Choosing an appropriate base is one of the most important parts of planning your hot tub installation. The combined weight of the tub, water, and occupants can reach up to 6,000 lbs, so it's imperative that you have a stable support structure under it.
But with so many different hot tub foundation options available, how do you choose the right one?
This guide will outline the most common base types, their strengths and weaknesses, and the ideal scenarios for each. It will also provide information on the installation procedures, and compare the bases on the most important factors such as ease of installation, cost, appearance, and durability.
1. Concrete (best in most cases)
Cost for 8'x8' spa base: $400-$500 (around $8 per square foot)
A concrete pad is one of the most common types of hot tub foundation. This is what most manufacturers or hot tub stores will recommend.
Concrete is a strong and durable choice for a hot tub base, but it is permanent. You'll need to hire contractors to install the pad (unless you’re handy enough to do this yourself) and be comfortable with the fact that your hot tub's location won't be able to change easily once it's in place.
Depending on where you live, you may also need a permit to install a concrete slab, as it may be considered a permanent change to your property.
How to build a concrete pad for a hot tub
For most DIY concrete slabs, the best material to use is a ready-mix crack-resistant concrete mix like QUIKRETE.
A concrete base for a spa should be around 4 inches thick. This means that for an 8'x8' spa pad, you'll need enough concrete to fill 21.33 cubic feet (that’s 36 80-lb bags of QUIKRETE).
The first step is to build a frame made from 2-by-4 lumber. You can also add a grid of Size #3 Rebar for extra stability and to stop any cracks from widening, which is recommended, but not strictly necessary. You can see an example of that here:
You can either pour the concrete directly onto dirt, or lay 4 inches of gravel down first. The concrete then needs to be poured into the frame, screeded, finished, and left to cure. Follow the manufacturer's instructions to know exactly when it’s safe to remove the frame.
What are the downsides of a concrete hot tub base?
Aside from being a permanent fixture in your yard, concrete isn't necessarily as durable as you might think if not installed and cured properly. It can also be susceptible to cracks, particularly if you have a lot of temperature variation where you live.
Why does concrete crack? Concrete absorbs moisture from the environment. In very cold temperatures, this moisture freezes and expands. Sometimes this is enough to cause cracks itself, but the thawing that occurs after is often more commonly the problem. When the ice melts, this causes shrinkage, which can result in cracks in the concrete.
Concrete is also not the most attractive choice if you care about aesthetics. If you want the stability of concrete, but are not a fan of the look, you can always cover it with something to give it a different feel. This could be something like paint, pavers, or something less permanent like these outdoor hardwood teak interlocking floor tiles:
Another alternative is to build the concrete pad just large enough so that the hot tub covers it completely. Then you won't even see it once your spa is in place.
2. Gravel (best for durability)
Cost for 8'x8' spa base: $250-$450 ($4 to $7 per square foot)
Gravel is an excellent choice for a hot tub base. It's relatively cost-effective and easy to install, plus it's very good for drainage. This is important because you need to make sure any rain water or overspill from your hot tub is able to drain away from the base. And unlike concrete, gravel won't crack over time.
How to build a gravel pad for a hot tub
You can't just place a hot tub on loose gravel; you need to build a wooden frame to contain it first. Then, calculate how many cubic yards of gravel you'll need, accounting for at least 4 inches of gravel. Finally, you pour the gravel into the frame and tamp it down to compact it until it forms a solid base.
Site Prep produced a very detailed guide on how to build a gravel base. It's for a shed, but the basic idea is the same:
What type of gravel is best for a hot tub base?
Although it's often called a gravel pad, it's important that you use crushed stone or gravel, and not pea gravel or river stone. Actual gravel is smooth and rounded which means it won't lock into place properly, and will be able to shift around under your spa. Crushed gravel on the other hand is jagged and textured, which is what allows it to form a solid base when compacted.
3. Pavers/Patio (best for aesthetics)
Cost for 8'x8' spa base: $600-$1,000 ($10 to $17 per square foot)
If you have an existing patio, this can be a great place to put a hot tub—as long as it's big enough. If not, but you do have space elsewhere, laying pavers can be a great option (and often more customizable than concrete).
Pavers are stone or concrete slabs, often square, which are usually around 1-2 feet in size (though you can also buy them in all kinds of custom shapes and sizes). You need to decide on a design, do some math to calculate how you're going to cover the area, then place as many pavers as you need on the ground next to each other to form the base for your hot tub.
You have a lot of options when it comes to the style and design of pavers, and the good news is they don't need concrete to sit on. That said, there is still base prep work to be done before you can actually lay the pavers. We'll take a look at that next.
How to install pavers for a hot tub base
When you need to support a heavy object like a hot tub, you’ll need to choose thicker paving stones—look for ones at least 2 inches thick.
And just like with a concrete slab, you should check local building codes to see if you need a permit to build a hot tub base using pavers. This will also be considered a permanent installation on your property, at least in much of the United States.
Typically, it's not a good idea to lay pavers directly on dirt. For pavers to last, the area under them should be excavated, leveled, and compacted.
Once you've chosen your pavers and come up with a design for your hot tub base, you need to build up several layers before you're ready to put the actual pavers down. This typically means a 6-inch layer of gravel, a 1-inch layer of sand, then the pavers themselves. Finally, you'll need to add joining sand between the pavers to keep them evenly spaced.
The full process is beyond the scope of this article, but you can see more information on how to build a paver patio in this guide from Lowe’s.
Overall, you can expect the process of building a paver patio to be a little more involved than building a concrete slab, but you'll end up with a supportive and attractive base for your hot tub.
Bear in mind though, if you're not actually planning to show much (or any) base around the edges of your hot tub, building a pretty design with pavers may not be worth your time as most of the finished result won't be visible under the spa.
