Water Under Fibreglass Pool

Water Under Fibreglass Pool - Why? How To Stop It?

Water under your fibreglass pool can seem like a significant issue. Especially when you look up this problem online, you are bound to find plenty of articles warning you of severe issues like pool popping, which can cause you to worry.

Luckily, in most cases, having water under your fibreglass pool is a normal and harmless condition.

In this article, we will discuss why there is water under your fibreglass pool, whether or not this water poses a threat to your pool, and how you can stop water from getting underneath your pool.

Why Is Water Under My Fibreglass Pool?

Inground pools have layers beneath them built during the construction phase known as subsoil. This layer is most often composed of compacted gravel and is designed to be an area that holds water.

Water in the subsoil layer of your pool can come from three primary sources. One of them is water flow from the ground; this typically occurs during and after heavy rain.

A second source of this water is from pool shell sweating, which occurs when your pool gathers condensation underground and this water drips off in pools.

The third and rarest source of water in your pool’s subsoil layer is pool leaks. This indicates a significant problem with the lining and shell of your pool that needs to be fixed by professionals.

Is Water Under Fibreglass Pool A Problem?

Water in your fibreglass pool’s subsoil layer is not a problem. This area is specifically designed to hold water so that it prevents settling.

Additionally, most pool subsoils contain a sump pump. This pump will be installed at the lowest area of your pool’s subsoil and automatically activates when water is present under your pool to expel it to a waste area.

How To Stop Water Getting Under Fibreglass Pool

Water getting under your pool is a natural occurrence. So long as your pool was constructed with a subsoil layer and an appropriate pump system, this water will be removed automatically.

If you live in a climate that receives little rain, your subsoil may not have a pump installed. Not to worry, - small portions of water will dissipate on their own, especially in dry and warm areas.

If you have a sump pump installed but suspect that it may not be working properly, contact a pool professional. If possible, we recommend contacting the company that constructed your pool as they will have the most relevant information about its construction.


Water underneath your fibreglass pool can seem concerning. Luckily, this condition is fairly normal, and so long as your subsoil and connected pump system are working properly, it poses no threat to the structural integrity of your pool.

Do you have any questions about having water under a fibreglass pool? Leave us a comment down below, we’d love to help answer any remaining queries you may have!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.