Every fall, pool owners stare at a fiberglass pool and wonder whether they’re supposed to drain the thing halfway down or just leave it alone. It’s one of those questions that sounds simple until you realize the wrong answer could crack your shell or float it right out of the ground.
For a fiberglass pool, leave the water level 1 to 6 inches below the bottom of the skimmer opening – that’s typically 12 to 18 inches below the pool deck, depending on your skimmer height. Do not drain below that. This article explains why that range matters, how to hit it correctly, what to do in deep-freeze climates, and the mistakes that send people to the repair shop every spring.
Why Fiberglass Pools Are Different From Vinyl and Concrete
Fiberglass shells are not attached to the ground. They sit in a bed of sand or gravel, and the only thing keeping them from floating upward is the weight of the water inside them. Groundwater is always present below the frost line, and when a fiberglass shell is empty or nearly empty, the hydrostatic pressure underneath it can push the entire pool upward – sometimes several inches. That’s called “floating,” and it causes cracks at the steps, tears at the plumbing connections, and permanent deformation of the shell.
Vinyl liner pools and concrete pools behave very differently: a vinyl pool can be drained more safely for liner work, and a concrete pool is rigid enough to resist most hydrostatic lift. Fiberglass needs water inside it all winter, full stop. River Pools and Spas lays out this principle clearly: draining a fiberglass pool below a safe level is one of the most common – and most expensive – winterization mistakes.
The Right Water Level: 1 to 6 Inches Below the Skimmer
The target is simple: the water surface should sit between 1 and 6 inches below the bottom edge of the skimmer opening. That range exists to protect the skimmer without removing so much water that you compromise the shell’s ballast.
- 1 inch below the skimmer (minimum): Enough to keep water out of the skimmer throat, which prevents a freeze-crack if you’re in a mild climate with only occasional light frosts. This is the floor – don’t leave water touching the skimmer bottom.
- 6 inches below the skimmer (maximum for most pools): The appropriate level for climates with sustained hard freezes, below 20 degrees Fahrenheit. It gives ice room to expand on the water surface without pressing into any fittings.
- Lower than 6 inches: not recommended unless a pool professional has assessed your specific groundwater situation and given you the green light in writing.
Here’s a concrete example. Say your skimmer opening starts 14 inches below the deck. Your water level needs to drop to between 15 and 20 inches below the deck – so you’re draining off roughly 1 to 6 inches of water relative to the skimmer, not relative to the deck or the pool floor. Measure from the skimmer, not the coping.
What Happens if You Leave Too Much Water In
If the water surface sits at or above the skimmer throat when a hard freeze hits, the water inside the skimmer body will freeze and expand. Ice expands by about 9 percent in volume. Even a partially full skimmer basket cavity holds enough water to crack the plastic skimmer housing or shatter the surrounding fiberglass collar. Skimmer replacement on a fiberglass pool runs $300 to $800 for parts and labor – an avoidable expense.
Return fittings face a similar risk. The return jets themselves are below the waterline, so the water around them will freeze, but as long as the lines are blown out and plugged, the fittings themselves are protected. The skimmer is the real vulnerability because it’s closer to the surface where freezing happens fastest.
Step-by-Step: Setting the Correct Level Before Closing
- Balance your chemistry first. Adjust pH to 7.2 to 7.6, alkalinity to 80 to 120 ppm, and calcium hardness to 175 to 225 ppm. Add a closing algaecide dose and a full winterizing shock. Do this 24 to 48 hours before you drop the water level. AquaDoc makes a pool closing kit that covers the shock and algaecide in one package, which some pool owners use to keep the checklist short.
- Turn off the pump and filter. Do not run the pump once you start draining – air can damage the seal.
- Use a submersible pump or partial backwash to lower the level. Drain slowly so you can stop at the right point. Remove 1 to 6 inches worth of water relative to the bottom of the skimmer opening.
- Blow out the skimmer line and return lines with a compressor. Push air through until no water comes out the returns, then plug immediately with winterizing plugs. For a detailed walkthrough on winterizing a fiberglass pool step by step, including plugging return lines, A-1 Pools and Spas has a solid breakdown.
- Install a skimmer plug or Gizzmo in the skimmer throat to absorb any ice expansion inside the skimmer body. This is a $5 to $15 piece of insurance.
- Install your winter cover. A solid safety cover anchored to deck hardware is the right choice for fiberglass – it keeps debris out and reduces evaporation, which means less makeup water needed in spring. While you’re at it, keep in mind that what accumulates on a winter pool cover over several months is worth planning for when you think about reopening.
