Pond underlayment is a material designed to go underneath a flexible pond liner and protect it from ripping and tearing. Not only does underlayment protect against tears during installation from rocks in the soil, it also keeps growing tree roots from penetrating through the liner to reach a ready supply of water. Underlayment is a thick, non-woven fabric material that is most commonly made from polypropylene. It’s not impermeable or waterproof like the liner that goes above it, but it’s designed to act like a cushion instead. It’s most commonly available in black since it’s hidden under the liner and it looks like a large mat of individual fibers all linked together.
The best underlayments will be thick enough to resist the efforts of rodents along with tree roots. Rats like to burrow in between the liner and soil to create nests, but they often puncture the liner while digging. An underlayment can prevent this from occurring far from the surface where it’s hard to spot a leak. These fabrics are designed to drain water rather than hold it so that mildew and mold don’t become problems under the surface. If you use another material like carpeting or newspaper in its place, you’ll experience unwanted settling and noticeable unpleasant odors coming from around the edges of the pond.
