Pond underlayment isn’t any harder to install than any other flexible liner material. However, it does require patience and attention to detail to get it properly spread and smoothed before you get to the next step of spreading the liner. Leaving the underlayment creased or wrinkled could damage it as the weight of the water presses down on it. The underlayment should come folded or rolled up, just like the liner material. Carefully unroll it and spread it out across the smoothed and prepared pond excavation. Trim the material carefully with a sharp utility knife where it reaches the edges of the pond.
Spend time carefully walking over the liner getting out as many of the wrinkles as possible. Small wrinkles are practically impossible to eliminate, but large creases will show through the liner and may become cracks that leak later. If you need to hold down the underlayment while spreading it out due to wind, use large rocks with rounded edges or bags of cement. Anything else with smooth edges that won’t tear the material will also work. Don’t spread gravel or anything loose since you’ll need the liner to rest just above the underlayment surface.
