Emptying a Fibreglass Pool: Risks, Limits & Refilling
A fibreglass pool is held down by the water in it. What emptying does to the shell, how far the level can safely drop, and who should do it.
Written by Tehbyn Nova, Head of Operations & Growth
Published
Can you empty a fibreglass pool?
A fibreglass pool can be emptied, but only as a controlled operation that manages the groundwater outside the shell, because the water inside the pool is what holds the shell in the ground. Draining a fibreglass pool is not a maintenance task. A fibreglass pool shell is light for its size and sits in a hole in the ground, and the weight of its own water counterbalances the soil and the groundwater pressing on it from outside. Take the water out and the shell still carries the soil load and the groundwater pressure, with nothing inside pushing back. An emptied fibreglass pool shell can shift, lift or pop out of the ground. Draining any inground pool carries risk. A fibreglass pool depends on the water inside it more heavily than a concrete pool does, because that water holds its shape as well as its position. Emptying a fibreglass pool is a job for the pool builder, planned from the groundwater outside the shell inward.
Our pool catalogue publishes a dry shell weight and a water volume for 34 fibreglass pool models. Those shells weigh between 341 kg and 1,286 kg, and they hold between 3,800 L and 52,780 L of water. One litre of water weighs one kilogram, so the water in a fibreglass pool outweighs the shell containing it by 11 to 43 times, with a median of 30 times.
| Pool model | Size | Shell dry weight | Water it holds | Water vs shell |
|---|---|---|---|---|
| Allure | 5.0 m × 2.5 m | 477 kg | 12,800 L (12.8 tonnes) | 27× |
| Bedarra | 7.5 m × 3.8 m | 838 kg | 30,500 L (30.5 tonnes) | 36× |
| Westminster | 9.0 m × 4.0 m | 988 kg | 39,490 L (39.5 tonnes) | 40× |
| Kensington | 11.0 m × 4.0 m | 1,223 kg | 51,700 L (51.7 tonnes) | 42× |
Emptying a Kensington takes 51.7 tonnes of water off a 1,223 kg shell. The groundwater under the floor then presses up against 1,223 kg of fibreglass, which is why the water outside the shell is pumped down before the water inside comes out.
What happens if you empty a fibreglass pool, and will it pop or crack?
Groundwater pressure is the primary risk to an emptied fibreglass pool: with no water inside to resist it, the shell can shift, lift or pop out of the ground, and the fibreglass can crack. Groundwater presses up under the floor of the shell and in against its walls, and that pressure is hydrostatic pressure. While the pool is full, the water inside balances it. With the water out, the shell carries the hydrostatic pressure on its own.
Ground pressure can damage an emptied shell in four ways. Ground pressure can bow the walls inward. Ground pressure can distort the shell out of shape, and fibreglass pool wall bulging is the form that damage takes on a wall. The same stress can open hairline cracks in the fibreglass that widen over time. Pressure against the side walls can also push the floor up.
The risk is a site condition, not a pool condition. Two things about the property decide it: how high the water table sits under the pool, and how saturated the ground around the shell is on the day. Where the groundwater outside stands higher than the water left inside, the pressure is unbalanced and the shell is the only thing resisting it.
A hydrostatic valve is a relief valve set into the floor of the pool. While the pool is full, the valve stays shut. When the level inside falls far enough that groundwater under the floor presses up harder than the water inside presses down, the valve opens and lets groundwater into the shell, which relieves the pressure underneath. A hydrostatic valve does not remove the groundwater and it does not hold the shell down. Professionals treat a hydrostatic valve as one component of a controlled emptying, alongside sump pumps and monitoring of the water standing beside the pool. A hydrostatic valve is not a clearance to let the water out.
How far can you drain a fibreglass pool safely?
No safe drain depth exists as a published number. No pool manufacturer states one for a fibreglass shell and no industry source states one either, because the stop point is not a property of the pool. The stop point is set by the site, and specifically by how high the groundwater stands beside the shell on the day the water comes out.
Depth alone shows why a rule of thumb cannot hold. Across the fibreglass pools in our range, the shallow end runs from 0.90 m to 1.37 m and the deep end from 1.47 m to 2.00 m. A drop of "halfway down" therefore takes a different volume out of every model, and leaves a different weight of water behind.
