Swimming Pool Leak Detection

A pool that needs topping up more often than it used to is losing water in one of four places: through the shell, through a fitting, through the buried pipework between pool and plant room, or into the air. Telling those apart is the whole job. They lead to completely different repairs at completely different costs, and you can narrow it down considerably before anyone drives to your house.

First, turn millimetres into litres

Pool owners describe a leak in millimetres and pay for it in cubic metres. The conversion is worth doing before you ring anyone. One millimetre of depth across one square metre of water surface is one litre. Multiply the daily drop in millimetres by the surface area of the pool in square metres and you have the litres you are losing each day.

A 10 m by 5 m pool has a surface area of 50 m². A 10 mm daily drop is 500 litres a day. That is the number to quote when you call, not "it seems a bit low".

Then work out how much of it is normal. SPATA, the UK pool trade association, gives 3 mm in 24 hours as a guide for an uncovered indoor pool losing water to evaporation, and says the figure is significantly higher outdoors. For an outdoor pool it puts normal loss at 25 to 30 mm a week once evaporation, splash, backwashing and water carried out by bathers are added together. It reckons roughly 0.75 of a litre leaves with every person who climbs out.

SPATA's own test is not the American bucket trick. Cover the pool. Stop people swimming. Turn off the automatic top-up. Close the plant room valves and stop backwashing. Mark the level on the front of the skimmer, leave the pool static for up to a week, and mark it again. A covered, isolated pool that still drops has a leak, and you now know its daily rate in millimetres.

Two extra readings are worth taking while you wait. Repeat the measurement with the circulation pump running and again with it off. If the loss speeds up when the pump runs, the pressurised return side is the first place we test. And read your water meter at the start and end of that week, because an isolated pool with the top-up off should barely move it. If the meter is spinning anyway, something upstream of the pool is leaking too.

What the water is actually costing you

Thames Water charges metered households £2.7346 per cubic metre for clean water and £1.4721 per cubic metre for wastewater from 1 April 2026. A cubic metre is 1,000 litres. Top-up water is charged at both rates, because metered volume is assumed to return to the sewer whether it does or not. That is just over £4.20 for every cubic metre your pool loses.

Pool	Surface area	10 mm a day	Water and sewerage, 30 days	Over a year
8 m x 4 m	32 m²	320 litres a day	about £40	about £490
10 m x 5 m	50 m²	500 litres a day	about £63	about £770
12 m x 6 m	72 m²	720 litres a day	about £91	about £1,100

Heated water costs more than that. Lifting 1,000 litres from mains temperature to a pool at 28 degrees takes about 18.6 kWh of heat. At the 7.97p per kWh gas unit rate in Ofgem's price cap for 1 October to 31 December 2026, that is roughly £1.48 of gas before any boiler or heat exchanger losses, on top of the £4.21 of water and sewerage. Call it close to £6 for every cubic metre a heated pool loses.

One thing worth reclaiming while you sort the leak out. Thames Water assumes 90% of your metered water returns to its sewers. If you can show that more than 10% does not, and it lists keeping a swimming pool among the qualifying reasons, you can apply for a wastewater abatement. You need a meter and at least twelve months of readings. That takes the sewerage charge off evaporation you were never going to avoid. It is not the same thing as a leak allowance, and it does not fix anything.

The hosepipe ban turns a slow leak into a legal problem

A hosepipe ban is in force across the Thames Water area and, as of 1 September 2026, it remains in place. Filling or maintaining a domestic swimming or paddling pool with a hosepipe is one of the restricted uses listed in the Water Use (Temporary Bans) Order 2010. Thames Water's own exemptions allow one initial fill after construction or major refurbishment, and topping up from a hand-held container filled at a tap. Nothing else.

Contravening a temporary ban is an offence under section 76 of the Water Industry Act 1991, punishable on summary conviction by a fine up to level 3 on the standard scale, which is £1,000. So a pool losing 500 litres a day cannot lawfully be kept level with a hose this year, and carrying it in by bucket is not a plan. Repairing the leak is the only route back to a full pool.

Liner, concrete and one-piece shells fail in different places

Where we look depends on how the pool was built, because in each type of construction a different component is doing the waterproofing.

Liner pools. The liner is the waterproof layer. The blockwork or steel panel behind it is not. Punctures and seam failures happen, but the usual failure is at a clamped flange, where the liner is squeezed between a faceplate and a gasket at a skimmer, return, light or main drain. Two things give it away: a liner that has gone slack or wrinkled, and a soft or hollow patch underfoot where escaping water has washed out the sand or vermiculite base. Dye testing on a liner pool is done with the circulation off, because a running pump drags the dye away from the point you are trying to find.

