How to build a pond with a liner
The short answer
Mark out the shape and check the rim is level. Dig the deep section and cut marginal shelves, sloping the walls rather than cutting them vertical. Remove every stone and root, lay underlay, then drape the liner in loosely on a warm day. Fill slowly so the water settles the liner into the contours, folding the slack into pleats as you go. Trim only when the pond is full, then edge the rim so no liner is left exposed to sunlight.
1
Mark out and check the levels
Lay a hose or rope on the ground to set the shape and live with it for a day before you dig. The one thing worth being fussy about is level: if the rim is not level the water line will sit high on one side and expose liner on the other. Check across the marked area with a straight edge and a spirit level before any soil moves.
2
Dig, including the shelves
Dig the deepest section first, then cut marginal shelves into the sides for planting. Slope the walls slightly rather than cutting them vertical, because a vertical wall makes the liner bridge and crease badly. If you want wildlife to get in and out, work one side into a gentle beach slope while you are digging rather than trying to add it later.
3
Clear the hole and soften it
Go over every surface and take out stones, roots and anything sharp. Punctures from underneath are the most common way a liner fails, and they are the damage a manufacturer guarantee will not cover. A layer of soft sand over the base and firmed into the walls helps, but it is not a substitute for underlay.
4
Lay the underlay
Underlay goes over the whole excavation at the same size as the liner. It is cheap relative to the liner and relative to draining a full pond to patch a hole, so there is no good reason to skip it.
5
Fit the liner
Work on a warm day if you can, because a warm liner is far more pliable and settles into the shape instead of fighting it. Drape it loosely into the hole with the overlap spread evenly all the way round. Do not stretch it tight and do not cut anything yet.
6
Fill slowly and let it settle
Start filling and let the weight of the water pull the liner down into the contours. Fold the slack into neat pleats as it goes rather than trying to eliminate it, because on any curved pond some folding is unavoidable. Keep the edges weighted while it fills so nothing drags in.
7
Trim and edge, in that order
Only trim once the pond is full and the liner has stopped moving. Trimming early is the mistake that cannot be undone. Leave a generous flap to secure under your edging, then finish the rim so no liner is left in daylight.
Finishing the edge
The last stage is the one most people rush, and it is worth its own guide. Exposed liner above the water line is where a pond starts to look homemade, and it is also where the material degrades first, because that strip is the only part in direct sunlight.
Questions
How deep should a garden pond be?
It depends what the pond is for. Wildlife ponds work with a shallow beach edge and a deeper central area. Fish need more depth so that the whole pond cannot freeze or overheat, and koi need considerably more than goldfish. Whatever depth you choose, measure the deepest point before working out your liner size, because depth is counted twice in the calculation.
Do I need underlay under a pond liner?
Yes, in practice. The liner sits on soil containing stones and roots, and punctures from underneath are both the most common cause of failure and the damage that guarantees exclude. Underlay should be the same size as the liner.
Should I put sand under my pond liner?
Sand is a useful extra layer over the base and pressed into the walls, and it helps smooth out minor imperfections. It is not a replacement for underlay, because it washes off vertical surfaces and does little against a root growing upwards.
When should I trim the liner?
After the pond is full, not before. The liner moves considerably as the water settles it into the contours, and material you thought was spare often turns out to be needed. Trimming early is not recoverable.
Can I install a pond liner in cold weather?
You can, but it is harder. Liners are much stiffer when cold and resist folding into the shape, which means more creasing and more fighting. A warm day makes the job noticeably easier.