In this article
When a pool owner sees a crack in a tile or wall, the immediate worry is whether it explains the falling water level and whether it is dangerous. The answer depends on the crack's type, depth, direction and location. Some cracks are hairlines in the finish that do not touch the waterproofing. Others run through the concrete and need structural repair.
This article helps you understand the cracks you see: where they come from, when they leak, how to tell surface from structural cracks, how to treat each type so it does not return, and how to prevent new ones.
Why cracks appear in a pool shell
- Drying shrinkage: concrete and render shrink as they dry. Without proper curing, fine hairline cracks appear, often in a map pattern.
- Thermal movement: sun on the coping and upper walls followed by cool nights or water causes daily movement that shows first at joints and edges.
- Soil settlement or movement: when one part of the ground moves more than another, stress opens longer, deeper cracks, often diagonal or running across the floor.
- Insufficient or rusting rebar: steel carries tension in concrete. If it is lacking or corroded by water, the area weakens and cracks.
- Construction joints: where a pour stopped and restarted is a natural weak line if not treated.
- Sudden draining and refilling: an empty pool baking in the sun and then filled with cool water suffers thermal shock.
Where cracks usually appear and what each location means
- Below the coping and at the waterline: usually thermal movement between hot stone and cool water. Often superficial, but they let water behind the tiles.
- Wall-to-floor corners: a natural stress zone, possibly showing different movement of wall and floor or an untreated construction joint.
- Diagonal from a step corner or fitting: sharp corners and openings concentrate stress, so these may be structural.
- A long line across the floor: often soil settlement beneath the pool, needing prompt assessment.
- Around the main drain: pipe movement or a weak junction between the drain body and concrete.
- In the deep end only: pressure is highest there, so weaknesses show sooner.
How cracks relate to leaks
A crack leaks when it passes through the waterproofing. If it is only in the tile or render and the membrane below is sound, it may not leak, though it lets water reach the membrane and stress it over time. If it passes through membrane and concrete, it becomes a direct path to the soil.
The reverse also holds: there can be a leak with no visible crack, because the crack is under the tiles or in a buried pipe. Do not assume the crack you see is the leak until testing confirms it.
Surface vs structural cracks
| Feature | Surface crack | Structural crack |
|---|---|---|
| Shape | Fine hairlines, often map-like or short | Continuous, diagonal or branching line |
| Depth | In finish or render | Through concrete, may reach rebar |
| Change over time | Roughly stable | Lengthens or widens |
| Leak link | Rarely leaks directly | Usually leaks, more as it widens |
| Other signs | Usually none | Raised tiles, sinking surroundings, rust stains |
Photograph the crack beside a ruler and repeat later. A crack that grows needs prompt engineering assessment.
Treating cracks to stop leaks
Surface cracks
- Clean the crack and remove loose material.
- Fill with a flexible repair product suitable for pool water.
- Regrout locally if the crack is in the joints.
- Check again later to confirm it has not returned.
Structural cracks
- Find the cause: soil movement, rusted rebar or a construction joint. Repairing the crack without the cause means it returns.
- Drain to reach and dry the crack.
- Open the crack into a channel to anchor the repair material.
- Repair by injection or structural filler, treating any rusted steel found.
- Re-waterproof with flexible coats extending well beyond the crack, with reinforcing tape.
- Fill test before finishing.
- Finish with tiles and grout.
If the cause is ongoing soil movement, a site assessment and soil treatment or local strengthening may be needed first.
Preventing new cracks
- Avoid unnecessary draining, and never leave an empty pool in the sun for long.
- Fix leaks promptly, since leaking water changes soil behaviour and reaches the steel.
- Keep water balanced, pH roughly 7.2 to 7.6, to protect grout and render.
- Manage surroundings: direct rain and irrigation water away from the pool and avoid deep-rooted trees nearby.
- Monitor existing cracks with dated photos. Change is the early warning.
When to act immediately
Act without delay if a crack is widening, bleeding rust, accompanied by heavy water loss, sinking paving or lifting tiles beside it. These show the problem is no longer superficial. To learn which cracks require draining, read structural pool cracks and when they require draining.
YAM Pools inspects each crack to identify its type and cause before proposing treatment, provides a written quotation, and carries out repairs under engineering supervision with a written warranty on workmanship and waterproofing. We serve Riyadh and all cities of Saudi Arabia.
Frequently asked questions
Are hairline cracks in tiles dangerous?
Usually not, if they are stable and there is no water loss. Photograph and monitor them, and treat them locally so water does not reach the membrane.
Can a crack be repaired with the pool full?
Small surface cracks can be treated with underwater products. Structural cracks usually need draining to dry, repair and re-waterproof them.
Why did the crack come back after repair?
Because the cause, such as soil movement or rusting steel, was not treated, or the repair material was not flexible. A lasting fix starts with the cause.
How do I know a crack is growing?
Photograph it beside a ruler or mark its ends, then repeat after a few weeks. Any increase in length or width calls for assessment.
