Pond liner underlay is a tough, geotextile layer that sits between your pond liner and the ground, protecting it from stones, roots, and sharp edges. Our selection includes Swell and Gordon Low underlay, including 150gsm and 250gms rolls, sold by the meter on a 2m wide roll. Use the calculator below to work out how much liner and underlay you will need.
Pond Liner Underlay
Help and advice on shopping for: Pond Liner Underlay
Protect your liner before filling your pond
A puncture is the most common cause of a pond losing water. Underlay is a low-cost layer that takes the strain from stones, roots, and uneven ground, so your pond liner can last for years.
What is pond liner underlay?
Underlay is a thick geotextile felt laid over the base and sides of the excavation before the liner is installed. We recommend using it under any liner, including HDPE, Rubber, or PVC. Some of our HDPE liners are sold with underlay included, so be sure to check the product page before ordering.
Why should I use underlay?
If a liner is punctured, the pond can drain, leaving fish and plants at risk and your pump running dry. Underlay costs very little compared with the liner, so it's worth protecting your pond life and having peace of mind your liner is safe.
Which thickness should I choose?
Underlay is measured in grams per square meter (gsm). Gordon Low Pond Grade underlay is 150gsm and rot resistant, Gordon Low Heavy Duty is a thicker 250gsm felt, and our Swell underlay is also a heavy duty thick material. Choose a heavier underlay if your ground is rocky or full or roots, as 150gsm suits softer, stone-free soil.
How much underlay do I need?
Underlay is sold by the running metre from a 2m wide roll, so use the number of meters you need as the quantity when purchasing. Divide the area of your liner by two: a 10m x 5m liner is 50 square metres, so you would order 25 metres. If you are not sure what size liner you need, try our pond liner calculator, and see our guide to installing a pond liner for laying tips.