Build an above-ground wooden pool: structure, waterproofing, and water weight

A wooden pond has a special charm: it blends into a deck, can be placed where digging is not possible, and brings the water up to eye and hand level. But it is also one of the projects where mistakes cost the most, because water is heavy and exerts strong pressure on the walls. In a successful wooden pond, the wood never holds the water: it forms a sturdy box, while an inner liner provides watertightness. Here is how to design the whole setup.

Understanding the load before cutting

One cubic metre of water weighs approximately one tonne. A tank measuring 1.5 m by 1 m, filled to a depth of 60 cm, already contains around 900 litres, or nearly one tonne resting on the ground, not including the wood and stones. This weight presses down on the bottom, but the water also pushes laterally against the walls, increasingly strongly with depth: the pressure at the bottom of the wall is much higher than at the top. This is why wooden tanks almost always deform in the same place, in the middle of the long sides, near the bottom.

Two practical consequences: the support must be level, stable, and capable of bearing this load (slab, compacted soil, inspected deck structure), and the walls must be held by corner posts and braces. On a raised deck or balcony, have the load-bearing capacity checked before starting any project: this is a matter of safety, not aesthetics.

Choosing wood

Regardless of the wood species, the liner separates the wood from the water. This keeps the wood dry on the inside and prevents the water from becoming loaded with tannins or treatment products.

Building the tank

1. Prepare the support: a slab or a compacted gravel bed, perfectly level.
2. Build the walls from thick timbers or boards on a frame, assembled on sturdy corner posts.
3. Reinforce the long sides with intermediate uprights and, for long tanks, with one or two tie rods running across above the water or with an exterior band.
4. Build the base from panels or boards laid on joists, or directly on the protected slab.
5. Sand the inside and remove any screw heads or splinters that could damage the liner.

For a first project, a water depth between 40 and 70 cm is a good rule of thumb: deep enough for plants and some cooling, while keeping the pressure on the walls manageable.

Waterproofing: felt and liner

Line the bottom and walls with protective felt, then lay the liner. In a rectangular tank, the challenge is at the corners: make a portfolio fold, press it firmly into place, and fold it back in the same direction at all four corners. Fill halfway so the water presses the liner against the surfaces, adjust the folds, then finish filling. The liner reaches the top of the walls; secure it along the upper edge and conceal it beneath a coping board, which also serves as a seat or shelf.

Plants, fish, and equipment

An above-ground wooden tank behaves like a small, highly exposed pond: the water closely follows the air temperature, in summer as well as winter. It is well suited to aquatic plants, a dwarf water lily, and small wildlife. Fish should only be considered with sufficient volume, a shaded area, and a winter plan; koi have no place in it.

To keep the water clear in this small volume, an all-in-one submersible filter combining a pump, mechanical filtration, and UV-C is concealed at the bottom, with a single cable discreetly routed out under the coping. Plants can be placed in floating baskets if the depth is uniform.

Maintenance and lifespan

Each spring, check that the walls are plumb and that the hardware is tight. A slight bulge appearing in the middle of a long side is a warning sign: add a reinforcement before it gets worse. Outdoors, deck oil maintains the appearance; indoors, nothing, since the wood does not come into contact with the water. In winter, in cold regions, a small tank may freeze solid: lower the water level, remove the pump and filter, and shelter the fragile plants.

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Frequently asked questions

— What wood should be used for a wooden pond?

A durable outdoor wood such as larch, Douglas fir, oak or black locust, or wood treated for outdoor use. In all cases, an inner liner prevents contact between the wood and the water.

— Is a wooden pond watertight without a liner?

No, not reliably. Wood moves, splits and rots with permanent contact with water. The liner provides the waterproofing; the wood serves as the structure.

— What water depth for a wooden pond?

Between 40 and 70 cm is a good rule of thumb for a first project. Beyond that, the pressure on the walls increases significantly and requires more substantial reinforcements.

— Can you put a wooden pond on a terrace?

Yes, if the support can bear the load: approximately one tonne per cubic metre of water, plus the structure. On a raised terrace or balcony, have the load-bearing capacity checked.

In summary

• The wood provides the structure, the liner provides the waterproofing.
• One cubic metre of water weighs approximately one tonne: a stable, level support.
• Corner posts and reinforcements on the long sides to withstand the water pressure.
• Underlay then liner, wallet folds at the corners, secured under the coping.
• Small exposed volume: plants yes, koi no, all-in-one submerged filter for clear water.

Last updated: September 29, 2026.