The best way of waterproofing your pond is to line it with a flexible sheet. This may range from 1000 gauge black polythene to the most expensive butyl lining. The latter is the best and it lasts for at least 50 years. Polythene liners must be protected from the sun's ultraviolet rays which make them brittle and this can cause cracking and leaking within a summer (see step 5). Make sure you buy a large enough sheet of liner to allow for plenty of overlap. Stretch the liner over the hole and ease it into place under its own weight. Put smooth-based heavy objects e.g. bricks, on the edges to weigh them down.