For most ponds, a smooth layer of freshly graded soil is the best base layer. Applying a packed layer of sand may help smooth out uneven areas that can’t be graded and cover rough bedrock. Underlayment is a geomembrane designed not to hold water but rather to protect the liner that does. It’s a thicker material that cushions the thinner liner from rocks and uneven ground. You can’t easily add underlayment under an existing liner, so make sure you know if you need it or not before installing.
Large commercial ponds used for intensive purposes like waste water storage may require underlayment, especially to meet federal or state regulations for hazardous waste. Most home and backyard ponds won’t need a specific underlayment, but you may want one even for small ponds if your soil is very rocky or abrasive. Reinforced liners like ours are less likely to rip or tear than non-reinforced materials, but we still recommend underlayment if you’re concerned due to soil conditions. Underlayments are also a good idea for ponds built in very loose and sandy soil since the thicker material helps stiffen and stabilize the surface under the liner. Skip repurposed materials like old carpet since they’re not very environmentally friendly.
