There’s an old story that’s been passed down for generations about a wise builder who chose to construct a house on solid rock, while another chose to build on sand. When storms came, the house on the rock stood firm, while the house on the sand collapsed.
It’s a simple but powerful lesson: a strong foundation matters.
Yet even today, some in the pool industry insist on using sand as the base and backfill for fiberglass pools. Now, setting sarcasm aside for a moment, let’s talk about why this matters. Because when you actually break down how these materials behave under pressure—literally—the difference between sand and gravel becomes impossible to ignore.
Just think about it for a second - In order for sand to come close to compaction, it must be saturated with water and tamped. But gravel? That's right, gravel only needs to be tamped and it's compacted. Heck, it can achieve 95% compaction upon placement! More about this below, but this is why it's used as the base and footer for incredibly heavy and large structures all over the world.
In fact, when was the last time you saw concrete, asphalt, basements, block walls, etc, etc built on sand?? For example, my home has a basement constructed with large concrete wall panels. These incredibly heavy panels line up vertically and interlock to form our basement and do you know what they are sitting on? That's right; you guessed it- a gravel footer. In other words, my entire home is sitting on a 12" gravel base.
Yet, unfortunately, there are still many in the industry who insist on using sand as a base and backfill material.
Why Gravel Is Better Than Sand for Fiberglass Pools
But enough of common sense. Let's just throw that out for a moment and look at the scientific reasons as to why gravel is better as a base and backfill for fiberglass pools. One of my business partners, Jason Hughes, wrote an incredibly respected and popular article about fiberglass pool problems, one of which discussed sand versus gravel. This is an excerpt from the article:
"If you talk to enough people or scroll through enough online forums you'll probably come across someone who has developed a bulge in the wall of their fiberglass pool. We encountered this problem years ago with several of our pools. This caused us to scrutinize our installation process and thoroughly inspect the pools where these problems occurred.
What we found flies in the face of the standard and accepted method of installing fiberglass pools across the country. The problem in our case stemmed from the backfill material that we used.....namely sand. Tens of thousands of fiberglass pools have been installed with sand backfill without incident, and most of our sand backfilled pools were fine as well. But when the right (or wrong) combination of circumstances align fiberglass pool walls do bulge.
Question: What happens to sand when it becomes saturated with water? It liquefies. That would be fine if the liquefied sand were resting against something other than fiberglass which is renowned for its flexibility. Fiberglass pools are structurally engineered to remain full of water. The outward pressure from the water works in conjunction with the structure of the pool to stabilize the vessel. When sand liquefies it is heavier than the water on the inside of the pool and if the wall is not strong enough to maintain its shape a bulge develops. High water tables as well as soils that hold water both lead to situations where a significant amount of water surrounds the pool structure and saturates the backfill material.
So does this mean that fiberglass pools are not suitable for installations with high water table or certain soils? Quite the contrary; I would submit that there is no better pool to have in such circumstances. You just need to make certain of two things. First, that your pool is strong enough to prevent bulges. And second, that you eliminate the sand backfill and use gravel.
Is there a difference between the strength of fiberglass pools? Yes, but from the perspective of the consumer it will be virtually impossible to rate the "bulge-ability" of the various pools on the market because all manufacturers claim to be the strongest and most stable.
The one thing you CAN control is the backfill material, and based on our experience, using gravel backfill is the only insurance against wall bulges in any situation with any pool. Clean crushed blue stone ¾" or smaller is the gravel used in our installations because it has several distinct advantages over sand or even round pea gravel. First, its properties do not change when it becomes saturated with water so it performs the same wet or dry. This makes it ideal for any situation and especially in areas with a high water table or difficult soil conditions.
Second, the point on point friction that occurs between the pieces of gravel make it a very stable material on the sides of the pool; much better than saturated sand that liquefies or pea gravel that acts like little ball bearings.
Third, the gravel compacts upon placement unlike sand which needs to be placed in lifts and compacted with water. This is further addressed in the discussion on plumbing problems below. You will see that this is a major advantage as well. Opponents to gravel backfill pitch against it by saying that it's a cheaper material and used in order to cut cost. In reality it actually costs us significantly more to use gravel.
We feel it is well worth the investment because you can't put a price tag on peace of mind........... Why would the plumbing settle? Good question! The plumbing doesn't settle, the backfill material around the fiberglass pool settles and takes the plumbing with it. Yes we're back to our sand vs. gravel discussion again. This is great because this is a topic that needs attention. The problem is that it's very difficult to fully compact sand around a fiberglass pool during installation.
