Underlay can be used underneath pond liner to provide additional protection for the liner. The weight of water in deep ponds results in considerable pressure on the pond liner
and underlay gives additional protection from sharp stones etc in the ground.
The use of underlay is not always necessary. It is possible to lay pond liner on a layer of sand instead of using underlay. Where pond liner is being used
underneath a preformed water course it may not be not necessary to use underlay since the liner is not supporting the weight created by a depth of water.
Where underlay is necessary it should be laid in strips to provide a cushioned surface for pond liner.