Fiberglass pool problems and how a good install avoids them

Not often, but they are not indestructible either. A fiberglass shell is a single factory-made piece with a smooth gelcoat, so it avoids the liner tears of a vinyl pool and the resurfacing of a plaster one. The problems that do come up tend to share a cause. River Pools, a fiberglass manufacturer, says most fiberglass issues are tied to how the pool is built and how it is installed. Thursday Pools points to installation, soil movement, improper draining and water chemistry.

That is good news for a buyer, because installation and water care are things you can control. Here are the problems worth knowing about, what causes each one, and what prevents it on Northeast Ohio ground.

Usually the backfill. River Pools explains that when sand is used as backfill and becomes saturated, it can behave almost like a liquid and push on the pool wall. In some conditions that pressure exceeds the push of the water inside, and the wall bows inward. River says this matters most where water tables are high or soils hold moisture.

That describes much of Northeast Ohio. The USDA lists Mahoning silt loam, one of the most widespread soils on the region's till plains, as holding a perched water table 6 to 12 inches below the surface from October through June in most years. Saturated backfill is a real risk here, not a hypothetical.

The prevention is stone. Gravel drains and does not change its behavior when wet. Latham, which makes the Liberty shells we install, describes backfilling in lifts: six inches of gravel around the shell, then six inches of water inside, alternating so the pressure on both sides of the wall stays equal until the pool is full and the backfill reaches grade.

It can, almost always when it is drained or partly drained while the ground around it is saturated. A full pool stays put because the weight of the water balances the groundwater pushing up from below. Take the water out and that balance is gone.

Two things prevent it. During the build, Latham describes a sump line or daylight drain at the deep end to help manage hydrostatic pressure, which gives groundwater a way out and gives a service tech a way to check the water level under the pool. For the life of the pool, the rule is simple: keep it full. Liberty's warranty says the pool is designed to remain full of water at all times, and letting the level fall below the skimmer voids the coverage. Thursday Pools likewise advises never draining a fiberglass pool without professional help.

In Ohio that includes winter. The pool is closed with water in it, the lines are blown out and the equipment is winterized.

Spider cracks are fine hairline cracks in the gelcoat surface. River Pools describes them as typically cosmetic rather than structural, often hard to see, and caused when pressure on the shell exceeds what the gelcoat can flex. It lists shipping damage, manufacturing mistakes and improper installation as the usual sources.

On the installation side, the prevention is a flat, properly prepared base. Latham describes a gravel base under the shell and a set that meets the level requirements of the industry standard, noting that it may take several attempts to prepare the hole and set the pool level, and that this is normal. A shell sitting on a high spot or a void is a shell under stress. Gelcoat repairs are possible if cracks do appear, and your installer can arrange them.

Leaks in a fiberglass pool are more often at fittings and pipes than in the shell. River Pools says the cause is usually the material around the plumbing, not the plumbing itself: sand backfill that settles over time can pull pipes down and stress the connections.

Good practice covers it from both sides. Stable gravel backfill keeps the pipes supported. And the plumbing gets pressure tested while it is still exposed. Latham's sequence has the builder connect PVC to the fittings as the backfill and water reach them and pressure test the lines for leaks before burying them. A leak found then costs minutes to fix. A leak found under a finished deck costs far more.

Mostly water chemistry. Thursday Pools says a fiberglass shell turning chalky white is often the result of a chemistry imbalance, and that high calcium combined with high pH can leave white deposits and permanent damage to the finish. It also warns that chlorine above 5 ppm can damage the surface.

Manufacturers do not all publish the same targets. Thursday recommends calcium below 120 ppm for its pools, while Liberty's 2025 warranty lists calcium hardness of 250 to 350 along with chlorine of 1 to 3 ppm, pH of 7.2 to 7.6 and total alkalinity of 80 to 120. Follow the owner's manual for your specific pool. For a Liberty pool the warranty asks for weekly testing by a pool professional, with the results kept on file.

Fading and discoloration are not covered by Liberty's warranty, so good water care is the protection.

Small bubbles in the gelcoat that form when water works its way into the laminate behind it. Leisure Pools explains that water absorbed into polyester laminate can get trapped between layers and build pressure. Several manufacturers use vinyl ester resin behind the gelcoat, or throughout the laminate, to resist it. This one is a manufacturing issue more than an installation issue, and Liberty's written warranty covers osmotic blistering of the finish for 20 years.

Gelcoat is tough but not armor. Thursday Pools lists dragging metal, glass or other sharp objects across the surface, and cleaning with wire brushes or metal scrapers, as common causes of scratches and gouges. Use nylon brushes and soft cleaning tools made for fiberglass, keep glass out of the pool area and be careful with metal-framed furniture near the steps. Shallow scratches can often be buffed out. Deeper damage needs a gelcoat repair.

Water again, this time from above. Rain that runs toward the pool soaks the ground beside it, softens the base under the deck and feeds the groundwater around the shell. Liberty's warranty excludes damage from runoff and from failing to drain rainwater away from the pool and deck, and it voids coverage if groundwater is allowed to build up around the pool.

The cure is grading and a well-built deck. The finished yard should slope away from the pool, downspouts should discharge away from it, and the concrete deck needs a compacted base suited to our freeze-thaw winters. The deck also covers the top edge of the shell. Liberty expects that edge, which it calls the beam, to sit under a cantilever deck, pavers or other coping.

Every pool loses some water to evaporation, more on hot, windy, dry days and more still with the heater running and no cover. So a falling level is not proof of a leak. The classic check is a bucket test: set a bucket of pool water on the top step, mark the water level inside the bucket and on the pool wall, and compare them after a day with the pump running normally. If the pool drops noticeably more than the bucket, water is going somewhere besides the air.

Do not solve it by letting the level fall. On a fiberglass pool, low water is its own risk, and the Liberty warranty treats water below the skimmer as grounds to void coverage. Top the pool up and call for a leak check. Most leaks show up at fittings, lights or buried lines and are repairable without touching the shell. We handle repairs and inspections as part of our pool service.

• What goes under the shell, and how do you level it?
• What do you backfill with, and do you fill the pool with water as you backfill?
• How do you relieve groundwater, and will there be a sump or drain I can check later?
• When do you pressure test the plumbing?
• How will the finished grade carry water away from the pool and the house?

A builder who works in Northeast Ohio clay should answer all five without hesitating. We plan the base, backfill and drainage for every pool at the site visit, before we dig.