How To Replace a Pond Liner

Now that your excavation is fully renovated and ready for action, it’s time for the main event: installation. By this time, you’re probably anxious to get going with your helpers arriving, your barbeque set up for the post-installation celebration, and lots of fish waiting in their temporary tanks. Still, check your weather. High winds or rain can make this a much harder job than you’d expect. Liners tend to be pretty slippery when they’re wet, and if your pond is on the large side, holding up an expanse of impermeable liner to adjust it is akin to attaching yourself to a giant kite. Your safety is the primary concern, but damage to your liner in these circumstances is likely as well. Best to wait for a calm, dry day.

Start with the Trench

Actually, there is one more preparation step, and that’s to make sure your anchor trench is ready. The trench primarily secures your liner in place but also protects vulnerable edges from damage. Small trenches start at about 6” wide by 4” deep for the smallest ponds and increase in size from there. Larger ponds or steeper slopes demand a more substantial anchor, up to 30” wide and 24” deep. Digging the trenches must be completed before installation begins, but it shouldn’t be addressed too far in advance, lest erosion or other damage causes them to collapse.  IS IT POSSIBLE TO MAKE THIS A CALLOUT?

There are several variations on the anchor trench. On smaller ponds, you’ll usually have steeply sloped sides or vertical walls and a plant shelf. The lining and additional layers extend across the trench bottom while a fill material covers the edges in a typical configuration. You can even choose a decorative fill material to disguise the liner and keep it securely in place.

First Layer: Underlayment

We’ve talked about underlayment, including those circumstances when it’s necessary (PVC and EPDM) and when you can pretty much ignore it (BTL’s AquaArmor). We’ve also covered when you may choose to take advantage of an old liner and leave it in place to act as an extra layer of protection for your new liner. Whichever route you’ve chosen, now is the time to implement it.

If you’re installing a new underlayment fabric, start by carefully laying out your pieces to cover every surface area of your pond. You probably won’t cover it all with a single panel, so be sure to overlap each piece generously and use waterproof tape to anchor it in place, if necessary. Smooth the underlayment as much as possible and fold or pleat any excess amounts neatly wherever necessary, like along a curved border or a sharp corner. Don’t stretch it taut but leave some allowance for the underlayment to adjust as the pond fills with water. You do want to avoid large bumps and lumps that could cause wear over time, though. You will also need to leave a 6-8” border around the perimeter so you can fold it into the trench with the liner. It’s best to simply wait until the installation is complete and the pond is full before doing the final trimming.

Second Layer: The Liner

If you have a custom liner from BTL Liners, it will probably be in a single panel and will arrive marked with clear instructions for unfolding it. These details aren’t crucial for smaller backyard ponds since the liner can easily be picked up and reoriented. In the case of a larger pond with a liner that’s particularly heavy or unwieldy, though, getting it oriented correctly right from the start will save a lot of time and effort.

If you have a liner that will require joining two or more pieces to span your pond, you’ll need to join those seams before the installation (unless you’re working on a massive project, like a reservoir). This work is best done indoors on a hard, flat surface with a heat-proof pad. You want your seams to be perfect, so take your time and make sure you use that roller to ensure that every millimeter bonds tightly. Remember, seams are the point of highest vulnerability when you’re fighting leaks. Depending on how many panels you have to join, be sure to allow the seams to cure (dry, cool down) before rolling them up and continuing to the next one.

As you unfold and spread the liner across your pond bed, make sure it’s centered so you have an even overlap on each side. Press the liner into each corner and any variations in the pond bed — the liner should be in direct contact with the substrate at every point. Make sure you don’t pull the liner taut; it needs wiggle room as well. The liner will need to shift slightly and spread as you fill the pond with water, and there must be room to accommodate any small voids or gaps. In the end, the ground itself (not the liner) must be supporting the weight of the water.


No pond is perfectly straight and regular, so you’ll have some areas where there will be wrinkles and excess liner. Smooth it out as best you can, but don’t stretch or cut it! In any tight sections, carefully fold or pleat the liner to lie as flat as possible (use inverted pleats for the best result). You can set some rocks down to hold the folds in place while you get everything just right. Once you fill the pond filled with water, the pressure will keep folds and pleats in place. If the appearance bothers you, rocks or plants can hide the folds. If you’re worried about them, you might consider using aggressive single-sided or double-sided tape, such as TapeCoat brand tape, to hold them together. You still need to allow some flex room, but this tape will secure your pleats in place and won’t loosen when exposed to water. Keep in mind that this level of support is unnecessary, but it’s a viable solution if you are very particular about appearance. 

Third Layer: Geo Ribbon

Of course, the primary liner won’t be visible if you choose to install a Geo ribbon. Geo is typically an eight-ounce-per-square-yard non-woven needle-punched polypropylene. It’s installed over the liner along the shore's upper perimeter and secured in the anchor trench along with the liner itself. A small amount of fill material, like rounded gravel, must be spread across the entire ribbon so that it's not prone to float. For example, you may choose to install layers of smooth stones or relatively flat river rocks over your ribbon and liner to resemble a natural pond bottom environment. This attractive treatment will keep your ribbon anchored and provide a natural look without risking seepage or water loss.

Adjusting and Anchoring

Once your liner and any other layers are correctly positioned, it’s time to add a couple of feet of water. The weight of the water will put some pressure on the liner, making it conform to the substrate. This step is probably your last chance to make adjustments without much trouble, so take a good look before you move forward.

Whether your pond has steep or vertical walls or a gentler slope, you’ll need to anchor the liner so that it doesn’t shift and allow water to leak out. You should have already chosen the type of anchor you plan to use and have prepared the site accordingly. For a basic anchor trench, carefully extend each layer across the divide, extending onto the trench floor. As you move around the edge, pressing your liner gently into the corners, you can backfill the trench with the excavated soil, gravel, sand, or whatever works for you. The weight of the backfill will hold your liner in place. This type of anchor is very effective and also hides the liner’s edges. You can cover any exposed parts with soil, gravel, plants, or even grass if that seems like an attractive option. Remember that PVC, EPDM, and many other liners are susceptible to UV rays. Your choice of cover here can expose (literally) the liner to rapid degradation and even void the warranty. If you’ve chosen one of those options, you may have to cover it with as much as 12” of soil to meet the manufacturer’s requirement. This addition could drastically alter your aesthetic vision, so keep that part in mind.

Liners by BTL

AquaArmor Pond Liner

The most versatile liner on the market today, AquaArmor maximizes protection from harmful UV rays, tear resistance and punctures that cause leaks. Simply the best liner on the market.

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