Freeze-Climate Considerations: Colder Than 20 Degrees Fahrenheit
In climates where temperatures drop below 20 degrees Fahrenheit and stay there for days at a time – think the Midwest, New England, or mountain West – push toward the 6-inch end of the drain range, not the 1-inch end. You also want to make sure every line is blown and plugged, not just the skimmer. Mock Property Services outlines the full fiberglass pool closing process with freeze-climate specifics, including how to handle pool light niches and main drains.
One common mistake in cold climates: relying on a skimmer plug alone without blowing the lines. The plug keeps ice from cracking the skimmer body, but if the skimmer line itself still has standing water in it, that pipe can freeze and split underground. Blowing the lines is not optional in freeze country – it’s what prevents the $800 to $2,500 repair that comes from a cracked underground pipe.
Common Mistakes That Lead to Spring Repairs
- Draining to the shallow end floor: Some homeowners assume “more drained is safer.” For fiberglass, the opposite is true. A pool drained below the skimmer level has lost significant ballast weight and is vulnerable to lifting, especially after a wet winter with high groundwater.
- Skipping the water level check in early spring: Evaporation and cover leaks can drop the water level further over winter. If the level falls far below the original drain point, refill before reopening. Check the level once a month if you can.
- Leaving the pump running during the drain: Running a pump with a dropping water level pulls air through the skimmer, which can cavitate and burn out the seal plate. Shut the pump off before draining begins.
- Not treating the water before closing: Winter water that goes in unbalanced comes out green. Properly balanced water with a closing algaecide holds much better over four to six months. If you’re curious about how algae establishes itself and what it looks like when chemistry goes sideways during a closure, this breakdown of the mysterious green tint explains the chemistry behind it.
How to Verify You’re at the Right Level
Don’t guess by eyeballing the skimmer from across the deck. Crouch down and look at the skimmer opening straight on. The bottom edge of that rectangular opening is your reference point. The water surface should be visibly below it – not touching it, not millimeters from it. If you’re aiming for 3 inches below the skimmer bottom, use a tape measure and mark it. Albert Group Landscaping’s nine-step winterizing guide includes a similar reference-point approach that makes it easy to confirm your level before covering.
After setting the level, take a photo on your phone with the skimmer visible in frame. That photo becomes your baseline if you check back in February and wonder whether the level has dropped further than expected.
Frequently Asked Questions
How much water should I leave in a fiberglass pool for winter?
Leave the water level 1 to 6 inches below the bottom of the skimmer opening – typically 12 to 18 inches below the pool deck. This protects the skimmer from ice damage while keeping enough water in the shell to counterbalance groundwater pressure from below. Closer to 1 inch for mild climates, closer to 6 inches if you get sustained hard freezes.
Can I drain a fiberglass pool completely for winter?
No. Draining a fiberglass pool completely risks the shell floating, bowing, or cracking. Fiberglass shells rely on water weight to resist upward groundwater pressure, and an empty shell can shift within hours of being drained. Even a short-duration full drain during high groundwater conditions has caused permanent damage to fiberglass pools.
What happens if I leave too much water in a fiberglass pool and it freezes?
If water sits at or above the skimmer opening and freezes, the expanding ice can crack the skimmer body or the surrounding fiberglass collar. Ice expands roughly 9 percent by volume, and even a small amount of water trapped in a skimmer cavity generates enough pressure to split the housing. Drain to 1 to 6 inches below the bottom of the skimmer to give ice room to expand safely away from the fittings.
How low should I drain a fiberglass pool in a climate with hard freezes?
In climates with sustained temperatures below 20 degrees Fahrenheit, drain to 6 inches below the bottom of the skimmer opening. Also blow out and plug all return lines and the skimmer throat with an expansion plug or Gizzmo – the drain level alone is not enough protection when temperatures stay below freezing for multiple days at a time.
Do I need to drain a fiberglass pool below the return jets for winter?
No. The water level itself only needs to drop 1 to 6 inches below the skimmer – not below the return jets, which typically sit 12 to 24 inches below the waterline. What matters for the returns is blowing out the lines with a compressor and sealing them with winterizing plugs so no standing water remains inside the pipes where it can freeze and crack.
The bottom line: keep your fiberglass pool at 1 to 6 inches below the skimmer opening all winter, blow out and plug every line, and never drain the shell below that range. That one rule prevents the majority of fiberglass pool winter damage.