Beckett Non-Woven Brown Underlayment 6 ft. X 12 ft., Protects Pond Liners From Sharp Rocks, Gravel, and Tree Roots. Pond liners are susceptible to punctures and tears. The underlayment is designed to cushion the pond liner, providing an extra layer of protection. It also protects the liner from damage caused by insects and rodents. If you have very rocky soil, you may install a 2 to 3-inch layer of sand first, then the underlayment, followed by the PVC pond liner, for additional protection. The underlayment covers 72 sq. ft. Simply overlap the underlayment fabric to cover larger areas based on your pond design.