When self-installing an above ground swimming pool, people can have varied ideas on what to put down on the ground under the pool. In reality, most pool sites don’t need anything to go down as the liner is designed to sit directly on top of the earth.
Above ground swimming pool liners transfer moisture a lot like a soda or beer can does. This means that moisture underneath the liner has to be able to go into the earth. Materials like Visqueen, tarps, and pearlite can trap moisture and eventually cause the pool’s wall to corrode.
Any material that can trap moisture should not go under a pool liner.
NOTE: This article only applies to metal-walled above ground pools. Soft-sided pool (Intex, Coleman) types have different designs
TRAPPED MOISTURE IS A BIG ENEMY WITH ABOVE GROUND POOLS
A few decades ago, it was a common practice for people to put Visqueen or plastic sheeting down just underneath an above ground pool liner. This would often cause the inside of the wall of the pool to start and eventually rust out.
This is because a vinyl pool liner will transfer moisture or “sweat” moisture through it when there’s a temperature difference.
In a normal situation, the moisture that transfers under the liner will just move away and into the earth and eventually dry or move away from the pool bottom. If something like plastic is placed under the liner though, this moisture can get trapped and have nowhere to go.
Eventually, the moisture will make its way over to where the ground meets the pool wall, and some of it will hang out against the metal wall. If this moisture cannot go anywhere, it will stay next to the pool wall and will soon get past any protective coating the wall has. Once the coating is lifted by the excessive moisture, the metal wall will begin to corrode. This is not at all good.
ONCE METAL BEGINS TO CORRODE, IT DOESN’T STOP
I don’t want to be too scientific here, but I will mention that most things are in a state of either building up or breaking down. This is the main dynamic of everything at least on earth.
When it comes to steel and aluminum (the two metals that above ground pool walls are made of) the rate that they can break down can be greatly increased if there is constant moisture present.
Better quality above ground pool walls are coated to help prevent the beginning of corrosion caused by moisture. This coating can only protect for so long. If the wall is exposed to moisture long enough, then it can penetrate the coating and the metal wall will start corroding.
It’s important to note here that once corrosion begins, it doesn’t stop. You can slow the rate of corrosion down by keeping the metal dry, but it will still be at an elevated rate of breaking down through oxidation.
This is why it’s important to not allow an above ground pool to begin corroding in the first place.
MATERIALS YOU SHOULD NOT PUT UNDERNEATH A POOL LINER
Visqueeen or plastic sheeting
Some will do a great job of completely covering their pool’s bottom with a single piece of plastic. They will even tape the ends to the bottom of the pool wall to help keep it in place for the installing of the liner.
This is a mistake as plastic will trap any moisture from the liner sweating.
Vinyl or poly tarps
These will also disallow moisture from dissipating into the earth. Avoid these.
Note: The less expensive soft-sided Intex/Coleman type of pools come with super cheap tarps that the instructions say to put down just before erecting the pool. This article is not about soft-sided above grounds, They are completely different and you can put down anything you want with them.
The old/existing liner
It’s common for people to want to leave their original liner in the pool and then place the new one on top of it. Yeah, don’t do that.
There are three main reasons not to leave in the old liner. The biggest is the potential for moisture to get trapped (see above). The second is that the old liner may make it hard or impossible to re-smooth the bottom, which is really nice to be able to do during a liner change. The third is that the old liner may cause you to have some wrinkles in your new liner.
Pull that old liner completely out. You’re not gaining anything by leaving it in anyway.
More than twenty years ago, a material called “perlite” was commonly used to make bottoms for above ground swimming pools. I installed maybe a couple of dozen pools with this and it wasn’t a bad material to work with other than its fine dust would make you cough.
What I and other professional installers discovered some years later was that, when applied thick and thoroughly, perlite would trap moisture. After seeing a few pools rust out from having a perlite bottom, I stopped endorsing its use.
NOTE: If you must use the above materials for your pool’s bottom for one reason or another, you still can, but you must keep an open space all the way around the bottom or make holes in the material to allow the moisture from sweating to find its way into the earth.
OTHER MATERIALS YOU SHOULDN’T USE UNDER AN ABOVE GROUND POOL LINER
Occasionally, I will see that someone has put down sheets of plywood under the pool. Then maybe put a layer of sand on top and between it and the liner.
This is not recommended because the wood will eventually breakdown and decompose. And when it does, it will make the pool bottom lumpy and bumpy. Then later, when it’s time to change the liner, it will be a pain to clean out all of that rotted wood and make a nice new pool bottom.
You don’t want to use anything that will quickly decompose in the earth for this reason.
Too much sand
It kills me when I see an above ground pool install manual say that a pool like a 24’ round needs a minimum of 3-4 yards of sand put down. It is true that sometimes this much sand is needed, but in most cases, it’s too much soft material.
In Central Florida (where I install pools), I almost never add sand to a pool bottom. THE REASON – In most yards here, the earth is already mostly sand based. By adding sand to an already soft base will make the bottom too soft and “footprinty”.
Opposite of what most people think, sand doesn’t pack very much. Sand is only made up of aggregate (small grains rock, seashells, minerals), air, and water. You can add water to it and pack it down to help remove the air, but that’s it. It doesn’t pack.
I bring this up because if you have too much sand, you’re more likely to have a bumpier bottom than if you didn’t have as much.
Bad or dirty fill dirt
There have been so many times in my 35 years of installing pools where the homeowner had a truckload of fill dirt dumped in their yard so I can use it for the pool install, but the fill dirt wasn’t clean.
Fill dirt needs to be clean. It can have a bunch of rocks or sticks or construction trash in it. Using dirty fill dirt for an above ground pool is inviting you to have something sharp left under the liner.
NOTE: In almost all cases, a hard base material is NOT needed for installing an above ground pool.
USING FOAM BOARDS TO MAKE A BOTTOM FOR AN ABOVE GROUND POOL
It has become a fairly common practice for people (and some installers) to lay down foam boards under their pools. They will buy as many 4’x8’ sheets of foam. Then duck tape them together to make a smoother and protected pool bottom.
I usually don’t recommend do this. Yep. You heard me right. This is because the foam sheets can trap moisture. After seeing a couple of dozen rusted walls caused by a well-done foam board bottom. I stopped thinking it was a good idea.
If you must use foam boards, you can install them and not cause a moisture barrier. You will just have to have openings that allow moisture to escape into the earth. This is best accomplished by leaving a gap all the way around the pool bottom.
To do this, lay out and tape your foam boards together to a shape smaller than the dimensions of the pool. Afterward, make your pool cove with earth or sand which will cover the gap you have in the boards. This will (hopefully) give room for moisture to escape.
If you are installing a foam board bottom along with foam coving, then you can’t do it this way. You’ll have to make gaps within the pool bottom instead. NOTE: Foam sheeting will NOT prevent nutgrass from growing through the liner.