Written by the Pondliner.org team · reviewed
You can overlap two pond liners. You cannot rely on the overlap alone to keep water in. The reason is simple: water finds its level, the upper sheet floats off the lower one under pressure, and any debris that washes between the two sheets opens a slow channel from one side of the lap to the other.
Why overlap alone doesn't hold water
A lapped joint in a flat sheet works fine for shedding rain off a shed roof, where gravity does the sealing. Inside a pond the same lap sits underwater with several hundred millimetres of head pressure pushing through the gap. Without a positive bond between the two sheets the water will find a route. Most "leaking" liners with two sheets are leaking at the lap, not the membrane.
Three legitimate ways to join two pond liners
- Heat welding — hot-air or hot-wedge fusing of two thermoplastic membranes. Produces a seam that tests stronger than the parent sheet. Works on RPM, PVC and HDPE.
- Seam tape — a primer-and-cured-rubber tape system used on EPDM. Reliable when laid by a trained installer; remains the inspection point over the life of the pond.
- Solvent-glued lap splice — a contact-adhesive lap joint used on butyl and PVC. Reliable on a flat lap, harder to keep clean at corners.
What works on each material
- EPDM — primer + cured-rubber seam tape. Not heat-weldable.
- Butyl — solvent-glued lap seam. Not heat-weldable.
- PVC — solvent adhesive or hot-air weld. Welded seams are stronger than glued.
- HDPE — wedge-welded, with a double track and air-test channel between the welds.
- Reinforced polymer (RPM) — hot-air welded. Seam tests stronger than the parent sheet.
Why our RPM is welded, not taped
Reinforced polymer is thermoplastic, which means it can be heat-fused. We pre-weld every liner in the workshop to the customer's dimensions, so most ponds arrive as a single seamless sheet. On large reservoirs that exceed the largest transportable size, on-site hot-air welds finish the job. Either way the seam is part of the membrane rather than something stuck to it — covered by the same 45-year lifespan as the rest of the sheet. The RPM spec sheet has the full background, and made-to-measure box-welded liners show what is possible on formal shapes.
Frequently asked questions
Can you just overlap two pond liners and hope for the best?
No. Overlap alone won't hold water under any meaningful head pressure — the upper sheet floats off the lower one and debris opens a channel through the lap. Every legitimate join is either heat-welded, taped or solvent-glued.
Can you connect two EPDM pond liners?
Yes — with the manufacturer's primer and cured-rubber seam tape. EPDM cannot be heat-welded, so the tape system is the standard joint. Done well it is reliable for decades.
Are welded pond liner seams as strong as the parent sheet?
On thermoplastic membranes (reinforced polymer, PVC, HDPE) yes — a properly executed hot-air or wedge weld tests stronger than the parent sheet in pull tests. Taped and glued seams on EPDM and butyl are reliable when laid correctly but remain the natural inspection point over time.
Why don't you supply pond liners in two pieces with a lap?
Because a pre-welded single sheet is more reliable than any on-site lap. We pre-fabricate every liner to the customer's dimensions in our UK workshop, so the pond arrives as one seamless membrane wherever transport allows.
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