Should you really keep your pool full at all times? Myth or reality
It's a question we hear often, from future owners and people who already have a pool: “Do I really need to keep my pool full of water all year, even in winter?” Some see it as a myth pushed by pool companies; others think it's overstated. For a fiberglass pool, the answer is clear: it's reality, not a myth.
It's a question we hear often, from future owners and people who already have a pool: “Do I really need to keep my pool full of water all year, even in winter?” Some see it as a myth pushed by pool companies; others think it's overstated. For a fiberglass pool, the answer is clear: it's reality, not a myth.
Why a fiberglass pool must stay full
The reason has nothing to do with looks or water care — it's purely structural, and it's called hydrostatic pressure.
A fiberglass pool is a rigid shell set in an excavation, surrounded by backfill and, most of the time, groundwater that naturally seeps around the hole. That water pushes constantly inward on the shell, in every season.
When the pool is full, the weight of the water inside balances that outside pressure. The two forces cancel out, and the shell stays stable in its excavation.
If the pool is emptied — even partially — that balance disappears. Nothing inside resists the groundwater pushing on the outer walls. Result: the shell can shift, deform, lift, or even crack, depending on the water table and soil conditions.
It's not about the brand or quality
This is not a design flaw or a problem with fiberglass quality itself. It's physics and installation: every fiberglass pool, regardless of brand or model, is vulnerable if emptied, because pressure balance applies the same way to every buried monocoque shell.
That's an important difference from some beliefs about inground pools in general: hydrostatic risk hits monobloc shells like fiberglass especially hard, where the whole structure acts as one rigid piece.
What about winter?
Winter is exactly when risk is highest — when the ground is saturated (fall rains, snowmelt, high water table). An emptied pool through a Quebec winter sits exposed when hydrostatic pressure is often at its peak. That's why we recommend lowering the water level slightly for winterization (to protect equipment and avoid freeze in the plumbing), without ever fully draining the basin.
What to remember
| Myth | Reality |
|---|---|
| “You can empty a fiberglass pool with no problem” | False — the shell loses its counterweight against hydrostatic pressure |
| “It's just an exaggerated precaution from installers” | False — it's documented physics, not a marketing tip |
| “The risk only exists in summer” | False — risk is often higher in winter and spring, when the water table is high |
In short: keeping a fiberglass pool full at all times is neither a myth nor an exaggeration — it's a structural requirement tied to how the shell interacts with the soil and groundwater around it. Fully draining a fiberglass pool, even temporarily, exposes the structure to a real risk of movement or damage.
Questions about seasonal care for your fiberglass pool? The Piscines Riviera team can guide you to protect your installation, winter and summer.