Overflow for a preformed pond: 3 methods for draining rainwater without dislodging the shell

A preformed pond receives all the rain that falls on its surface, and often the runoff from the surrounding area as well. During a heavy storm, the level rises by several centimeters and the water overflows where the edge is lowest. The overflow itself is not a problem. What is a problem is water seeping into the backfill around the shell, softening it and eventually causing the shell to shift. A well-designed overflow solves the problem once and for all.

How much water needs to be drained?

Each millimeter of rain provides one liter per square meter. A 30 mm storm over a 4 m² pond therefore provides 120 liters, meaning a 3 cm rise in water level, or more if the surrounding lawn drains toward the pond. This is not a torrent: a simple overflow is almost always sufficient, provided the water is directed to the right place.

Method 1: the notch toward a wetland area

This is the simplest and most natural solution. Choose a point on the edge, lower it very slightly compared with the rest (by adjusting the shell's bedding or the edging stones), and direct the excess toward a low area planted with wetland plants: irises, loosestrife, meadowsweet. It is a true rain garden that makes the most of every storm.

Place this notch on the side where the ground slopes downward, and keep the wetland area at least one metre away from the shell. Otherwise, the water will flow back and infiltrate the backfill.

Method 2: the watertight wall fitting

For a pond with perfectly level edges, or if you want to set the maximum level precisely, drill the shell just below the edge and install a watertight wall fitting. A pipe then carries the water to a soakaway, wetland area, or stormwater drainage system, depending on what your local authority allows.

1. Drain the pond below the drilling height.
2. Drill with a hole saw, using the diameter specified in the instructions.
3. Install the wall fitting with its seals on both sides of the shell.
4. Protect the inlet on the pond side with a grate to prevent leaves or fish from being sucked in.
5. Give the pipe a consistent slope toward the outlet.

Method 3: the soakaway

If you have neither a slope nor room for a wetland area, a soakaway collects the excess water and allows it to infiltrate slowly. Dig a pit away from the pond shell, line it with geotextile fabric so the soil does not clog the stones, fill it with draining stones, close the geotextile fabric, and cover it with soil or decorative pebbles. Lava rock, which is highly porous and lightweight, is particularly well suited to this use.

Limit the water coming in

The best overflow is the one that is rarely used. If the lawn or a path slopes toward the pond, create a slight ridge of soil or a diversion ditch around it. This will also prevent fertilizers, soil, and treatment residues from entering the pond and feeding algae.

Special case: pond with pump and filter

If your preformed pond is equipped with a gravity filter or a waterfall, the overflow must take the circulating water into account. When the pump stops, due to a power outage or cleaning, the water contained in the filter, pipes and waterfall flows back into the pond and raises the level by several centimetres. Therefore, set the operating level slightly below the overflow to absorb this returning water without any loss. Another pitfall: an overflowing gravity filter discharges muddy water onto the lawn. Make sure its own overflow returns properly to the pond or to the designated outlet.

You may also like

To ensure the shell is properly supported from the outset: How to install a preformed pond shell?.

READ THE ARTICLE

Frequently asked questions

— My pond overflows every time it rains; is this serious?

Not for the water, but for the shell: waterlogged backfill can settle, causing the shell to shift or crack. An overflow prevents this risk.

— Can the overflow be connected to the sewer system?

Rules depend on your municipality. In many places, on-site infiltration is preferred for rainwater. Check before proceeding.

— Can fish escape through the overflow?

Yes, if the inlet is not protected. A fine-mesh grille over the inlet of the wall fitting or notch solves the problem.

— Does the overflow lower the water level in summer?

No, it only drains water above its level. In summer, evaporation is what lowers the water level.

In summary

• Overflow mainly threatens the backfill and the stability of the shell.
• Notch towards a wetland: simple and natural.
• Watertight wall fitting: precise maximum level, connected to an outlet.
• Soakaway pit made of drainage stones under geotextile, away from the shell.

Last updated: September 28, 2026.