How do you repair a crack in a concrete or fibreglass pool?

A fine web of hairline cracks in the plaster, or in the gelcoat of a fibreglass pool, looks worrying but rarely lets water through. It sits in the finish, not in the structure. The same goes for shrinkage cracks: thin, often straight lines that formed while the plaster cured and have not changed in years.

A structural crack runs through the shell. You recognise it by its width, by a slight step where one side sits higher than the other, and by its location: often running diagonally down from a corner, around the skimmer or straight across the floor. A crack like that almost always comes with water loss.

If in doubt, draw two pencil marks across the crack and measure the distance between them. If it grows over the following weeks, the structure is still moving. Measure the water loss as well with the bucket test; our article on pool leaks explains how.

The ground here is the main cause. Many plots sit on clay or on filled ground on a slope. Clay shrinks in a dry summer and swells after the first autumn rain. A concrete tank weighing tens of tonnes moves with it, and concrete is strong in compression but weak in tension.

Then there is temperature. An empty pool in full sun gets far hotter on the sunny side than on the shaded side, and a pool left empty in winter gets night frost on damp plaster. Older pools also often have less reinforcement than is usual today.

A leak can also be the cause rather than the result. A leaking pipe washes away the ground under the floor, and only then does the shell crack. That is why we always have the pipework pressure-tested first by a company specialised in that work; otherwise you repair the crack and it comes back.

In a fibreglass pool the crack usually sits in the gelcoat, the smooth coloured outer skin. It cracks through ageing, an impact or stress around the steps and the edges. Osmosis blisters, small bubbles that open up, are the second common problem.

A spot repair of the gelcoat almost always stays visible and holds poorly, because new gelcoat bonds badly to weathered material. If the crack goes through the laminate, the laminate first has to be rebuilt from the inside with glass fibre and resin.

A polyurea coating can also be applied over a fibreglass pool, provided the surface is sanded, degreased and given a suitable primer. The result is a new, elastic skin over the whole pool in a colour of your choice, without patchy repairs.

The joint between a plastic skimmer and a concrete wall is the most common leak point in a pool. Two materials that expand differently, a rigid mortar between them, and movement every summer and winter. A cracked skimmer, or a crack in the mortar around it, often lets through more water than the owner expects.

The same logic applies to underwater lights, return inlets and the main drain. During a renovation such a connection is taken apart, a cracked skimmer replaced, set again in repair mortar and the transition sealed elastically.

The sprayed coating then runs right onto the flange of the fitting. The transition between concrete and plastic is no longer a joint but a continuous membrane.

When we treat the whole shell, the work follows fixed steps. Skipping them is exactly why so many earlier repairs failed.

A quick repair with sealant, cement or a two-part filler is rigid, or bonds poorly to the old, damp plaster around it. At the first movement of the shell the bond breaks and the pool leaks again, often exactly along the edge of the repair.

Then there is the water pressure. At two metres deep the water pushes hard into every opening. A filler that only sits at the surface gets pushed out over the course of a season.

An emergency repair under water with epoxy putty is useful as a stopgap: it limits the loss until the real renovation. Just never treat it as the solution.

If there is one crack, it has stopped moving and the rest of the pool is in good condition, a targeted repair is enough. If there are several cracks, the plaster is weathered, the pool loses water in more than one place or the ground keeps working, spot repairs are money down the drain.

In that case we treat the whole shell. The polyurea membrane stretches to several times its own length before it tears, so it bridges movement that plaster and tiles cannot cope with. One closed skin, with no joints for water to get through.

What determines the price is not the crack itself but the surface area of the pool, the condition of the plaster, the number of pipe entries and whether a levelling layer is needed first. The quote sets that out item by item.

A coating makes a pool watertight, but it does not stop a structure that is moving. If the crack has a step of more than a few millimetres, keeps growing measurably, the edge or the surrounding terrace is sinking, or the pool sits on a slope that is visibly sliding, the ground is the problem.

In those cases we advise bringing in a structural engineer or a specialist foundation-repair company first. A new finish only makes sense once the structure has stopped moving. We would rather say that up front than have you pay twice.

A targeted crack repair, without coating the whole shell, takes a few days. A complete renovation of the shell takes five to ten working days on average, of which the spraying itself is only one day.

The rest is preparation: removal, repair, levelling, drying and priming. The pool has to be dry enough before the machine starts, and that cannot be forced. After the top layer is applied you wait seven days before filling, so that it cures fully.