The Proper Backfill for a Fiberglass Pool
- Michael Bowers
- Mar 9
- 3 min read
Why ¾" Clean Fractured Stone Is the Only Proper Backfill for a fiberglass pool!
Just as important as the installation method is the material used to properly backfill a fiberglass pool.
Some fiberglass pool installers use sand or excavated soil, but these materials come with serious drawbacks. Professional fiberglass pool installers almost universally use ¾ inch clean fractured stone instead and here is why!
Clean Stone Provides Structural Support
Using the proper backfill for a fiberglass pool like clean fractured stone creates a stable, compactable structure around the pool shell.
Because the stone pieces interlock, they provide consistent support against the outside of the fiberglass shell.This helps keep the pool firmly in position and prevents shifting over time.
Stone Allows Water to Drain
Clean fractured stone contains no sand, dirt, or fines. This means water can move freely through the material rather than becoming trapped. Proper drainage is extremely important for pools installed in climates like Wisconsin where: freeze-thaw cycles occur• heavy rain events are common• groundwater levels fluctuate If water becomes trapped around a pool shell, it can create pressure against the structure.
Stone backfill helps prevent this by allowing water to move away from the pool.
Sand and Dirt Can Settle Over Time
Sand and native soils may appear stable initially, but they often settle or shift after installation.
When that happens, voids can develop around the pool shell.
These voids can lead to:
• uneven support
• pressure points on the fiberglass shel
l• long-term structural stress
Clean stone avoids this problem because it maintains its structure over time.
The Backfill Process During Installation
During a proper fiberglass pool installation, the process typically looks like this:
The pool shell is set on a properly prepared stone base.
Water begins filling the pool.
Clean ¾" fractured stone is installed around the shell.
The water level and backfill level rise at roughly the same rate.
This balanced approach ensures the fiberglass structure remains properly supported during installation and there is no undue stress on the pool in either direction.
Why Installation Matters Just as Much as the Pool Shell
Fiberglass pools are engineered to last decades, but their performance depends heavily on how they are installed. Two fiberglass pools may look identical when completed, but the quality of the installation beneath the surface can be dramatically different.This is why experienced installers follow strict installation standards that focus on:
• proper excavation
• engineered stone base preparation
• balanced water and backfill installation
• clean stone structural backfill
These steps help ensure the pool remains stable and performs properly for many years.
The Bottom Line
When installed correctly, fiberglass pools are extremely durable and perform well in a wide range of climates. But proper installation requires two key principles:
Backfill must be installed at the same rate the pool is filled with water.
¾ inch clean fractured stone should be used as the structural backfill material.
These practices help ensure the pool shell remains properly supported, stable, and protected long after the installation is complete.
THE MORAL OF THE STORY
If you’re considering a fiberglass pool and want to understand more about how installation affects long-term performance, it’s always worth asking your builder what materials and installation methods they use.
For best results they should be using a process that mirrors " The Pool Pros Way? Because in the end, a pool that is installed properly is a pool that will perform for decades.
SHAMELESS CALL TO ACTION
If your builder insists on using flex pipe or flxible pvc it may be time to look for a second opinion on how a fiberglass pool shoul be built. Better yet call Pool Pros in house designers today to go over how we build pools The Pool Pros Way! Pool Designers are standing by @ 1-920-771-0107.
