat with a neutralising sealant.

    Where the cracks are more severe, it may be possible to use a liner to cover the whole pool, if there are no sharp obstructions. Otherwise it is more satisfactory to completely demolish an old pool and start again from scratch.

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