How to build a Concrete Slab or base for your Hot Tub?

Hot Tub Concrete Base

If you’re wondering how to build a concrete slab or base for your hot tub, this step-by-step guide is the resource you’ve been looking for! There are many benefits of having a hot tub in your backyard. It is the perfect place to relax after a long day, and it can be used for entertaining guests as well.

However, before you get started with installing your new hot tub, on indeed building one, you have to make sure that you have done all of the proper prep work too! The first thing that must be taken care of is building the base for the hot tub. This post will cover how to build a concrete base for a hot tub so that everything else goes smoothly from here on out! Whether you are building one or buying one, you need something for it to sit on!

Measuring Up for your Base

First, you will need to measure the area where your hot tub is going. This includes measuring how deep and wide it needs be so that there are no issues with stability or safety later on. No one wants their new hot tub to collapse, so you have be sure that it is sturdy enough for the job! You also might want to consider where you are going to place the hot tub cover when it is removed. They are quite large so it is a good idea to have a space ear marked for where it will sit whilst you are in your tub.

The next step is to decide what kind of foundation you would like. If your budget allows, a poured concrete slab will be the best option for stability and longevity! This can also help with drainage issues as well since it makes water pooling less likely than if there were just dirt or other materials underfoot; this could lead over time too subsidence of your tub into the ground which we clearly want to avoid.

Clear the Area for a Concrete Slab or Base for your Hot Tub

Next up: make sure all of this space has been cleared out for construction purposes too (this means removing any trees/bushes from around). It’s important not just because they can get damaged during installation but also due their roots getting into concrete which could cause problems over time as well…and we don’t want anything like a tree root coming through our new flooring now do we? 😉

Ok, so once the area has been cleared, next it ‘s time to start building the foundation for your hot tub. The first thing that you need to do is to cover the area with a type one ballast or small gravel that you will then pound flat with a “whacker” or similar machine.

Gravel Base

Once the gravel has been pounded down, the next stage is to create a form or mould, which will be made out of wood and then filled with concrete for your slab or base for your hot tub. The form will need to be a little bit larger than the hot tub you are installing or building. When you make the form, it is essential that it is perfectly level. In that way, when you pour the concrete, you know when it is full to the top of your form it will be level and not sloped.

Build a Hot Tub Base

Wood Frame & Waterproof Membrane

The next step is to add a waterproof membrane to the form. This will ensure that the concrete does not leak out and ruin your deck or patio. The membrane can be made of a variety materials, but it is best to use one with high-density polyethylene (HDPE) as this material has excellent resistance against water damage.

Build a Hot Tub Base

Placing a membrane beneath a concrete pad serves multiple important purposes, one of which is preventing the ingress of dirty water. Here’s a detailed explanation:

Waterproofing and Protection:

A concrete pad membrane, often referred to as a moisture barrier or waterproofing membrane, plays a crucial role in safeguarding the area beneath the concrete from water infiltration. Concrete is a porous material, which means it can absorb water and moisture from the surrounding environment. Without proper protection, this can lead to various issues, including:

  1. Dirty Water Ingress: When rainwater, groundwater, or even surface runoff carries dirt, sediment, or contaminants, it can seep through the concrete and accumulate beneath it. Over time, this can create a muddy or even waterlogged environment, making the area unsuitable for various purposes, such as building foundations, flooring, or even hot tub installations.

  2. Damage to Concrete: Excessive moisture can weaken concrete structures and cause cracks, spalling, or erosion. Additionally, freezing and thawing cycles in colder climates can further exacerbate concrete damage, leading to costly repairs or replacements.

By installing a membrane beneath the concrete pad, you create a waterproof barrier that effectively blocks the ingress of dirty water, preventing the issues mentioned above. This protective layer ensures that the area remains structurally sound, clean, and suitable for its intended use, whether it’s supporting a hot tub, serving as a foundation, or providing a stable surface for other purposes.

In essence, the membrane acts as a shield against moisture and contaminants, preserving the integrity and functionality of the concrete pad over the long term.

