Pond liner underlay protects your pond liner from threats from below - what about threats from above? You can sandwich the pond liner between two layers of pond liner underlay. This second pond liner underlay is known as an overlap and this article looks further into the idea behind this process.
Depending on the type of pond project being embarked on you may wish for additional pond liner protection. Standard use of pond liner underlay is to line the pond base and then to lay the pond liner on top. This method is by far the most common but it only protects the pond liner from below. If the pond is likely to come under threat from above – for example wildlife or (unfortunately) vandalism, then an extra layer of pond liner can be placed on top of the pond liner itself. This second layer on top of the pond liner is called an overlay. This overlay can act as a primary barrier from threats from above – eg, falling stones, animal claws or sharp bird beaks. The added benefit is that pond liner material tends to have a grippier surface than the pond liner itself which tends to be very smooth. The benefit here being that any pebbles designed to cover the base of the pond would be more likely to stay in position as the pond liner underlay material gives the surface a ‘key’.
Sometimes the overlay itself is covered by a layer of earth. Six to twelve inches of substrate on top of the pond liner overlay should certainly offer a high level of protection. It is this method that is often used by companies building public ponds in the community. However, introducing soil into the pond can create problems. Soil may well be nitrate and phosphate rich which may make it more likely to end up with green water. In other countries, for example theNetherlands, it is more common for ponds to be covered in specially designed substrates that do not carry high levels of nitrate/phosphate. We are looking to introduce these ranges to pondkeepers in theUKsoon. Gravel, or pebbles could be used to cover the base and these will not leech out toxins (wash thoroughly first). There is a benefit in using soil if you are considering a wildlife pond full of plants as the plants will have a place to grow their roots into.