Overlap two EPDM liners in a leak-free stream: overlap direction, thresholds, and welding

A somewhat long stream, a bend, or simply a liner that is not large enough: sooner or later, two EPDM sheets have to be joined. This is where most stream leaks originate, often invisibly because the water escapes beneath the stones. The golden rule can be summed up in one sentence: the upstream liner must always overlap the downstream liner, like roof tiles, and the seam must be located where water cannot rise underneath it.

Why water flows between two liners

In a stream, water does not simply flow along the surface. It follows every possible path: between stones, under pebbles, and especially through the gap between two overlapping liners. If this overlap is installed the wrong way round, the current pushes water under the upstream liner and sends it straight into the soil. Even when installed the right way round, a seam placed on a flat area where water pools above it will often eventually leak simply by rising between the two layers.

Three factors therefore determine watertightness: the overlap direction, the seam location, and the height of the edges.

The overlap direction: upstream over downstream

Always install the liners starting from the bottom. Install the receiving basin liner first, then the lowest stream section, and continue upward. Each upstream liner must overlap the downstream liner already in place. The water flowing downhill will then pass over the seam, never underneath it.

The overlap must be generous: as a general guideline, several dozen centimeters for an unbonded joint, and more if the slope is gentle. On the sides, the upstream liner must also cover the raised side edges of the downstream liner; otherwise, water will bypass the joint around the edges.

Placing the joint on a threshold

The best location for a joint is a small waterfall. The downstream liner rises behind the step, as high as possible, then the upstream liner comes over it and drops into the lower section. The edge of the downstream liner is therefore clearly above the water level of the downstream section: even without adhesive, water that seeps beneath the flap of the upstream liner cannot pass behind it.

Building the step with a flat stone or a small stable wall, placed on felt, helps maintain this raised edge. A threshold stone laid across the stream, slightly higher on the upstream side, also creates a small pool that slows the current and makes the waterfall more attractive.

When to weld the two liners

Sometimes the liner cannot be placed on a threshold: a very flat stream, a lagoon pond, or a connection at the bottom of a basin. The overlap is then permanently submerged and must be welded. EPDM jointing tapes are specific double-sided adhesive strips, used with a primer from the same manufacturer.

1. Clean both faces to be joined: new EPDM often has a release talc that must be removed.
2. Apply the primer to both surfaces and let it dry according to the instructions.
3. Apply the tape to the lower liner without stretching it, remove the protective film, then fold the upper liner back over it.
4. Press it firmly with a roller, first perpendicular to the seam and then along the seam.
5. Secure the ends with an EPDM-compatible sealant.

Bathroom silicone and contact adhesive alone are not waterproofing solutions for a submerged EPDM seam. They may hold for a few weeks, then come loose due to water, sunlight, and temperature fluctuations.

Leaks unrelated to the seam

A common case: the seam is perfect, but the stream still loses water. The culprit is often the wicking effect. A buried liner edge, a stone overlapping the edge, or a bank plant whose roots extend over it can be enough to draw water out of the channel through capillary action. Another classic: a stone deflects the flow toward an edge that is too low, especially at waterfalls, where splashes clear the edge.

To locate them, turn off the pump: if the level in the basins no longer drops, the leak occurs during operation, caused by overflow or splashing. Raise the edges, recenter the flow, and check the height of the side returns in each section.

And what about the connection between the stream and the pond?

The same principle applies: the stream liner goes over the pond liner and extends down inside, below the pond's water level. Never stop it at the edge; let it extend a good distance into the pond, held in place by a flat stone or slab. This way, both liners are in the same body of water, and a small imperfection in the overlap no longer matters.

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Frequently asked questions

— Which way should two EPDM liners overlap in a stream?

The upstream liner always overlaps the downstream liner, like roof tiles, so that water flows over the joint and never underneath it.

— Do I need to glue the overlap of two EPDM liners?

Not necessarily if the joint is placed on a threshold with the downstream edge raised above the water. If it remains permanently submerged, sealing it with jointing tape and primer is recommended.

— Is silicone enough to make an EPDM joint watertight?

No. A compatible sealant helps secure the ends, but watertightness relies on proper overlapping or on a jointing tape designed for EPDM.

— My stream is losing water even though the joint is properly made, why?

Often caused by the capillary effect (soil, stones, or roots extending over the edges) or by splashing that crosses a ledge that is too low.

In summary

• Install the liners from bottom to top: the upstream liner always overlaps the downstream one.
• Place the joints on thresholds, with the downstream edge raised above the water level.
• A permanently submerged joint is sealed with EPDM tape and its primer.
• Watch out for the capillary effect and splashing: they cause many stream leaks.

Last updated: September 29, 2026.