How do you detect and find a leak in a swimming pool?

Sun, heat and above all wind evaporate a great deal of water here. In summer a pool loses several millimetres a day without any defect, and strong wind, warm water, low humidity and an uncovered pool push that higher.

Convert millimetres into litres using the water surface: one millimetre of drop is one litre per square metre. A pool of 8 by 4 metres has 32 square metres of surface, so 3 millimetres a day is close to 100 litres. A cover keeps most of that water in and doubles as a test: if the level still falls quickly under a blanket, evaporation is not the explanation.

These steps tell you whether your pool is losing more than evaporation alone:

Run the test a second time, this time with the installation switched off completely. The difference between those two measurements usually points you in the right direction.

If the pool mainly loses water while the pump is running, the pipes are under pressure and water is being forced out where a fitting or a pipe no longer seals. That is nearly always in the pressure line between the plant room and the inlets.

If the level also drops with everything off, the leak is in the shell itself or in a component sitting at water level, such as the skimmer or a light housing. If it drops at roughly the same rate with the pump on and off, something in the shell is at fault and it has little to do with the installation.

Note where the fall stops as well: water settles at the height of the leak. Just below the light points to the light niche, under the skimmer to that connection, and draining all the way down to something low in the shell.

If you are torn between the shell and the pipework, you do not have to work that out yourself. With every pool we offer a pressure test as standard: a company specialised in that work puts the pipes under pressure one by one. If a pipe does not hold the pressure, the leak is in that pipe.

That way it is established beforehand whether the problem is in the shell or in the pipework, before anything is broken open or finished. It prevents the most expensive mistake with a leak: repairing in the wrong place.

The skimmer is by far the most common spot. The joint between the plastic skimmer box and the surrounding concrete moves with temperature changes and eventually cracks open. You often cannot see it because it sits behind the coping.

Next come the light housings. The seal around a light is a rubber that hardens after ten to fifteen years. The inlet nozzles and the main drain are equally common places for a seal to give up.

In the shell itself it is usually hairline cracks, often in a corner or along the steps. They appear because concrete moves and the tile adhesive or grout cannot follow that movement. In a tiled pool the water does not even need a visible crack: it creeps behind the tiles along the grout lines. Watch the transition between floor and wall as well: that corner takes the most stress and cracks first. In a liner pool it is the seams and welded joints.

Finally there are the buried pipes. Those are the hardest because there is nothing to see. A wet patch in the garden that never dries out, or one strip of grass growing noticeably faster, is sometimes the only clue.

One backwash of the filter easily costs several centimetres of water height. Anyone who backwashes weekly and counts that as loss is hunting a leak that does not exist.

Splash out adds up too. A pool with children in it every day loses noticeably more than the same pool sitting quietly. An automatic top up makes it treacherous: it refills unnoticed, so you only spot the loss on your water bill.

So always check first how often you backwash, whether the pool tops up by itself, and how heavily it is used. Only once all of that is accounted for and the water keeps dropping is there reason to look further.

The most common mistake is patching one spot: mastic in a joint, a crack chased out and filled, a sealing product poured into the water, or the whole pool regrouted. The leak disappears for a while and returns a few metres away weeks or months later.

That is because the cause is rarely in one place. A tiled pool has hundreds of metres of joint, and those joints are the finish, not the waterproofing. If the shell moves through temperature swings, outside pressure or slight settlement, that movement transfers to the next weak point as soon as you close one off. You are not sealing a leak, you are relocating it.

A leak is rarely just a water problem. The escaping water carries sand and soil out from under the shell. As a result the pool starts to settle unevenly and a hairline crack grows into a real one.

On top of that, damp concrete attacks the reinforcement, and chlorinated water saturates the surrounding ground. Money runs away daily as well: with every top-up you pay again for the chemistry needed to restore the balance and, in a heated pool, again for warming cold mains water.

If you are losing more than roughly one centimetre a day, it is wise not to wait until the end of the season.

With one clearly identifiable leak, for example a worn seal around a light, a targeted repair is perfectly fine. Nothing else needs to happen.

If there is more than one leak, or the water runs behind the tiles without you being able to point at where, targeted repairs become a losing battle. Every time you close one spot, the water finds the next weak point.

In that case a seamless sprayed polyurea membrane is the approach that removes the cause: no joints, no overlaps, no welded seams. The membrane stays elastic and bridges movement in the substrate.

The build-up: first the repair of old layers, cracks and irregularities, where necessary with a levelling layer of cement-based mortar, around 15 kilos per square metre for a layer of one centimetre. Then a bonding primer suitable for permanent immersion in two coats, around 500 grams per square metre in total, which also acts as a vapour barrier. On top of that the sprayed membrane, around 2 kilos per square metre in one continuous layer, final thickness at least 3 millimetres. Finally a UV and wear layer for pools, in two coats with a roller, around 400 grams per square metre.

A complete renovation takes on average 5 to 10 working days; the spraying itself is a single day's work. The pool may be filled after seven days. Waterproofing and adhesion carry a ten year guarantee. The expected service life is fifteen to twenty-five years, although that is an expectation and not a promise.

A coating waterproofs the inside of the shell, and nothing beyond that. If the leak is in pipework outside the shell, in the ground or in the plant room, coating changes nothing about it.

Waiting is also wiser where a structure is still settling, and where expansion joints are not treated as such. The same applies to water pressure from outside: if high groundwater presses against the shell, that must be solved first. And sometimes the shell is finished: a failing structure, a cracked prefabricated polyester shell or an above-ground pool with a thin wall calls for repair or replacement.

Water loss feels urgent, but hardly ever needs action the same day. First measure for a few days how much water really disappears, do the bucket test, and note whether the level behaves differently with the pump on or off.

If you would like us to look with you, send a few photographs of the pool, of the skimmer and of the height at which the level settles. We assess the situation on site or from those photographs and send a quotation within 24 hours.