The 7 Installation Mistakes that Cause 90% of Fiberglass Pool Failures

Knowing these 7 mistakes makes it much easier to judge whether a contractor is cutting corners.

Fiberglass pools rarely fail because of the pool shell. Almost all long-term problems come from installation mistakes. Installers who know fiberglass well avoid these mistakes, but inexperienced contractors repeat them over and over again.

The base supports the entire pool structure.

Common mistakes:

• Setting the pool on excavated soil
• Using sand as a base
• Too little base material
• Base is not appropriately leveled

Correct method typically includes:

• 4–6 inches of clean crushed stone (#57 or #8)
  • Could be more if conditions warrant
• Laser leveling
• Screeding and compacting

A bad base causes:

• Crooked waterline
• Pool settling
• Deck cracking

Fiberglass pools must be extremely level. Even 1 inch off can be noticeable and places stress on the fiberglass pool.

Common causes:

• Poor base preparation
• Not checking level after the shell is set
• Not adjusting stone under the shell

Good installers:

• Check level before backfill
• Check again during water filling

This is one of the most common and damaging mistakes.

Bad installers use:

• Excavated dirt
• Clay soil
• Sand

These materials:

• Hold water
• Shift over time
• Create uneven pressure on the shell

Correct materials:

• Clean crushed stone
• Pea gravel

These allow drainage and structural support.

Fiberglass shells rely on balanced pressure.

Correct method:

1. Fill pool 12–18 inches
2. Backfill 12–18 inches
3. Repeat

If installers:

• Backfill the whole pool first
• Fill water later

…the shell can flex or distort.

Steps, benches, and tanning ledges need solid support underneath.

If there are voids:

• Steps may flex
• Gelcoat can crack
• Structural stress develops

Good installers:

• Hand pack gravel under these areas
• Sometimes use structural foam

Groundwater pressure can push on the pool shell.

Good installations often include:

• French drains
• Perimeter drainage pipe
• Sump pits with pumps; automated pumps are preferred.

Ignoring drainage can cause:

• Pool lifting slightly
• Deck movement
• Water pressure against the shell
• Bulging of walls, which places stress on the whole shell

This is especially important in the Midwest where the excavated soil often weighs more than the pool shell with water.

Common plumbing mistakes include:

• Using flex pipe instead of Schedule 40 rigid PVC
• Not pressure testing lines
• Burying pipes before testing
• Poor equipment layout

Leaks under a pool deck can be very expensive to fix later.

Professional installers:

• Pressure test plumbing
• Use Schedule 40 PVC; not flexible pipe
• Install separate lines for skimmers and drains

When installed correctly:

• Fiberglass pools last 30–50+ years
• Maintenance is lower than concrete
• Structural problems are infrequent

And most problems trace back to one of these seven mistakes.