It’s easy to assume that a single layer of pond liner is all you need to keep water in and protect your pond from erosion. In many cases, it’s actually necessary to install another layer of material as an underlayment to the liner for extra protection. Failing to install an underlayment when it’s necessary will result in leaks or even a total loss of water from the pond. Yet it’s all too easy for a new pond owner to miss the underlayment requirements or suggestions when shopping for a liner. Find out when underlayments are needed and how to avoid them to reduce the cost of lining your pond.
Certain Liner Materials
Many flexible materials sold as pond liners simply don’t hold up well to the stresses of in-ground or buried installation. Underlayment is often necessary to increase the puncture resistance of the top liner so that roots, rocks, and cleaning equipment won’t create tears. Materials like PVC and EPDM are prone to tearing when dragged over the ground during installation. Yet even if they stay intact after placement, pressing against exposed rocks or roots can slowly wear a hole in the material over time. Installing an underlayment is the only way to keep PVC and EPDM pond liners intact through years of service. Materials like reinforced multi-layered RPE don’t need underlayment in most cases. When underlayment is needed, a non-woven geotextile is generally recommended.
Extremely Rough Surfaces
If you’re working with high concentrations of sharp-ended rocks or exposed bedrock for the base of your pond, you may need an underlayment material regardless of the top liner you choose. Even RPE liners need a section cushioning layer of underlayment when installed over rough enough surfaces. It can be hard to tell when rocky soil is rough enough to demand a layer of protective geotextile. If you’re in doubt, contact a soil engineer for a professional’s opinion. Trying to guess if you need underlayment could result in either overspending when it’s not needed or a leaky pond if it was required.
When a flexible liner is installed under a layer of poured or cast concrete, it’s generally referred to as an underlayment. This is regardless of the thickness or material used for the flexible liner. The concrete on top is considered the primary liner and the flexible material serves as an impermeable underlayment to control leaks. A liner covered in soil or ballast isn’t an underlayment unless there’s some other flexible liner or concrete installed over it. Despite being buried, a flexible liner remains a liner rather than an underlayment when it’s the primary method of water containment.
Potential for Stretch and Slump
Some of the more flexible liner materials, including EPDM, tend to stretch a surprising amount after installation. This causes the material to settle and pull away from the sides where it is installed over the banks. An underlayment of a material less likely to stretch or shrink is usually required to keep a pond stable when covered in EPDM. If you’re already adding more materials just to counteract stretching and slumping, you might as well start with a material less prone to these issues for your primary liner. Multi-layered RPE doesn’t need an underlayment just to prevent stretching, reducing installation costs significantly.
Careful liner selection can cut your installation efforts in half since you’ll have to stretch underlayment over the same amount of ground you’re covering with liner. Even if a better liner costs more per square foot, don’t forget to include underlayment costs when comparing the costs of various materials. Underlayment is usually about half of the cost of the liner material again, offering significant opportunities for savings through elimination.