Americans rarely shy away from a good debate.

And it’s no different in the swimming pool industry, where argument swirls over topics such as proper water chemistry, or the psi number that’s sufficient for shooting a pool.

Oftentimes there is no clear-cut winner; instead, opinions are established based on the perceived merits of a particular philosophy or technique.

In the realm of fiberglass pools, one hotly contested issue is the installer’s choice of backfill. Indeed, proponents of the two leading materials — gravel and sand — insist their medium is best.

So who, ultimately, is right?

To help decide, we’ve framed a mock trial, and selected a strong advocate for each side to present an opening statement, discuss the evidence, and provide their closing argument.

Speaking in support of gravel are Marcus Sheridan and Jason Hughes, co-owners of River Pools and Spas in Warsaw, Va.

Ric Reineke, president of The Pool Guyz in Virginia Beach, Va., makes the case for sand.      

As for who emerges victorious in the court of public opinion — that’s for you, the jury, to decide. 


Opening statement

“Therefore whoever hears these sayings of Mine, and does them, I will liken him to a wise man who built his house on the rock: and the rain descended, the floods came, and the winds blew and beat on that house; and it did not fall, for it was founded on the rock.

“But everyone who hears these sayings of Mine, and does not do them, will be like a foolish man who built his house on the sand: and the rain descended, the floods came, and the winds blew and beat on that house; and it fell. And great was its fall.”

(Matthew 7:24-27)

It’s a funny thing to me that man was told thousands of years ago that building structures upon rocks was much more intelligent than building upon sand. Notwithstanding, there are still people that insist upon the use of sand as the base and backfill for a fiberglass pool.


Clean, crushed blue stone ¾-inch or smaller is the best type of gravel to use with a fiberglass pool because it has several distinct advantages over sand.

First, its properties do not change when it becomes saturated with water, so it performs the same wet or dry. This makes crushed stone ideal for any situation, but 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 exceptionally stable material against the sides of the pool — much better than saturated sand.

And third, the gravel compacts upon placement, unlike sand, which needs to be placed in lifts and compacted with water.

Anyone that has been around the fiberglass pool industry long enough is aware that pools can, on occasion, suffer from bulging walls. We encountered this problem years ago with several of our pools, and it caused us to scrutinize our installation process and thoroughly inspect the pools where these issued occurred.

The problem in our case stemmed from the backfill material we used at that time, 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.

High water tables, as well as soils that hold water, lead to situations where the backfill becomes saturated. When sand fills with water, it liquefies.

Fiberglass pools are renowned for their flexibility. And when sand liquefies, it becomes heavier than the water on the inside of the pool. If the wall is not rigid enough to maintain its shape, a bulge develops.

So does this mean that fiberglass pools aren’t suitable for environments with high water tables or certain soils? No! I would submit that there is no better pool to have in such circumstances. Installers just need to make sure the pool is strong enough to prevent bulges, and that gravel is used as backfill instead of sand.

There is a definite difference in strength between the various brands of fiberglass pools. But for a consumer it’s virtually impossible to rate the “bulge-ability” of the various products on the market, because all manufacturers claim to be the strongest and most stable. The one thing an installer can control is the backfill material. And based on our experience, using gravel is the only insurance against wall bulges in any situation with any pool.

Opponents to gravel backfill claim that it’s a cheaper material utilized in order to cut cost. But in reality, for most places around the country it’s significantly more expensive to use gravel.

Another point involves the settling of pool plumbing. In reality, the plumbing doesn’t settle — the backfill material around the fiberglass pool settles, and takes the plumbing with it.

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-inch to 12-inch 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. Many installers tie their plumbing up to help hold it in place, but notwithstanding, that’s still a lot of pressure. Settled plumbing manifests itself in one way — leaks.  But if the backfill doesn’t move, the plumbing doesn’t move, and this becomes a non-issue.

In terms of a proper base for pool patios, let’s suppose you have a 6-foot-deep fiberglass pool to install. When excavating for a fiberglass pool, the over-dig is typically about 12 inches. Therefore, the deep end of the pool will be backfilled with either 6 feet of sand or 6 feet of gravel to fill in the over-dig. Keep in mind here that your pool’s patio (concrete deck) will be partly resting on top of that backfill. Obviously, it’s very important that this first 12 inches of concrete decking is on solid ground that does not settle over time.

Nonetheless, many pool builders will backfill the deep end of the pool with 6 feet of sand, and then add about 3 inches 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 is more stable: the first 12 inches of concrete decking resting on 6 feet of gravel, or 6 feet of sand with 3 inches of gravel on top?