Partial draining is the safer and more practical option for a fibreglass pool owner. The drop most maintenance work actually needs is to the skimmer or to the returns, well short of the floor, which leaves most of the counterweight in the pool. A partial drop reduces the hydrostatic risk while still allowing water-quality problems and repairs to be dealt with, and full draining is reserved for repairing or refinishing the interior. What the work needs and what the site tolerates are two separate questions: on a saturated block with a high water table, even a partial drop is the pool builder's call.
How do professionals drain a fibreglass pool?
A professional emptying is planned around the groundwater outside the shell, not around the water inside it. On our installs, the shell is craned into the hole, bedded on gravel, and backfilled with gravel while the pool is filling with water. Each wall ends up held by water on one side and gravel on the other. Emptying takes away one of those two supports, and recovering a shell that has lifted out of position is a crane job again.
Three things define a controlled emptying. The groundwater beside the shell is monitored and pumped down before and during the drop, so the pressure under the floor falls as the water level inside falls. The shell is watched while the level comes down, so any movement is seen while there is still water in the pool. The discharge point is settled before anything starts, because the rule for where pool water may go is set by the local council and is confirmed before the job begins.
When does a fibreglass pool have to be braced before it is emptied?
Bracing a fibreglass pool before it is emptied is triggered by an engineering judgement made on the site, from the water table under that property and the ground condition on the day. No Australian source publishes the water level at which bracing becomes necessary. The bracing is then specified for the pool it is going into. A bracing method copied from another pool, another country or another water table is not a specification.
Bracing and dewatering are two different jobs, and an emptied fibreglass pool needs both. Bracing props the pool walls from the inside so that soil pressure cannot push them inward. Dewatering pumps the groundwater down beside and below the shell so that it is not pressing up under the floor. Propping the walls does nothing about the water under the floor, and pumping the ground down does nothing about the soil against the walls.
How long can a fibreglass pool stay empty?
No safe period for an empty fibreglass pool has been published anywhere: not in hours, not in days, and not separately for a braced and dewatered shell against an unbraced one. Damage accumulates for as long as the shell stands empty. Professionals refill the same day where they can, which keeps the exposed period as short as possible. Same-day refill is a working practice, and no source publishes it as a rule.
An empty shell degrades in two ways. Structurally, it carries ground pressure with nothing inside resisting it, for every hour it is empty. At the surface, the pool loses the water that was regulating the temperature of the laminate, so the fibreglass expands and contracts further through seasonal swings. The gelcoat is the decorative and protective outer coat of that laminate, the interior layer facing the water. An empty pool exposes the gelcoat to UV and heat directly, which can fade or crack it into a repair.
Older shells are reported to tolerate an empty period worst. An older fibreglass pool may not withstand the stress of standing empty at all, up to the wall collapsing, so age is one more reason the empty period is planned to be short.
How do you clean a fibreglass pool while it is empty?
The gelcoat lining the inside face of the shell determines what an empty fibreglass pool can be cleaned with. The gelcoat is a thin coat, and acid washing, abrasive pads and pressure washing all take surface material off. On a fibreglass pool that surface material is the gelcoat itself.
The second limit is the clock. The time available to work in the shell is set by the emptying operation, not by the state of the surface. An empty fibreglass pool carries ground pressure for as long as the cleaning takes.
Most of what prompts an empty-pool clean does not need the water out at all. The gelcoat surface of a fibreglass pool is smooth and non-porous, so it resists algae, cracking and staining. A fibreglass pool also needs fewer chemicals than the other main pool types to hold water balance and pH.
How do you refill a fibreglass pool, and what water level should it sit at?
A fibreglass pool is refilled from the top with clean water, run in as the groundwater control comes off, with the bracing the last thing removed. The order follows from what each part does: the water going back in is what restores the counterweight, so the props stay in place until the water inside is carrying the load again.
Refilling is not a garden-tap exercise. On our installs, where a property has no hydrant available, we order a water truck to fill the pool.
MFP Easy sets a fibreglass pool's standing water level at the middle of the skimmer throat. The water is left there at handover, and that is the level a pool goes back to after any work on it.