Many times, despite a fiberglass pool installer's best efforts to bring the sand up in 6"-12" lifts and fully saturate the sand with water, the sand still settles some over time. The plumbing is encased in this sand and as the sand settles, it exerts downward pressure on the plumbing of the swimming pool. Many installers tie their plumbing up to help hold it in place, but not withstanding, that's still a lot of pressure.
Settled plumbing manifests itself in one way....leaks. "Leak" is a dirty word! We want to avoid saying that word, and we do so by using gravel backfill that just doesn't settle! If the backfill doesn't move, the plumbing doesn't move. It's as simple as that!"
There are a couple of other points I'd like add to Jason's explanation. Let's suppose you have a 6' deep fiberglass pool to install. When excavating for a fiberglass pool, the over-dig is typically about 12". Therefore, the deep end of the pool will be backfilled with either 6' of sand or 6' of gravel to fill in this 12" over-dig. Keep in mind here that your pool's patio (concrete deck) will be resting on top of that first 12" of backfill.
Obviously, it's very important that this first 12" of concrete decking is on solid ground, ground that does not settle over time. Notwithstanding, many pool builders will backfill the deep end of the pool with 6' of sand and then add about 3" of gravel on top of that sand to form the base of the concrete decking. Are you starting to see just how little sense this makes? Tell me, which would you rather have the first 12" of your concrete decking resting on- 6 feet of gravel or 6 feet of sand with 3" of gravel on top? I can guarantee that 3'-8' of sand backfill (depending on the depth of the pool) will settle at least slightly over time. It doesn't take a genius to quickly realize this is not healthy for the structural integrity of a pool's concrete decking.
Plumbing Leaks
The other point I'd like to add deals with sand's ability to follow moving water. Jason mentioned this a little bit and I want to further delve into the subject. Let's say that after 6 years of owning your fiberglass pool (that was backfilled with sand) you fall victim to a plumbing leak at one of the pool's return fittings. This means that huge amounts of water are blasting away behind the pool wall.
Before long, because sand so easily follows moving water, the force of the water creates a tunnel effect and an empty void is created behind the wall. Obviously, voids in the backfill of a fiberglass pool can have major negative ramifications, with the fiberglass bending to the shape of the void. As you can well imagine, this is not a good thing, but it can be almost entirely prevented with the use of gravel. Unlike sand, gravel will not wash and follow moving water very easily. Therefore, if leaks ever do unfortunately occur, at least the pool's backfill will likely not be altered.
Wondering about prices for a complete project? Make your own decisions and get a price range before speaking with an installer.
So, there you have it folks. I could go on and on about this topic, but I'll stop there. Everything written here is simply the result of having installed our first 300 pools with sand and our last 700+ pools with gravel. The reality is that anyone who insists sand is a better base for fiberglass pools is simply living in an antiquated state of denial and refusing to build a fiberglass pool to a modern standard. Such people are likely still using flip phones and sending faxes.
Mark my words: Within 10 years of this little blog, over 90% of the fiberglass pool industry will be using some type of gravel as the base and backfill for their pools. Until then though, I, along with my business partners Jim and Jason (and anyone else who likes to mix common sense with pool education), will continue to do everything we can to be a source for positive change in the fiberglass pool industry.
About the Author and a 2025 Update
Marcus Sheridan - Speaker, Author, Pool Installation Company Owner
When I first wrote this article in 2009, I made a bold prediction: within 10 years, over 90% of the fiberglass pool industry would be using gravel as the base and backfill material. While we can’t verify that exact percentage today, here’s what we can confirm: this method is no longer considered 
“radical”—it’s now being taught in one of the pool industry’s most respected training programs, Genesis by the Pool & Hot Tub Alliance.
That’s right—the same approach we adopted after hard-earned lessons in the field is now part of the curriculum used to train and certify future fiberglass pool professionals. If that’s not validation, I don’t know what is.
Read more about Marcus and his pool installation company here.
From the Manufacturer
At River Pools, we don’t just manufacture fiberglass pools—we advocate for the long-term success of every installation, no matter who’s doing the work. That’s why we believe the materials and methods used to install our pools matter just as much as the shell itself.
While installation practices vary across the industry, we support the use of clean, crushed gravel backfill as a best practice for stability, drainage, and long-term structural performance. We’ve seen firsthand how this method helps reduce issues like wall bulging, plumbing stress, and settlement around the pool.
And we’re not the only ones. Today, gravel backfill is the preferred material by independent installers around the country, a clear sign that the industry is embracing smarter, science-backed methods.
Whether you're working with an independent builder in your area or researching installation best practices on your own, we encourage you to ask informed questions and choose a builder who prioritizes long-term performance—just like we do.
Editor's note: This article was updated June 30, 2025
Up Next:
Answering Your Fiberglass Pool Installation Questions
Topics:
