By far the most popular choice of pond building materials today, flexible liners give builders the option to fashion almost any shape and size of koi pond or water garden. The most significant advantage of flexible liners is their versatility, and includes PVC, vinyl, EPDM and butyl.

The low-cost liners are PVC and vinyl compositions. Commonly used for aboveground swimming pools, these liners have been around for decades and have a loyal following among pond builders. They cost considerably less than other liner types and have a shorter life span. Vinyl and PVC liners are thinner (typically 20 mils [0.5 cm]) than EPDM and butyl (45 or 60 mils [0.11 cm or 0.15 cm]), but this makes them lighter and easier to manipulate.

Originally blue in color (from their beginning as swimming pool liners), the standard PVC and vinyl liners are usually black, but sometimes other colors, too. The knock against PVC and vinyl pertains to low puncture resistance and brittling over time. Most PVC and vinyl liners contain UV inhibitors to protect against sun damage. A bigger concern revolves around damage from rocks, roots and animal visitors.

The next step up on the liner hierarchy is EPDM liners. The majority of pond liners in the United States today are EPDM formulations. Look for 45- or 60-mil (0.11- or 0.15-cm) liners to ensure you get the benefits of using EPDM. Pond kits often include the thinner 20-mil (0.05-cm) liners prone to punctures and root penetrations. Quality varies greatly, so it pays to buy a recognized liner brand with a clearly stated warranty.  

If you follow internet discussions, you may be familiar with the debate about EPDM pond liners. The original purpose for EPDM liners was waterproofing flat roofs. Some now argue that EPDM pond liners are nothing more than the clever work of a marketing executive intent on squeezing additional money from concerned koi and goldfish owners.

Many stories, myths and misinformation exist ranging from the use of algaecides and fungicides in roofing liner to claims that all EPDM liners are of identical chemical composition. Simply put, EPDM describes a wide family of rubber products and involves a host of different formulations depending on the end use. To be safe, buy a recognized name brand, fish-safe liner even if it costs a little more than a roofing liner. Without question, many pond owners have installed roofing liner with no detrimental side effects. Nevertheless, those who do experience trouble with roofing liners bring to mind the principle of tempting fate. Naturally, the choice ultimately rests with you, the pond builder.  

While EPDM costs more than either PVC or vinyl, it is a small price to pay for the longevity and strength of the material. However, EPDM is far from foolproof. Take great care during excavation to leave no sharp stones, sticks or roots exposed that could penetrate the liner (this applies to all flexible liners). Like the less expensive liners, extended sun exposure weakens EPDM liners.

Some pond installers use this fear to justify covering every square inch of liner with stones to “protect” the liner from sun damage; this concern is unfounded. Without delving into a physics seminar on light refraction, liner exposed to sunlight is safe from UV rays except the part above the water’s surface. Exposed liner suffers from UV rays, but damage takes far more time to occur than with PVC or vinyl liner.

Butyl is a close relative of EPDM. It is a little more durable and commands a higher price tag. Like EPDM, it typically comes in 45- and 60-mil (0.11- or 0.15-cm) thicknesses and installs in the same manner. Butyl may be hard to find at a local pond dealer in the US, but it is readily available on the internet. Commonly used in Europe, butyl maintains a small but loyal following among American ponders.

In short, butyl enjoys the same advantages as EPDM and suffers from the same weaknesses, only to a lesser extent. Butyl tolerates a slightly wider range of temperature extremes and exhibits better tensile strength. For most applications, neither factor comes into play. Perhaps the most notable attributes of butyl are excellent weathering characteristics and a very long life span. Many people say that butyl is the liner of choice if you could only build one pond in a lifetime. Despite this, many opt for EPDM due to lower cost with almost identical performance, especially in the short and medium time ranges.

The newest innovation in liner technology is a urea-based spray-on liner. Just like the spray-on bed liner for trucks, spray-on pond liners are highly durable semi rigid systems. Like concrete ponds, apply spray-on liners to an excavated area and allow it to cure. This system requires stable soils -- not a good choice for sandy or loamy areas. A brush-on formula repairs concrete structures and waterproofs other rigid structures.

Literature and websites indicate that spray-on urea costs more than other flexible liners. It offers long-term durability and wonderful design capabilities. Being so new, the jury is out on its performance and durability parameters. However, it proffers great promise. I believe its popularity will increase as installation techniques improve and the price falls.

Regardless of liner selection, the first step in successful liner installation entails using an appropriate underlayment. Many do-it-yourself pond books suggest using sand, multiple layers of newspaper, carpet padding or even carpet. The top-of-the-line underlayment material is a non-woven geo-textile fabric available from water garden retailers or landscape supply companies.

While newspaper may not be your first choice, sand, carpet padding and carpet do have a place in some applications. Carpet padding or carpet functions well as underlayment over flat, hard surfaces like concrete. Carpet also works very well over rocky soils. Sand can be an effective underlayment, but suffers from the weakness that it does not work on vertical surfaces. In addition, it tends to migrate downhill on slopping surfaces.

Non-woven geo-textile underlayment is an extremely tough fabric with high tensile strength. It limits root penetration and damage from sharp rocks. Use it under the liner for external protection. Inside the pond, it affords an extra measure of protection from heavy or sharp items in the pond. Non-woven geo-textile underlayment works well for building bogs and lining marginal and lily pots to keep soil from leaching into the pond water.

Unfortunately, aggressive marketing by some “pond companies” has given fabric underlayment a bad reputation. Claims that only non-woven geo-textile fabric facilitates essential gas exchange in the soil below a pond do not help matters (sand, carpet and padding are all gas permeable). Yet the fact remains that no other underlayment combines the strength and versatility of fabric underlayment. Furthermore, it pays to read the liner warranty terms. Some suppliers require an “approved” underlayment as a condition of proper liner installation (and of honoring the liner warranty). Overall, the minimal cost of fabric underlayment is well worth the price. Combine sand, carpet, or padding with fabric to provide the utmost in liner protection.