Can I empty afiberglass pool?

Your pool is a membrane, not a bathtub

This is the part that surprises people, and once you understand it the rest of the article is obvious.

A fiberglass pool is not a rigid container sitting in a hole. It is a thin composite shell that works as a membrane between the earth and the water. The soil pushes in from the outside. The water pushes out from the inside. The shell lives in the balance between those two forces, and that balance is what keeps it exactly the shape it left the factory in.

Take one side of that equation away and the other side wins.

The force you cannot see: hydrostatic pressure

Groundwater is any water sitting in the soil around and underneath your pool. After a wet spring in Central Ohio, that can be a lot of water. It presses in on the shell from every direction and, more importantly, it presses up from underneath. That upward push is hydrostatic pressure, and it is the same buoyant force that floats a boat.

A full pool weighs a great deal. Twelve thousand gallons is roughly fifty tons of water, and all of it is pinning the shell down. The groundwater outside never stood a chance.

Empty that same pool and you have removed fifty tons of ballast from a shell that weighs a few hundred pounds, while the groundwater underneath it has not gone anywhere. Now the only thing resisting that upward force is the shell itself, and it was never designed to do that job alone.

What actually goes wrong

People imagine the failure mode is dramatic, and sometimes it is. More often it is quiet and expensive.

• The floor lifts. The bottom of the pool rises in the middle. Sometimes you only notice once it is refilled and the water sits strangely.
• The walls bulge. Without water pushing out, soil pressure pushes in, and a straight wall stops being straight.
• The floor splits. Enough upward force in one place and the laminate gives.
• The plumbing breaks. The shell moves, the pipes bonded to it do not, and now you have a leak underground as well as a damaged pool.
• The deck cracks. Anything poured tight against a shell that has shifted comes along for the ride.
• The pool pops. The worst case. The whole shell rises out of the ground. It is rarer with fiberglass than with concrete, but it absolutely happens, and it is not a repair. It is an excavation.

Why Central Ohio makes this worse

We install in Columbus and the towns around it, and this is not a generic warning for us. Central Ohio sits on a transition zone of glacial till, outwash sand and gravel, and limestone-influenced clay. Clay is the problem. It holds water rather than letting it drain, so after a wet stretch the soil around your pool can stay saturated for a long time.

Add our freeze and thaw cycle, a spring melt, and the kind of week where it rains for five days straight, and you get exactly the conditions where an empty pool is at maximum risk. If somebody tells you to drain your pool in March, they are giving you the worst possible advice at the worst possible time of year.

The pipe next to your pool that nobody explains

There should be a capped pipe near the shell. It is called a standpipe, a sump tube or a groundwater access pipe, and it runs down alongside the pool to the level of the floor.

It has one job: it lets you find out how much water is in the ground around your pool, and it lets you get that water out. Pull the cap, drop a tape or a stick in, and if it comes out wet you have groundwater. Drop a small pump in and you can remove it.

So when does a fiberglass pool ever get drained?

Rarely, and always for a real reason. Structural repair. Certain resurfacing work. Relocating a pool. Occasionally, water so chemically wrecked that partial replacement will not fix it.

When it does happen, the process looks nothing like putting a hose in and walking away. Roughly:

1. Get written consent. From the manufacturer, before anything else, or your warranty is gone regardless of how carefully the work is done.
2. Wait for dry weather. Not just the day of. The drain, the work and the refill all have to happen in a dry window. One heavy storm mid-job is exactly the scenario that floats a pool.
3. Check and clear the groundwater. Standpipe first. If there is water in the ground, it gets pumped out and kept out for the whole job.
4. Brace the shell. Braces across the width hold the walls where they belong once the water is not doing it.
5. Lower it slowly and watch the floor. If the floor starts to lift, the job stops and water goes back in.
6. Send the water far away and downhill. Pumping it out and letting it soak into the yard next to the pool is how you create the exact problem you were trying to avoid.
7. Refill immediately. An empty pool is a liability every hour it stays empty. Nobody drains one and then goes on holiday.

Read that list again and ask yourself whether it sounds like a Saturday project. It is not. It is a controlled procedure, and it is why every manufacturer in this industry, including ours, says the same thing: let an authorised professional do it.

What to do instead

Almost everybody who asks us about draining has one of five reasons. Here is the better answer for each of them.

The water is green

Do not drain it. Green is a chemistry and circulation problem. Get the pH into range, shock it, brush the walls and run the filter. Draining a pool to fix algae is solving a weekend problem by risking a structural one.

The chemistry is out of control

Usually cyanuric acid or total dissolved solids that have crept up over years. The fix is a partial replacement, which stays above the skimmer and inside the 30 percent limit. Talk to somebody before you start, and keep your monthly record either way, because the warranty requires it.

There is a white haze or scale on the surface

That is calcium coming out of the water, and it comes from the pH sitting above 7.4. Caught in the first few months it can be treated chemically. It does not need the pool emptied, and it is far easier to prevent by holding pH between 7.2 and 7.4.

It is time to close for winter

Winterizing lowers the level a little and blows out the lines. The pool stays full. In Ohio, an empty pool over winter is the single riskiest thing you can do to it.

Something is genuinely broken

Then call your installer and let them plan it properly. This is the one case where the answer might be yes, and it is also the one case where doing it yourself is most likely to turn a repair into a replacement.

If your pool is already empty

Maybe you inherited it that way when you bought the house. Maybe somebody drained it for you without knowing better.

Do not refill it on instinct. If the shell has already moved, putting water back in can set that movement permanently and hide it under the water line where nobody sees it for a season. Get the groundwater checked, get the shell's position checked, and then refill under supervision.

Call us if you are in Central Ohio and this is you. We would rather look at it for free than watch somebody turn a fixable problem into a new pool.