If you choose the right liner, underlayment shouldn’t be necessary. For thinner underlayments or underlayment more prone to tears and punctures, sand or a non-woven geotextile can be used to protect the liner itself from stones and other issues that may damage the liner. Use underlayment when you use PVC or EPDM as your liner.
If you use RPP and RPE liners such as those offered by BTL, then it’s unlikely that you will need underlayment for the pond liner. That’s because these materials are incredibly puncture-resistant. Though RPP and RPE may be more expensive than cheaper liners, eliminating the materials and labor costs associated with installing an underlayment beneath a cheaper liner generally ends up saving you money.
