After the soil has been properly prepared and tamped down thoroughly, you are ready for the “underlayment” and the liner. The underlayment is one of the most important steps for a water feature, and is often one that many people overlook. The underlayment is a thick fabric apx. 1/4″ thick and will provide a nice soft backing for the liner. This fabric will ensure the longevity of the liner and will help to prevent critters from damaging the liner and ruining your waterfall or pond. This protection should be installed underneath every part of the liner, and you will want to lay it long on all sides of the feature. By extending the fabric beyond the liner, you will prevent loose soil from surrounding areas from falling into the water falls or pond.
After the underlayment is secure, you can start to unroll the liner. The pond liner should be a 45mil rubber liner, and should be rated “EPDM”. This rating means that the liner is safe for fish and does not contain chemicals. (Some liners out there don’t mention this fact and soon after the liner is installed in the pond, the fish die.) After you unroll the liner you will want to gently shake the liner into the pond or pondless reservoir. By shaking the liner as you pull it out, you will not stretch or damage the liner. Even though the pond liner is strong, taking precautions is always important for a successful water feature. Keep tension to a minimum, and avoid sharp rocks and tools. My crew will also take our shoes off, just in case a pebble or rock is lodged in the treads, or if you find a rock under the liner you will feel it. These precautions are definitely not mandatory, but we have never had a rip in the pond liner.
After the liner is set into place in the pond and cascades, you will want to ensure that there is slack in the corners. By creating a 2″ or 3″ overlap, or fold, in the bottom corners of the pond you will give the pond liner room to move. Most soils in California are clay type and will in turn shrink and swell depending upon the season. So by working with mother nature you will always have a successful water fall and pond. The idea of a liner based water feature, is that it can breathe with the earth. By using a flexible membrane you will never have cracks and leaks, and by utilizing expanding foam instead of mortar, this makes the entire feature flexible.