Why We Never Backfill a Fiberglass Pool With Sand in Tulsa

The most important part of a fiberglass pool is the part you will never see. It sits under the shell and along its walls, buried before the deck goes on. That hidden material is called backfill, and it decides whether your pool stays level for decades.

Many builders use sand for that job, but we don't. We set every fiberglass pool on crushed stone, and we backfill every wall with the same stone. Here is why, and what to ask any builder before you sign.

What Is Backfill Under a Fiberglass Pool?

A crane lowers the finished shell into a hole dug a little larger than the pool. Two layers fill that extra space, starting with a flat base layer across the floor that the shell rests on. The backfill fills the gap between the shell walls and the surrounding soil.

A fiberglass shell is strong but flexible by design, so it needs firm, even support on all sides. When that support moves, the shell moves with it. The concrete deck also rests partly on the backfill, so a settling backfill means a settling deck. Our guide to  fiberglass installation steps walks through the full build.

Why Some Builders Still Use Sand

Sand has a long history under fiberglass pools because it is easy to spread, shape, and smooth into a level floor. Crews can shovel it by hand and soak it with water to pack it down. Many sand-based pools have held up for years.

Some manufacturers still allow it in writing. San Juan's current  IAPMO evaluation report permits structurally stable sand or pea gravel up to 3/8 inch under its shells. Latham's  ICC-ES report covers brands such as Viking and Blue Hawaiian. On expansive soils, it calls for clean sand in the final 6 inches under and behind the walls. A builder who uses sand may be following a published specification rather than cutting corners.

So why do we refuse? 

Sand performs well only while it stays dry and stays in place. Its problems appear when water soaks in and has nowhere to go, and in Tulsa, that happens often. Leisure Pools, which makes our shells, publishes its own  Leisure installation steps . That guide sets the shell on compacted gravel, names gravel as the usual backfill, and recommends a groundwater access pipe.

What Happens to Sand When It Gets Wet

Dry sand grains press against each other and carry weight well. When water fills the spaces between them, the water pressure pushes the grains apart. They lose their grip, and the sand starts to behave more like a thick liquid than a solid.

Scientists call this effect  soil liquefaction . The U.S. Geological Survey studies it as an earthquake hazard in loose, water-soaked sand. Under a pool, the trigger is slower, but the idea is the same. Rain saturates the sand, moving water carries grains away, and a slow chain of failures begins:

1. Saturation: Rain fills the space around the shell and has nowhere to drain.
2. Movement: Softened grains wash into low spots or out through soil gaps.
3. Voids: Empty pockets form, and nothing supports the shell or deck above them.
4. Settling: One side of the shell drops lower than the other.
5. Deck damage: Concrete over the void sinks and cracks.

Jason has seen this pattern on pools before. He has found sand that softened, washed out, and left a large void along one side of the pool. The concrete deck above it had already started to sink.

Soaked backfill can also press on the walls, and inward bulging is a well-documented weakness of fiberglass shells. One  patent filing for a fiberglass pool relief valve blames improperly placed backfill and groundwater.

Why Tulsa Raises the Stakes

Tulsa receives about 41 inches of rain in a normal year. National Weather Service data show May as Tulsa's wettest month, with more than 5.5 inches. Much of the metro also sits on clay, which drains slowly. A pool hole dug in clay can act like a bathtub, holding rainwater around the shell instead of letting it drain.

Any pool type can go into Tulsa clay, and we build all three types in it. The soil does not pick a winner. Construction does, through the base, backfill, and drainage system. Our article on  Tulsa clay soil covers how clay shapes digging and drainage costs.

What We Use Instead: 3/8 Chip Stone

Every fiberglass pool we build rests on 3/8 chip stone, which is crushed rock in small, sharp pieces about the size of a pea. Here is our specification:

• The base: At least 4 inches of 3/8 chip across the excavation floor. Some spots get 5 or 7 inches to match the shell's curves.
• The backfill: The same 3/8 chip around the walls, from the floor to the top.
• The compaction: We compact the stone dry and never flood it with water to settle it.
• The quantity: A typical fiberglass installation takes 80 to 100 tons of stone.

Crushed stone has sharp, angled edges. When compacted, the pieces lock together like puzzle pieces, so they stay where we placed them. Water drains freely through the gaps, and the stone stays just as firm when wet as dry.

Some pool contractors use 5/8 chip stone, which is fine. Jason uses 3/8 because the smaller stone compacts more tightly, leaving fewer gaps and giving the shell more even support. We use the same 3/8 chip stone around our semi-inground pools, too.

The Groundwater Pipe Most Buyers Never See

Groundwater pushes on anything buried in it, and that force is called  hydrostatic pressure . A full pool pushes back with the weight of its water. An empty or low pool cannot, and that is when shells can shift or even rise out of the ground.

