Excavating and lining
- Lay out a rope or a length of hose to the shape of the pool, including its planting pockets, checking that it is level all the way round. You can do this initially with a spirit level on a long straight piece of wood or with a transparent hose filled with water. You may need to do some preliminary digging at this stage, but it is essential to establish exactly where the water level will be.
- It will help when you come to digging and installing the edging if you hammer pegs in at intervals just outside the final outline, with their tops say 10 cm above the planned water level, and level them using a spirit level. When you have finished, look at the outline from all possible angles, including from above if you can, such as from an upstairs window.
- Mark all round with a spade just inside the outline. Take out the turf or topsoil first and stack it or barrow it away. Then excavate the subsoil and do the same with that. Finally, cut back further to accommodate your chosen edging. Check carefully all over the base for protruding roots or sharp stones that might puncture the liner, and remove them. At the same time, make a final check of dimensions and slope angles.
- When you are completely happy, line the hole with protective material. On a smooth clay soil 15cm or so of soft (builder's) sand and/or a layer of old newspapers may be enough, but on stony soils or to maximise liner life, use a layer of polyester matting. It is also wise to line with PVC or plastic sacks the upper edge of the pool where it borders any rough ground. While butyl seems able to resist most threats, some vigorous plants – notably couch grass – grow straight through it.
- Drape the liner into the hole with an even overlap all round, weight the edge with stones or bricks, then start filling it with a hose. As it fills, move the stones to allow the liner to fit as neatly as possible into the hole. Some creasing is unavoidable but can be minimised by easing and stretching the liner.
- When the pool is full, secure the edge by pushing large nails through it into the ground, and cut off any surplus, making sure that you leave enough to anchor the edges securely. Where it terminates in the soil, take it up and over a small firm mound of earth and anchor the end with more soil or with rocks buried in turn in soil. Make sure the liner is high enough all round to prevent any water loss into the surrounding soil.
Finally, install any edging, making sure that it is horizontal and in exactly the right position relative to the planned water level. If using cement, take care not to drop any into the water, and it is also worth proofing cement exposed to pool water to stop salts leaching out. With a liner, you have four main choices for how you handle the edge. Often you will want to use more than one method, such as a paved edge for access and a planted edge round the back for wildlife value.
Run the liner up about 10 cm above maximum water level and bury it in the soil
- simple and natural effect, but liner shows when water drops;
- danger of damaging the liner when mowing or cultivating near the pond.
Set large stones or cobbles into a bed of sand/cement laid on the liner, running out of the pool up a shallow slope
- stones can just be piled loose on the liner, but will need regular maintenance to stop them silting up and becoming overgrown by plants;
- makes access by wildlife easy, and conceals liner completely.
A planting pocket is built on to the liner, edged with stones or bricks
- soil level should have a gradient, ending a few inches above water level, so that plants requiring a range of damp soil conditions can be grown;
- important that liner at outside of pocket is high enough to prevent water loss.
Concrete or stone slabs are laid on a sand/cement bed over the edge of the liner (ideally on a foundation of hard core)
- more formal but very practical;
- if you pave all the way round, make sure, e.g. by stacking loose rocks next to the edge, that very small animals like froglets can easily climb in and out.