The thickness of any particular liner, including pond liners, is generally measured in mils. Despite sounding like it’s derived from the millimeter measurement of the thickness, it’s actually based on the fraction of inches instead. A mil is a thousandth of an inch. This means that a 35 mil pond liner will be 0.035 inches thick. You don’t need to know the precise measurement of a pond liner in inches or millimeters because you can simply compare them by their mils. A 60 mil liner will be the same thickness regardless of what it’s made from or how you stretch it out. Once you understand how to compare mils, you’ll need to determine what thickness of liner is required for your particular purpose. Thicker liners are less flexible, heavier, and harder to shape, but they’re also more tear resistant and last longer. Find the perfect balance between thick and thin with these tips.
Most professional quality flexible pond liners you’ll find for sale, even to homeowners and backyard hobbyists, will start around 18 to 20 mils in thickness. Anything thinner than this simply won’t last long enough or withstand installation over rough ground, even when using durable materials like multi-layered RPE. Thin pond liners are often sufficient for decorative and hobby ponds that are shallow and only a few hundred to few thousand gallons in volume. More demanding applications call for a thicker liner.
When tree root penetration is a problem or there’s regular traffic in and out of the pond, liners 30 mils and thicker are recommended even for basic ponds. Even backyard ponds benefit from a 30 mil or greater liner to prevent rocks and roots from wearing holes in a thinner material. In general, you’ll only find EPDM in 30 mil and thicker sheets. This is because thinner sheets are too fragile and likely to tear on inground debris during installation. Tougher materials like multi-layered RPE can be used for high-demand applications like chemical storage starting at this thickness if they’re reinforced.
Commercial grade pond liners, for large scale installation and demanding applications, tend to start at 45 mils. These liners are thick enough to weigh quite a bit, adding to installation and shipping costs. However, that extra material also adds security against seepage and leaks. Loose soils that are prone to shifting will also need at least a 45 mil cover, whether that’s being used as a single liner layer or as an underlayment for a top liner.
For the most challenging and demanding oil and gas storage ponds, a 60 mil pond liner is often required. This thick liner is also commonly used as underlayment since it offers a cushioning effect for the thinner top layer. 60 mil liner is the heaviest and thickest material you’ll find commonly sold, but some manufacturers can produce liners up to 80 or even 100 mil in thickness if necessary. For most purposes, you’ll find that 60 mil material meets all the impermeability and durability requirements of even the biggest jobs.
Choosing an RPE liner can reduce the thickness of material needed by half or more. Working a soil engineer or pond designer is the best way to select the right pond liner mils from the beginning. Using a material that is too thin could result in the need to completely replace the lining within just a few months of filling the pond. The costs involved in draining and relining a failed pond are often so high that owners simply give up on the water feature instead. Make your budget stretch by choosing the right liner from the beginning.