Concrete and rendered shells. Reinforced concrete on its own is not watertight in most UK pool construction. The tile bed, the render or marbelite, and the puddle flanges around the fittings do that work. Leaks turn up at movement joints, at the bond beam, along grout lines, and very often in the conduit leaving a light niche. A genuine structural crack through the shell is rare, and when there is one it usually announces itself elsewhere as well, in the surround or on the plant room wall.

One-piece GRP and composite shells. The shell almost never fails. The leak is at the fittings or the pipe tails bonded to them, so the survey goes straight to the pipework.

Any verdict of structural failure reached before every fitting has been dye tested and every circuit pressure tested is a guess. Relining a pool that is leaking at a skimmer gasket is an expensive way of not fixing it.

How the survey runs

The sequence matters more than the kit.

1. Isolate the shell from the system. Plug the skimmers, returns and main drain, then measure the level again. If the loss stops, the structure is sound and the problem is in the circulation. If it carries on, the shell or a fitting is at fault.
2. Pressure test each circuit on its own. Every suction line, every return, the cleaner line, the plant room manifold, one at a time. A sound line is filled, pressurised and left alone, and the gauge does not move. A needle that creeps down identifies the leaking run, which narrows a whole garden to one pipe.
3. Dye test the fittings with the pump off. Skimmer throats, return inlets, light niches and their conduits, steps, main drain, and any grout line or joint that looks suspect. Dye drawn into a gap is the leak. Dye that hangs still is not.
4. Trace the buried run. Where a circuit has failed its pressure test, tracer gas and acoustic correlation place the escape point within a spade's width, so any excavation is one targeted hole rather than a trench across your terrace. The same methods we use on buried supply pipes work on pool lines.

Nobody should be digging before step four. Reinstating a lifted patio, deck or lawn is usually the biggest line on the final bill, and it is the one that is avoidable.

Ready to book? Call 020 7123 8560 with three things: the pool size, the daily drop in millimetres, and whether the loss changes when the pump runs. We will tell you how many hours we expect to need and confirm the rate before anyone travels.

Why we do not drain the pool to look

Draining is the quickest way to turn a small problem into a large one, and in London it carries more risk than most places. An empty shell in wet ground has nothing pushing back against the groundwater outside it. Concrete pools can lift. Liners float and wrinkle badly enough that they will not lie flat again. Basement pools sitting in London clay below the water table are the worst case of the three. Then there is the cost of refilling and reheating, which you carry while the pool is out of use and, while a temporary use ban is running, cannot lawfully do with a hose.

Nearly every pool leak can be found with the pool full. We drain only when a test genuinely requires it, and we say why first.

Who actually calls us about a pool

Indoor and basement pools. Most of these are in Kensington and Chelsea, Westminster, Camden and Hammersmith and Fulham, sitting in the same excavated box as the plant room, gym and cinema. The water bill is not the point here. What matters is where the water is going and what it is doing to the structure, which is why these jobs usually need moisture mapping of the surrounding slab and walls. It is worth saying plainly that in a pool hall, not every damp problem is the pool. Across our last 400 leak detection surveys, 3% turned out not to be a plumbing leak at all but roofs, gutters and structural defects letting water in. A humid pool hall under a cold roof will condense, and condensation gets blamed on the pool a great deal.

Garden pools. Richmond, Barnet, Bromley, Kingston and Havering, typically, where the pool sits twenty or thirty metres from the plant room and the pipe runs cross lawn, patio or driveway. These are pressure testing and pinpointing jobs, and the pinpointing is what saves the money.

Small commercial pools. Hotels, gyms, spas and schools, where the binding constraint is that the pool cannot close. These sites have balance tanks, which hide a loss well, and automatic top-up, which hides it completely. If you have an auto-fill, switch it off before you measure anything, or you will be measuring the valve rather than the pool. This work is normally scheduled outside opening hours.

What it costs, and what happens if we cannot find it

Standard leak detection is charged at a fixed hourly rate of £160. Most surveys run two to four hours. A pool with several buried circuits usually sits at the top of that band, so three to four hours, £480 to £640. If the job needs trace and access work, lifting a section of surround or opening a plant room wall to reach the fault, that is £200 an hour. Moisture mapping starts at £175 an hour and emergency attendance at £220. The rate is agreed before anyone travels and it does not rise on the day. The full rate card is published.

Behind the rate sits no find, no fee. If we attend a confirmed live leak and cannot locate it, the detection time is not charged. Across our last 400 leak detection surveys we identified the cause in 95%. In 2% the water was coming from a neighbouring property, and in 3% it was not a plumbing leak at all.

If pool water has already damaged the house, ask to see a sample report before you book rather than after. Escape of water claims turn on cause, origin, method and evidence, and an invoice reading "found leak, repaired" is not evidence.

Pool dropping faster than the weather explains? Call 020 7123 8560. We cover London out to the M25. You will get the hourly rate, the hours we expect to need and an arrival window before you commit to anything, and if we cannot locate a confirmed live leak, the detection time is not charged.