An overflow edge only works if the water flows evenly along its entire length. A perfectly level edge decides the result. A difference of a few millimetres is enough to destroy the effect.
What happens when the edge is not level
The consequences differ depending on the type of pool.
On an infinity pool, an uneven edge means that part of the overflow profile remains visible. Water does not cover the entire edge, and the vanishing effect is broken.
On a mirror (zero-edge) pool, the problem appears differently. The water surface is meant to sit flush with the terrace. When the edge is not level, some sections stay dry while others overflow. The continuous mirror effect is lost, and the dry areas become clearly visible.
In both cases, the visual result is compromised. The difference is mainly in how the defect appears.
The tolerance to aim for
The level of the overflow edge must be held to about 1 mm. This applies along the entire length of the overflow, not at a few points only.
The tolerance is tight, but the effect depends on it. Any larger variation shows: dry sections, or a profile that stays visible.
How the level is checked
The level is checked on site, before the concrete is poured. A theodolite can be used. A simple water level also works very well, and it needs no special equipment.
The reading is taken every 50 cm, along the whole length of the overflow. This is demanding. It is also the only way to be sure the edge is level before the pour.
Why this cannot be fixed later
These irregularities are difficult to correct once the concrete has been poured. Reworking an uneven edge after the fact usually requires significant demolition and reconstruction. This is why the level must be controlled before the pour, not after.
Morana’s advice
A common belief on site is that adding a layer of mortar on the concrete before laying the final finish will put small errors right. It is not a solution: under the flow of water, this mortar will probably not hold for long. The pool may seem fine at commissioning, but the owner will find the problem again sooner or later. Whether it is tiles, polyester or a membrane, no finish can durably correct a level error in the concrete. The check is made before the pour. After that, any repair is risky or difficult.
Common workarounds that don’t solve the problem
Some contractors attempt to compensate for an imperfect edge by increasing the pump flow to create a thicker water sheet. The idea is that a higher volume of water will mask small differences in level.
In practice, this approach only hides the problem temporarily. It increases energy consumption, generates more noise, and accelerates wear on the equipment. The visual effect remains fragile. Increasing the flow does not correct the root cause.
What really matters
A consistent overflow depends on geometry, not on hydraulic compensation. The overflow edge must be designed and built to the tolerance stated above, so that water flows evenly without relying on excess pump capacity.
To go further on how an overflow pool works as a whole, read our pillar article: Overflow pool.
Frequently Asked Questions: Overflow edge level
What level tolerance should an overflow edge meet?
About 1 mm, along the entire length of the overflow. The tolerance applies to the whole edge, not to a few points only.
How is the level of an overflow edge checked?
With a theodolite or a water level, taking a reading every 50 cm along the whole length. A water level needs no special equipment and works very well. The check is demanding, but it is the only way to be sure.
Can the final finish correct an uneven overflow edge?
No, not durably. Whether it is tiles, polyester or a membrane, no finish can durably correct a level error in the concrete. A mortar layer added before the finish may pass at commissioning, but it will probably not hold for long under the flow of water. The level must be right before the pour.
