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.
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.
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.
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).
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.