I guarantee that 3 to 8 feet 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.

Closing argument

So there you have it. Everything stated here is the result of having installed our first 300 pools with sand and our last 400 pools with gravel. It also comes from many discussions with fiberglass pool builders and manufacturers alike.

This is also why the percentage of builders using gravel were once in the minority, but are now a majority. These professionals base their decisions not on the concept of “it’s always been that way” but rather on what science and common sense tell us to be true.


Opening statement

In today’s fiberglass market, there is a misnomer that sand is in a battle against gravel as to which substance is the better backfill material. The fact of the matter is that only a tiny fraction of the fiberglass swimming pools throughout the world are installed using gravel, or any other material besides sand.  

This is primarily because every major manufacturer worldwide, and the vast majority of smaller firms as well, recommend that their product be installed with a sand substrate. This is based on more than 40 years of experience and hundreds of thousands of pools.

The following argument will focus on a builder’s perspective as to why sand is a superior backfill material over gravel and other products currently being used.


I have had the privilege of installing several hundred fiberglass swimming pools over a number of years. I have used sand as the backfill material in every project. Why? Because the manufacturer that supplies my fiberglass shells recommends that sand serve as backfill for their pools. The company has produced tens of thousands of pools over a 40-year period, and they have seen several types of backfill used in the installation of their product. 

Because of their experiences with the different types of backfills, they have developed an installation specification for their product using sand. This is a direct result of the problems that have arisen when other backfills are used to install fiberglass pools. This specification has been approved by the International Code Council (ICC) as a practice for the installation of fiberglass pools in the United States. Several other U.S.-based manufacturers have also received ICC approval for the installation of their products, and every one of them specs the use of sand as backfill material.

Additionally, there is a national standard in Australia and New Zealand (AS/NZS 1839:1994) that specifically states that a premixed cement/sand ratio of one part cement to 16 parts sand be used in installing fiberglass pools. Additional backfill materials that can be used in Australia and New Zealand all include high volumes of sand or very fine crushed stone. The objective is a very high compaction rate on the backfill around a fiberglass pool.

It is also interesting to note that more than 50 percent of the swimming pools installed in Australia and New Zealand are fiberglass pools.

There is no specification in the United States, Australia or New Zealand that I can find that approves any backfill other than sand, or a high concentration of sand, as a product for use in the installation of a fiberglass swimming pool. 

My personal experiences with sand have been outstanding. The key to using sand correctly is to start by digging the hole to the manufacturer’s specifications. This leaves a very tight area between the outside pool wall and the surrounding excavation. Prior to placing the pool in the excavated area, a sump drain system is installed at the deep end of the pool. Next, a base of sand about 4 inches thick is laid in the hole, wetted, and then screed to create a very level and solid bottom for the pool to set upon. 

The pool is then lowered into the hole, and sand is shoveled into the hole while wetting it to make sure it compacts completely. This type of compaction cannot be achieved with most other backfill materials, including gravel. Although the gravel may appear to be completely compacted, it is still subject to ground movement and settling over time. Sand, when compacted correctly, will form a very dense, and secure, barrier between the pool and the excavation walls, offering support without jagged edges to scrape against the plumbing lines and the pool shell. 

If the excavation was properly prepared and drainage was considered, the sand will hold a permanent space between the pool shell and the excavation wall. Even with ground movement, it is difficult for sand to shift without tremendous outside influence.

Other backfill materials cannot make this claim, as they do not compact as well as sand and therefore leave air space. This can cause settling and potential bulging of the pool walls, or spaces under steps and seats. Even at the initial installation, there will be air spaces at the bottom radiuses, which are structural points in a fiberglass pool. No support in these areas can lead to cracks. 

The entire process of using a non-recommended backfill creates a real dilemma between the manufacturer, builder and homeowner. The homeowner expects an installation based on the manufacturer’s

recommended practices. When this does not occur and trouble arises on the site, the finger-pointing begins and it becomes questionable whether warranties will be honored.

I want and expect my manufacturer to support me in any potential claim, so I follow their recommendations. I certainly would not want them opposing me because I did not follow their ICC-approved practices.

Closing argument

I look at the matter of sand vs. gravel and other backfills as an opportunity to give the homeowner peace of mind. If hundreds of thousands of swimming pools have been installed using sand, and are performing beautifully throughout the world, why tempt fate by trying to come up with my own installation method using other backfill materials? 

If manufacturers come out with new information that supports a backfill other than sand, then I will move to that product. I am just one of tens of thousands of builders across the world, and I will place the safe bet for both the homeowner’s and my sake by not trying to create my own guidelines when they have already been established and work fantastically.