Dropping a fibreglass pool down for the winter is a United States practice, and it does not apply in South East Queensland. A fibreglass pool here holds its operating level all year.
What happens if a fibreglass pool overflows or gets too full?
An overflow does not damage the structure of a fibreglass pool. What it costs is water quality: heavy rain dilutes the chemistry, the skimmer loses its surface draw while the water sits above the opening, and the storm carries debris in with it.
Recovery runs in that order once the weather clears. Clean the debris out first, run the filtration, then test the water and rebalance it. Queensland Health advises that a conventional filter alone should not be relied on to restore water quality after flood contamination, so filtration is one step in the recovery rather than the whole of it.
The level to come back to is the skimmer opening. Australian builder and industry guides put the operating level between the mid-point of that opening and about three-quarters of the way up it, and the sources vary within that band. Getting the level down means pumping the excess to waste, and only once conditions allow it. Queensland Health advises against emptying a pool while the ground around it is saturated, because of the risk of structural damage. Industry guidance is that a shell should not be drained until the ground has dried, since hydrostatic pressure can lift it.
Before forecast heavy rain, lowering the water slightly in advance is the uncontroversial move.
Do you actually need to empty your fibreglass pool?
Four situations call for a fibreglass pool to be emptied.
- Structural repair. Repairing the shell itself requires the water off the damaged area, so the level is dropped past the damage rather than to the floor.
- Resurfacing or refinishing the interior. Recoating the interior surface of a fibreglass pool is work on the whole shell, so it requires a full drain rather than a partial drop.
- Severe chemical failure. Water that has gone so far out of balance that testing, dosing and filtration can no longer bring it back is replaced rather than corrected. Severity is the whole test, because an imbalance that correction can still reach is a water-in problem.
- Relocation or removal. A pool being taken out of the ground is emptied for good, and getting the water out is the first step of a fibreglass pool removal, quoted separately from the demolition itself.
Every other reason has a water-in answer. A chlorine shock treatment kills bacteria, restores clarity and resets the water balance. Testing and adjusting pH, alkalinity and calcium hardness keeps the water safe and protects the gelcoat. Running the pump and filter for longer cycles clears the particles causing cloudiness and stops the water stagnating. Staining and water chemistry are the common triggers for reaching for the drain, and both are fibreglass pool maintenance jobs done with the pool full.
Pool lights typically have enough cable behind the fitting to be replaced without draining the pool at all.
MFP Easy builds and services fibreglass pools across South East Queensland, and we will assess the pool and the site before any water comes out of it.
FAQs
Can a fibreglass pool be left empty?
No. A fibreglass pool can be emptied under control, but standing empty is the condition the structural failures come from. Ground pressure works on the shell for the whole period, and the gelcoat takes UV and heat with no water regulating the temperature of the laminate. No safe duration is published, so professionals plan the work around refilling the same day.
Can you completely drain a fibreglass pool?
Yes, as a controlled operation. The groundwater beside the shell is pumped down and monitored, the walls are braced to a specification written for that pool, and the refill is planned before the first water leaves. A full drain is reserved for repairing or refinishing the interior. Every other reason is handled with a partial drop or with the water in.
How is a fibreglass pool emptied?
A fibreglass pool is emptied by pump. The water is pumped out under supervision, while the groundwater standing beside the shell is pumped down at the same time. The fitting in the floor that governs an emptied fibreglass pool is the hydrostatic relief valve, which opens to let groundwater in and relieve the pressure underneath, and takes no water out of the pool.
Does a hydrostatic valve make it safe to drain a fibreglass pool?
No. A hydrostatic valve relieves pressure under the floor by letting groundwater into the shell when the level inside drops, and that is the whole of its function. A hydrostatic valve does not lower the water table, does not dry the ground beside the pool and does not hold the shell down. Professionals use a hydrostatic valve alongside sump pumps and groundwater monitoring, never on its own.
Does draining a fibreglass pool yourself void the warranty?
Draining a fibreglass pool yourself can void the manufacturer's warranty. Most manufacturers require an authorised dealer to carry out the draining, and cover is lost when the owner does the work instead. Owner-performed draining also raises the chance of structural damage, and a repair bill for a shifted or cracked shell runs past the cost of engaging someone to do it properly.
