Guide to the protective benefits of installing a pond liner underlay
Many new pondkeepers may be unaware of the need for a pond liner underlay. Indeed, modern flexible pond liners are very tough. However there are very important reasons for installing a pond liner underlay before laying down the pond liner itself. Your new pond faces threats from both above ground and below.
The first reason for a pond liner underlay is to protect against small stones or sharp corners puncturing the pond liner. When you dig down through the topsoil you will often find that the earth becomes rougher as you dig down. Quite often a number of stones and rocks will be uncovered. In domestic garden situations it is not unusual to discover rubble from the remains of the original house building process that has been hidden by a couple of feet of top soil. If you do come across such stoney ground all you can do is get as much of the stones out by hand, constantly smoothing down the remaining surface (perhaps with the back of a spade) until you get to as smooth a surface as possible. Note: It is especially important to get the bottom or base of the pond level as this takes most of the strain, the sides will not be as vulnerable but any stones should still be removed.
Another danger for pond liners is root growth from below. Strong roots from vigourously growing plants such as bamboo have been known to push right through even the thickest butyl rubber pond liner. A good pond liner underlay helps protect against such root penetration of the pond liner.
A final benefit is that an underlay can protect against localised stretching of the pond liner. "Settlement" of the pond liner may occur as the pond takes on its long-term form - such settlement may place areas of the pond liner under increased duress through stretching, the pond liner underlay will help protect against this.