Part of the water loss is ordinary evaporation: on the Costa Blanca a pool loses several millimetres a day in summer with nothing at all wrong with it. Only when the loss clearly exceeds that, towards a centimetre a day or more, does it point to a leak.
You can tell the difference yourself with a bucket test. The way the level then behaves usually reveals where the problem lies: if it keeps dropping with the pump off, the shell itself is losing water, and if the drop stops at a fixed height, the leak sits exactly there.
For a repair that holds, keep to this order: first measure how much water really disappears, then establish whether the loss is in the pool or in the pipework, and only then repair.
How much evaporation is normal here
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.
The bucket test, step by step
Place a bucket on the second step of the ladder, fill it two thirds full with pool water and mark two lines with a waterproof pen: the level inside the bucket and the pool level on the outside.
Leave everything for 24 to 48 hours and change nothing: no topping up, no swimming, no backwashing, and start again if it rains. If both drop equally far, it is evaporation alone; if the pool has dropped further than the bucket, you have a leak. Repeat the test once with the pump off and once with it running. Pinpointing the leak itself is work we describe separately.
Where the leak usually sits
Top of the list is the seal around the light, above all the conduit carrying the cable: the level drops to just below the light and stops there. Then the skimmer, usually not the box but its connection to the shell, recognisable by a level that settles just under the skimmer mouth. Third are the return jets and their wall fittings, where the loss occurs mainly while the pump is running.
Next come buried fittings and pipework, showing as wet or unusually green patches in the garden. Further down are joints and loose mosaic, with a hollow sound when tapped and washed out grout, and cracks in the shell, often hairline and with the level stopping at the height of the crack. In a liner pool it is the seams and welded joints. Finally the transition between floor and wall: that corner takes the most stress and cracks first.
Is it the pool or the pipework?
If you only lose water while the pump runs, the leak is under pressure and is almost always in the return line or at the jets. If you lose water with the pump off, the pool is draining through the suction side, so the skimmer line or the main drain. If the level falls at the same rate either way, the loss is in the shell itself.
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.
Why postponing gets expensive
A leak does not only lose water, it moves ground. The escaping water washes sand and soil out from under and behind the shell, and you notice those voids only when the terrace subsides or wet patches appear in the garden. In concrete, corrosion of the reinforcement follows: rusting steel expands and pushes the concrete apart, and pipework suffers from permanent damp.
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. Leaving the pool empty in the meantime risks uplift: where groundwater stands high, an empty shell can be pushed upwards.
Sealant and patch repairs rarely hold
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.
The solution that removes the cause
If the loss is in the shell, the only structural approach is to remove every seam, with a seamless sprayed polyurea membrane: 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 2 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.
When coating is not the answer
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.
Start calmly, at the beginning
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.
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