So every inground pool we build gets a groundwater relief pipe. The pipe sits in the chip stone at the deep end and rises to a removable cap in the deck. If groundwater builds up, a pump drops into the pipe and pumps it out. Leisure recommends the same kind of pipe in its installation guide.

Many pools rely only on a floor relief valve. AQUA Magazine has reported on valves that jammed or let water in too slowly to protect the shell. Chip stone helps here too, because it lets groundwater travel freely toward the pipe. Sand tends to hold water right against the shell.

Sand vs. 3/8 Chip Stone at a Glance

Factor	Sand	3/8 Chip Stone
Grain shape	Small and smooth	Sharp and angled
When soaked	Can soften and shift	Stays firm
Drainage	Slow and holds water	Fast, water passes through
How it packs	Needs water and careful layers	Locks together when compacted dry
Risk of voids	Can wash out and leave gaps	Stays in place
Support for the deck	Can let concrete sink	Gives concrete a firm base
Works with a relief pipe	Holds water away from the pipe	Carries water to the pipe

Can You Fix a Fiberglass Pool Built on Sand?

Not once the pool is in the ground. The sand sits under and around the shell, and nobody can swap it for stone without removing the pool. A builder can tear out a sunken deck, fill the void with chip stone, and pour new concrete. That helps the deck but does nothing for the sand under the shell.

Watch for these warning signs if your pool sits on sand:

• Deck sections that sink or tilt toward the water
• New cracks or gaps between the coping and the deck
• A water line that sits higher on the tile at one end
• Walls that look bowed or pushed inward
• Steps or benches that feel hollow or shift underfoot

If you see these signs, call a pool professional and don't drain the pool. An empty fiberglass shell has no water weight holding it down.

Questions to Ask Before You Sign

Backfill gets buried before you ever see the finished pool, so builders rarely bring it up. Ask these questions and get the answers in writing:

1. What material will you set the shell on, and how thick is the base?
2. What will you use to backfill the walls?
3. Will your crew add water to help it settle?
4. How many tons of stone does my pool need?
5. How will you manage groundwater after heavy rain?
6. Which backfill materials does the manufacturer allow, and which will you choose?

The last question matters because some manufacturers permit sand, so a builder can follow the specification and still choose it. For more ways to size up a builder, use our list of  seven builder questions .

What Chip Stone Adds to Your Price

On a typical fiberglass build, our 80 to 100 tons of 3/8 chip stone cost about $3,000 to $5,000. But it's included in your price. A sunken deck or an out-of-level shell costs far more. Your quoted price is fixed unless the excavator hits buried rock, which can change the price. Crane service, permits, dirt hauling, and any electrical panel upgrade are extra. Our  fiberglass pool cost guide covers what drives the final number.

What Jason Recommends

One job sums this up. I replaced a deck in Broken Arrow around a fiberglass pool that was backfilled with sand. The sand was gone, and a big cavity had formed under the concrete. The shell had dropped 2 inches on one side, and the deck was sinking. We filled the cavity with chip stone and poured new concrete. That fixed the deck, but it didn't fix what's under the shell, and nothing will.

That's why I won't bend on this. We set every fiberglass pool on at least 4 inches of 3/8 chip and backfill it with the same stone. Each one gets a groundwater pipe at the deep end.

My advice: ask every builder what goes under your pool. If they say sand, ask what happens after a week of heavy rain. A Leisure shell carries a  lifetime structural warranty , so give it a base that lasts as long as the shell. Built right, the structure should last 50 years or more. The gel coat is what wears, and we cover that in our  pros and cons guide.

Frequently Asked Questions

Can you backfill a fiberglass pool with sand?

You can, and some manufacturers allow it, but we never do, because sand works well only when it stays dry. Once soaked, it can soften, shift, and wash away, leaving voids under the shell and deck. In rainy Tulsa clay, that risk is too high for a pool you plan to keep for decades.

How much gravel does a fiberglass pool need?

A typical fiberglass build needs 80 to 100 tons of 3/8 chip stone to cover both the base and the backfill. We spread at least 4 inches across the excavation floor, then fill around the walls to the top. The stone costs about $3,000 to $5,000.

Why is my fiberglass pool sinking on one side?

The most common cause is shifting backfill. When sand under or beside the shell washes out, it leaves a void, and the shell and deck settle into it. Watch for a tilted deck, a higher water line on one end, and new gaps at the coping. Call a pool professional, and do not drain the pool.

Can you drain a fiberglass pool?

Do not drain a fiberglass pool yourself, because an empty shell loses the water weight that holds it in place. Groundwater pressure can then push the shell upward or bow the walls inward. If a repair requires draining, have a pool professional check the groundwater first.

Find the Pool That Fits Your Family

The hidden layers decide whether a pool lasts. Get them right, and your summers go to cannonballs and late swims with your kids, not cracked concrete. Not sure which pool type fits your yard?  Find your perfect pool and see which pool and features match your yard, budget, and family.