Winterizing a Fiberglass Pool: Do It Right or Pay in Spring

To winterize a fiberglass pool: balance the water, lower it just below the skimmer (never more), blow out and plug the plumbing lines, protect the equipment, and cover the pool. The cardinal rule is that a fiberglass pool winters full, draining it for the season is how shells shift, bulge, and float.

Rule One: The Water Stays In

Some owners assume winterizing means emptying the pool so nothing can freeze. For fiberglass, that instinct is exactly backwards. The water in your pool is structural ballast, it holds the shell down against groundwater and braces the walls against soil pressure all winter, precisely the season when the ground is wettest. An empty fiberglass shell sitting through four months of winter rain is a leak, a bulged wall, or a floated pool waiting to happen. If anyone suggests draining your fiberglass pool for winter, read why draining is the most dangerous thing you can do, then get a second opinion.

And no, the full pool won't be hurt by surface ice. A sheet of ice on top of a full pool floats and expands upward and outward. What freeze actually breaks is water trapped in the plumbing and around fittings, which is what the rest of the closing procedure is for.

The Fiberglass Closing Procedure, Step by Step

• 1. Balance the water first. Close with pH 7.2-7.6, alkalinity 80-120 ppm, and sanitizer in range. The water will sit against your gel coat for months; closing with acidic or scale-forming water means opening to a duller surface. Full targets are in our chemistry guide.
• 2. Clean the pool and the waterline. Leaves left to steep all winter leave tannin stains; a scum ring left all winter bakes in. Ten minutes now saves a stain-removal project in April.
• 3. Add winterizing chemicals. A winter algaecide and a final dose of sanitizer keep the covered water clean through the off-season. Pre-dissolve anything granular, never let concentrated chemicals rest on gel coat.
• 4. Lower the water just below the skimmer mouth. A few inches below the skimmer opening, enough to keep freezing water out of the skimmer throat, no more. This is the only "draining" a fiberglass closing should involve. (Skimmer covers and plates that let you keep the water at normal level are also common; follow your equipment's approach.)
• 5. Blow out the lines and plug them. Water left in suction and return lines is what actually freezes and cracks. Blow each line with a compressor or high-volume blower until it runs dry, then seal returns with threaded winter plugs and the skimmer with a Gizzmo-style absorber that takes the expansion if any water sneaks back in. Swim-friendly pool antifreeze in lines that can't be fully cleared adds insurance. If you're not confident here, this is the step to hire out, it's where the expensive mistakes live.
• 6. Drain the equipment. Pump, filter, heater, and salt cell all have drain plugs for a reason. Store what can be stored indoors.
• 7. Cover it. A properly anchored safety cover is best: it keeps out debris and sunlight, holds up under snow load, and protects kids and pets. Make sure the cover doesn't bear on the coping in a way that grinds against the shell edge as it loads and unloads.

The Freeze-Thaw Damage We See Every Spring

Fiberglass handles cold well, flexible shells tolerate ground movement far better than rigid concrete. But every spring, our route crews across the colder half of our territory open the season with the same repair list:

• Cracked returns, skimmer throats, and fitting rings, lines that weren't fully blown out, or plugs that weren't sealed. Water froze, expanded, and split the plastic or the laminate around it.
• Spider cracks around steps, corners, and fittings, stress concentration points that flexed through freeze-thaw cycles, especially where the shell had voids in the backfill behind it or the water was left too low. See spider cracks in fiberglass pools for how we evaluate them.
• Waterline damage under ice sheets, pools closed with the water too high and no ice compensation, letting sheet ice work against the tile line or skimmer face all winter.
• Heaved or settled plumbing, saturated backfill freezing and jacking pipes, which shows up in spring as a pool that mysteriously loses water.

Caught in spring, most of this is a modest repair. Ignored for a season or two, water follows the cracks into the laminate and the fix grows into blister repair or resurfacing. If your pool opened this year with new cracks, don't wait: photos to our Free Pool Checkup get you a specialist diagnosis within one business day.

Winterizing Across 16 States: Climate Matters

Our routes run from the Gulf Coast to the Midwest, and "winterizing" means something different at each end. Broad strokes:

One warning for the warm half of the map: a single hard freeze in a normally mild state does more damage than a whole winter up north, because nothing was protected. If a rare freeze is forecast, run the pump continuously, moving water resists freezing, and insulate exposed equipment. Find your state's page under our service area for local specifics.