Crazing is a finish defect. A structural crack is movement. They look similar from across the pool and require completely different responses. Crack direction tells you what moved and why.
Learn the H.U.N.T.E.R. Method →Crazing is shallow cracking in the plaster or finish layer -- it forms a random network pattern, stays in the surface coat, shows no displacement, and usually does not leak. A structural gunite fracture has long, distinct lines caused by shell movement. Structural cracks can travel in any direction, but their direction gives evidence of what type of movement caused them. Movement is what causes all structural cracks in both gunite and fiberglass pools. A visible crack is a suspect -- only confirmed dye movement or matching water-level behavior makes it a confirmed active leak. Cracks at the pool bottom may indicate hydrostatic uplift. A pool that is no longer level is a serious structural sign.
Every structural crack in a gunite or fiberglass pool was caused by movement. The pool shell did not fail on its own -- something moved, and the shell responded. Identifying the type and direction of movement is what separates a meaningful diagnosis from simply calling something a crack.
Crack direction, location, and pattern all carry information about what moved and why. A technician who understands structural crack reading can make better judgments about whether repair is needed, what type of repair, and whether the underlying cause has been addressed.
In a pool with a shallow end and deep end, a vertical crack most commonly appears in the middle section trending toward the shallow end -- though this is not universal. This pattern is often associated with differential settlement where one section of the pool is moving at a different rate than another.
A vertical crack can also indicate soil movement on one side of the pool wall, particularly in pools near landscaping, slopes, or areas subject to soil erosion or root systems.
A U-shaped crack on the pool wall indicates wall movement. The direction of movement determines the cause. If the pool is always kept full and the crack is on the outward-facing side, the likely cause is soil erosion undermining the support on the outside of the wall -- the wall is moving outward because the soil support is gone. If the pool has been emptied and the crack is on the inward-facing side, the cause may be soil pushing inward -- the water load that normally resists the surrounding soil pressure was removed, and the soil moved the wall.
This is why draining a gunite pool without understanding the soil and water table conditions is a risk -- the water inside the pool provides structural resistance to the soil pressure outside.
A horizontal crack across the pool wall often indicates lateral pressure from outside the pool -- soil pressure, hydrostatic groundwater pressure, or both. These cracks deserve careful attention because horizontal wall cracks can indicate the wall is being pushed inward at that elevation.
A crack at or near the bottom of the pool floor -- particularly one that runs across the floor or radiates from the main drain area -- may indicate the pool is experiencing hydrostatic uplift, commonly called "pool pop." This occurs when groundwater pressure from below the pool floor exceeds the weight of the pool shell and water, causing the shell to lift or crack at the floor.
A pool that is no longer level -- where the waterline is visibly higher on one side than the other, or where the coping, tile line, or deck no longer runs parallel to the water surface -- is a serious structural sign that the shell itself has moved. This is beyond standard leak detection scope and requires a structural assessment.
Do not recommend simply patching a bottom crack or floor fracture without considering whether uplift is the cause. If the shell has moved, patching the surface does not address the underlying structural condition.
Fiberglass pool cracks are almost always caused by the loss of soil support beneath the shell, not by a failure of the fiberglass material itself. When soil erodes from beneath a portion of the shell, that section of the shell loses its support. The unsupported section flexes under water load and eventually cracks along the stress point.
This means that repairing the fiberglass surface without addressing the void or erosion beneath it will result in the crack recurring. The underlying soil condition must be addressed as part of any structural repair plan.
A visible crack is a suspect. Two pieces of evidence are needed to confirm it as an active leak source:
Both tests should be performed before any repair is recommended. A crack that neither pulls dye nor matches the water-level behavior is cosmetic until evidence shows otherwise.
Ask whether the pool loses water at a specific level, whether the pool has ever been emptied, and whether the coping or tile line appears level around the perimeter.
Know the difference between finish-layer crazing and structural movement cracks. Know what crack patterns indicate about the type of movement that caused them.
Use water-level behavior to identify the target elevation. Map visible cracks against that elevation. Identify whether the pool is level before focusing on individual cracks.
Dye test under still-water conditions at the confirmed water level with the pool full. Document dye movement or absence of movement. Note debris masking as a limitation.
Confirm active dye movement before calling the crack an active leak. Document displacement, staining, rust, and direction as structural evidence.
State clearly what the crack is, what the evidence supports, and the limits of the inspection. Flag structural signs -- unlevel pool, floor cracks -- that may require assessment beyond leak detection scope.
Jeff David is the founder of Leak Business Academy, LLC and Leak and Subsurface Locators, LLC. He is a licensed pool contractor in Florida and an FSPA Palm Beach board member. Jeff teaches crack pattern reading as a structural diagnostic skill -- including why crazing does not spider out like true structural cracks, why crack direction reveals the type of movement that caused it, and why a pool that is no longer level is a more serious finding than any individual crack.
LBA training covers crack pattern reading, dye testing under still-water conditions, water-level behavior analysis, and the structural signs that require disclosure -- including pool pop indicators and fiberglass support loss.
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