The material known as underlayment is a geotextile that is thicker than a flexible pond liner. It’s a felt-like fabric that is generally non-woven and made from compressed fibers rather than an extruded or calendered sheet. You’ll see the little fibers overlapping on the surface of the underlayment. While the actual polymer used for the geofabric can vary depending on its intended use, most underlayments are made from polypropylene or polyethylene. These two plastics are both durable, mildew and mold resistant, and designed to last for years under a liner.
While liners are designed to hold water and prevent leaking, underlayments tend to let water soak through instead so it doesn’t pool up in the material. These layers of felt are only designed as a cushion and protection against rips. Don’t confuse underlayerment installed in an existing pond for the liner layer. It can’t be installed alone because it will saturate the soil under the surface and cause wash outs. When in doubt about the material found in a pond, add a new liner layer to ensure you’re holding water properly. We can also help you identify liner or underlayment materials if you send us some photos of what you’ve found in an older pond.