4. Synthetic Hot Tub Pad (best for portability)
Cost for 8'x8' spa base: $595-$600
These are prefabricated hot tub bases made from interlocking squares of plastic, about 2 inches thick, and designed to be strong enough to support the weight of a spa. They are also modular, which means they will work for larger spas, or even swim spas.
However, bear in mind that the smallest size pad they can form is going to be 8 by 8 feet. That means that if you have a smaller spa, you'll have some visible plastic pad around the edge. This isn't necessarily a bad thing, but just know that you can tell they are plastic on close inspection, and you may not care for this aesthetic in your yard.
I personally have one of these, and I've been very happy with it.
Synthetic spa pads are a very cost-effective type of hot tub base. They're also pretty hard wearing, and won't crack like concrete can.
The real benefit of these is their temporary nature though. This makes them the perfect choice if you live in rented accommodation or cannot make a permanent change to your property for whatever reason—or if you want to be able to take the spa pad with you when you move.
However, you do still need an absolutely flat surface to lay the pad on, otherwise the plastic can bow and won't be able to support your spa properly.
How to install a spa pad
When it comes to the easiest spa base to install, ready-made spa pads win, hands down.
All you need is a screwdriver to attach the different sections of the spa pad together, and then provided you have a flat area, you can literally place the spa pad on top of any kind of surface: grass, dirt, patio, gravel, concrete, you name it.
However, if you don’t have a flat surface, you will either need to excavate an area for the spa pad to sit in, or you can fill dips with gravel until you have a level place for the spa pad to go.
5. Deck (best for the view)
Cost for 8'x8' spa base: $0-$1,000+ (this varies a lot depending on exactly what work you need)
Decks are a really popular feature to combine with hot tubs. Whether you place the hot tub on the deck or recess it into it, it's great to be able to access the hot tub easily from the deck where you already hang out.
While placing your hot tub on a deck is a great idea in many ways, there are several considerations that go into doing this safely.
For example, it is critical to make sure that the deck is capable of supporting the weight of the hot tub. This usually involves hiring a structural engineer. If they determine that the deck is not strong enough, there are ways to reinforce it with extra joists in the area where the hot tub will go.
But getting this work done will obviously add to the installation costs, so it's something you'll need to factor into your budget for the project.
If you're interested in recessing your hot tub or framing it within a deck instead, the deck itself won't have to support the weight of the spa. In this case, people usually cut or build an opening in the deck, and pour a concrete slab under the area where the spa will sit.
Then, you'll have to find hot tub movers who are able to safely get the spa into the opening.
Do hot tubs need to be perfectly level?
It’s important that a hot tub is installed on a completely flat surface, but a very minor slope is acceptable. This is because you need to ensure the weight of the spa is distributed evenly, so that the frame doesn’t shift or bow once it’s filled, or when it’s being used. If this happens, you risk damaging the spa and voiding the warranty.
A flat surface is also important so that you don’t have dips under or around your spa where pools of water can form. You don’t want stagnant water forming around your hot tub where mosquitoes could breed.
What is the maximum slope for a hot tub?
As a general guideline, the maximum slope for a hot tub should not exceed 1/8 inch per foot, which translates to a total of 1 inch over the course of 8 feet (a little larger than the footprint of most hot tubs).
This slope is considered safe, and will not cause any damage to the structure of the tub. However, for a more visually pleasing appearance, you may prefer to stick to a maximum slope of 1/16 inch per foot, which will result in a more even waterline.
It's important to note that even though slopes of up to 2 inches across the width of the hot tub are technically acceptable, this will result in quite a noticeable slope of the waterline, which many people find unappealing. This is one reason it's generally recommended to keep any slope to a minimum.
If you are dealing with a slope that's outside of these guidelines, I have another post which outlines some common methods for leveling your spa pad area.
What's the best base for an inflatable spa?
You'll be pleased to know that all the above bases will work for any kind of hot tub—inflatable spas included (yes, even gravel).
But as an inflatable spa owner, you also have an extra choice available to you: you can put an inflatable hot tub directly on grass. Bear in mind that this will not be the most energy efficient choice though, due to losing heat through the ground. What's more, if you leave the spa in place for a long time, you will kill the patch of grass underneath.
Something to bear in mind in general when it comes to inflatable spas is that they have less insulation on the underside to stop heat escaping into the base than then a regular spa, where the shell is raised off the ground.
You can help to offset this problem by investing in a thermal spa pad to put down as a barrier, which will minimize the heat loss. This can help with energy costs, as well as reduce the wear and tear on your spa pump by making it work less hard. A great choice for round inflatable spas is the CosySpa Hot Tub Flooring Protector:
Can a hot tub be placed directly on the ground?
In most cases, you cannot place a hot tub directly on the ground, whether on dirt or grass. It's just not going to be level enough to support your spa evenly.
However, there is one exception: inflatable spas don’t have a frame, and are therefore not picky about being placed on an even surface.
There are other reasons you might not want to place an inflatable spa directly on the ground, such as damage to underlying grass, scratches or punctures on the spa itself, or excessive heat loss. But if you don't have any other choice, it can be helpful to know that it is at least an option.
Can you put a hot tub on artificial grass?
Just like with real grass, placing a hot tub directly on artificial grass would result in permanent flattening of the grass fibers—and potentially cause damage to the hot tub, depending on the surface beneath.
If your artificial grass was laid over concrete, this would be fine, as long as you didn't mind damaging that patch of artificial lawn. However, if it is just on dirt, the ground can sink or shift under the weight of the hot tub, resulting in an uneven surface. This can cause the hot tub to warp, which is something we want to avoid.
In most cases, it's better to provide a solid and level base, such as concrete, gravel, or decking, to ensure proper weight distribution and prevent damage to both the hot tub and the artificial grass.