PLEASE NOTE: This product has not been tested for indoor applications, and is recommended for outdoor use only.
Our rectangular box-welded liners are strong, light and UV resistant. They are designed as flexible preformed liners that can be unpacked and installed quickly. The main advantage of a box-welded liner is that once it's installed, it's the exact shape you need, with no bulky folding of the liner required. In addition, the liner is light and flexible, making it much easier to ship compared with other preformed solutions such as rigid plastic or fiberglass. For formal shaped ponds, such as some japanese style ponds, our box-welded liners are the perfect option.
This product is available in two different liner materials. Both are fish, plant and human safe, with the upgraded EIA liner even being NSF certified as safe for holding human drinking water. Both liner materials are guaranteed not to leach any harmful chemicals into the water, which makes them excellent for use in fish farming, horticulture, koi ponds, or water filtration applications.
Something to keep in mind when ordering this liner that might not be immediately apparent from the photo, is the liner is flexible. It is not rigid, and as such the liner cannot be filled with water while freestanding - it must be supported in some way. It can be supported above or in-ground by being placed in some kind of a frame (a plywood frame works well). The circular shapes take a little more work to create a frame since the frame must be curved, but by building a frame out of 2 x 4 and then bending plywood or another material into an arc and then attching to the 2 x 4 frame, it is possible to create a curved shape to hold the liner.
Each liner is welded at our facility into the desired shape before shipping. We can usually have an order shipped out within 2 - 5 business days after receiving the order, and for any size listed above, free shipping is included. If you need a slightly different size, please send us a message for a quote.
Liner Option 1 - 45 mil Reinforced Polypropylene
Reinforced polypropylene is strong, light, flexible and UV resistant. Please note that reinforced polypropylene is different than reinforced polyethylene, which we also sell as a flat liner. The polypropylene is a premium product that is stronger, more UV resistant and has the quality that we can heat-weld it into the 3D shaped boxes. As a comparison, our reinforced polypropylene is around 4 times more puncture resistant than a standard EPDM rubber pond liner.
EIA Liner is a premium product in both quality and price to the polypropylene liner option above. It is around twice as strong and twice as puncture resistant as polypropylene mentioned above (which itself is already a strong liner). It is also NSF certified for holding drinking water meant for human consumption (potable water). However, the main advantage from our point of view is the ability for us to use a slightly different welding method which takes the reliability up another notch. While the polypropylene liners mentioned above can be hand or extrusion welded, EIA Liner lends itself to Radio / High Frequency welding (also called microwave welding). This is done with a different piece of equipment which drastically lowers the likelihood of human error while welding the liner together.
Which liner should you choose? That's easy - the EIA liner if you can afford it… but if you can't or are looking to save a little money, go for the polypropylene liner which is also a very reliable product.
Our box-welded liners are warranted to be free from defects in material and workmanship for 25 years from the day of purchase. This includes the ability of the liner to withstand normal weathering and environmental degradation when used as a primary containment membrane holding water. There are a few limitations however for the warranty to be valid: The liner must be covered at all times by soil or water, the warranty is not covered against damage resulting from acts of god, or catastrophic events such as earthquakes, tornadoes, or hurricanes. The liner is also not covered against punctures or damage resulting from improper installation, or use. Examples of improper installation or use include damage by machinery or other equipment, people, animals, ice, or sharp/rough objects in the soil or substrate.
45 mil Polypropylene
Fabrication & Warehouse
.21lb / sq ft
.21lb / sq ft
30 MILS min
Breaking Strength - Fabric, min (lbs)
550 lbs min
ASTM D 751
LOW TEMPERATURE FLEXIBILITY 1/8" MANDREL (°F)
Pass at -30 F
ASTM D 4833
Hydrostatic Resistance min (psi)
ASTM D 751
Ply Adhesion minimum (lbs/in)
Water Absorption (% wt. change, max)
0.025 kg/m2 max. @ 70º F/21º C
ASTM D 471
8000 hours min. with no appreciable change
ASTM G155 Xenon Arc
up to 3ppm (< 100 deg F)
up to 3ppm (< 100 deg F)
Frequently Asked Questions
What are the advantages of Polypropylene over EPDM Rubber?
The main advantage is the strength and weight of the liner. Polypropylene is considerably lighter than EPDM, and even though it's thinner, it is much more puncture resistant. In most cases, if the polypropylene membrane is being placed on regular soil or sand, wood or concrete, then no protective underlayment is needed.
What are the advantages of box welded polypropylene over preformed plastic liners?
While being inexpensive, preformed plastic liners are made from a lower quality material compared to box welded polypropylene liners. The main problem with the rigid plastic is it is prone to cracking which is can result in a very costly repair. Because polypropylene is flexible, it doesn't crack. However, it should be noted that because our polypropylene liner is flexible, it must be supported on all sides by compacted soil or some kind of frame in the case of an above ground application.
When is a protective underlayment needed?
Because of it's high puncture resistance, this liner doesn't need a protective underlayment if being placed on regular soil or sand. In the case that the soil contains some small rock, as long as the rock is not too sharp, then you could still get away with no protective underlayment. In the event of laying the membrane on sharper rocks, gravel, or soil with large aggregate, then a protective underlayment should be used to prevent a possible puncture once the weight of the water is added.
How is the liner transported and delivered?
If the liner is under 150 lbs, it is folded up into a box and shipped UPS ground. If over 150 lbs, it is folded up, placed on a pallet and shipped via freight truck. The liner includes free shipping no matter what the size is.
Once installed, can I walk on the liner?
Both the regular strength and extra strength liners are strong enough to handle some traffic as long as there's nothing sharp. That being said, it's always a good idea to protect the liner if you can, especially if the liner will come in contact with claws, sharp rocks, roots or digging animals. A popular way of protecting a liner is to put down a protective layer of geotextile fabric followed by some gravel, rock, brick or other material to prevent direct contact with the liner.
Can I place a heavy fountain inside the pond, directly on the liner?
As long as base of the fountain doesn't contain any sharp points or edges, the liner can handle quite a bit of compressive strength. Just be careful when placing the fountain to prevent dragging the fountain over the liner which could cause damage due to abrasion.
Can I cut the liner to install a skimmer, bulkhead fitting or bottom drain?
Definitely yes. You can cut this liner just like any other flexible liner. You would install skimmers, fittings or drains using the manufacturers recommendations.
What are the flaps / flange around the top for?
The upper flange is there to help secure the liner during installation. If being installed in a frame, the flange can be wrapped around the top of the frame and secured, or if installing in the ground, the flange can be run over the ground and covered with rocks or landscaping material to help hold the liner in place. You can also trim the flange off if desired.
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Rectangular Box Welded Pond Liner Insert (Flexible)