The right thickness liner for your pond depends more on the product than on a one-size-fits-all magic number. You’ll also need to consider several factors, including the design of your pond, the location and environmental factors, the longevity you need, the size of the installation, and cost. Let’s go ahead and clarify something about numbers, too. Sheet goods like pond liners are usually designated by their thickness in mils. A mil is not a mm or even a fraction of a mm. It is 1/1000 of an inch. So when you’re shopping around, remember that 40 mil is just about equivalent to 1mm so you can be sure you’re comparing apples to apples.
- PVC is traditionally the cheapest option, and is available in thin sheets, around 20 mils. However, it is vulnerable to both punctures and tearing and degrades quickly when exposed to sunlight. If your budget is very tight and you’re building a small ornamental pond that you don’t expect to last very long, it could be a viable inexpensive solution. - Butyl rubber and EPDM are usually offered in a range of thicknesses between 30 and 45 or even 60 mils. These liners can be quite heavy, meaning that you’ll need to literally weigh your options between thicker and more durable vs thinner but relatively easier to ship and install. In these cases, 30 mil is generally acceptable for most hobby ponds, while large or commercial installations generally need to start at 45 mil or even higher. - RPE (reinforced polyethylene) is a newer liner type that uses multiple layers of woven materials to produce an exceptionally strong liner that is not only resistant to tears, punctures and UV damage, but is much lighter as well. Generally, most hobby ponds are well suited to choices available in the 20-30 mil range. 30 mil is tough enough to withstand most tree roots, rounded rocks and foot traffic for maintenance activities, even without underlayment. Large commercial installations may want to move up to 45 mil.