Now you are ready to add a metal rebar constriction to your form. This will give the concrete a sturdy and strong structure. The rebar should be placed every two feet around your form, with one end of each piece sticking out about six inches from where it is inserted into to make sure that they do not bend when you pour in the concrete.

Metal Rebar for your Hot Tub Slab

Rebar can be purchased in sheets as you can see above. This makes it much easier to lay. It should be kept off the ground so that the concrete can flow underneath it when you pour.

When constructing a hot tub concrete slab, the incorporation of rebar becomes a vital aspect of the process to ensure the structural integrity and longevity of the foundation. Rebar, made from steel, is an essential reinforcing material that adds tensile strength to the concrete, preventing it from cracking and enhancing its load-bearing capacity.

The primary purpose of rebar in a hot tub concrete slab is to distribute and withstand the pressure and weight exerted by the hot tub, especially when it is filled with water and occupants. The weight of the water and people inside the hot tub can put significant stress on the concrete slab. Without proper reinforcement, the slab could develop cracks and compromise the stability of the entire structure over time.

To effectively utilize rebar in the concrete slab, it is commonly laid in a grid pattern before the concrete is poured. This grid pattern consists of intersecting horizontal and vertical bars, forming a mesh-like reinforcement throughout the slab. The size and spacing of the rebar depend on the dimensions of the hot tub and the anticipated load it will bear.

By incorporating rebar in the concrete slab, several benefits are achieved:

  1. Increased Strength: Rebar reinforces the concrete, providing additional tensile strength and making it more resistant to bending or flexing under pressure.

  2. Crack Prevention: The presence of rebar helps distribute loads evenly, reducing the risk of cracks forming in the concrete, even under heavy loads.

  3. Enhanced Durability: With rebar reinforcement, the hot tub concrete slab gains increased resistance to wear and tear, resulting in a longer-lasting foundation for your hot tub.

  4. Improved Load Distribution: Rebar evenly distributes the weight of the hot tub, ensuring that the load is adequately supported throughout the entire slab.

To further ensure the strength and durability of the hot tub concrete slab, proper concrete mix design is essential. The concrete mix should be appropriately proportioned, and the placement and compaction of the concrete must be carried out with precision.

In addition to rebar, other reinforcing methods, such as fiber-reinforced concrete, can also be used to complement the structural integrity of the slab. The combination of these reinforcing materials ensures that the hot tub concrete slab remains robust and resistant to various environmental factors, such as temperature changes and ground movement.

The next step is to mix your cement. You will need a wheelbarrow, shovels and hoes. Better still a cement mixed will save you some back ache and conservable amount of time.

Build a Hot Tub Base

Cement Mixing

You should mix the cement in batches of about 50 pounds at time to make sure that it is mixed thoroughly before you add more water or sand as this can cause lumps if not done properly . You want your concrete mixture wet enough so when squeezed together with hands there are no visible dry patches but also firm like cookie dough without being too sticky where they stick on each other’s fingers (this means adding just right amount of sand and water). The ratio for mixing mortar usually ranges from one part cement to one and a half parts sand.

The next step in building an outdoor hot tub base is pouring cement into the form until full level on top. To get a perfectly level finish you can use a trowel to smooth the surface of your concrete.

When you are done, cover it with plastic and let dry for at least 24 hours before removing the form or adding any water . You can also use an electric sander if desired but be careful not too sand off all that hard work!

Build a Hot Tub Base

Build a Hot Tub Base

Remove the Wooden Frame / Mold

The next day remove from mold by lifting up on one side then pulling out gently while using other hand as support under base until fully removed (be sure there is no excess cement left in corners). If needed add more mortar mix into corner gaps so they don’t crack later when drying completely which could cause leaks around edges where hot tub sits against wall/ground over time due lack proper waterproofing sealant applied during construction process;

It is highly advisable that you add waterproofing agent to your cement mix before pouring the base. Concrete as a material by nature is porous. This means it is not waterproof. You should try and make the concrete waterproof by adding a waterproofing agent into the mix. This will help prevent cracks later down the line.

The best way is adding a waterproofing agent like “Sika Sarnafil” (available at most hardware stores) into your cement mix before you pour it, but if not available then just apply some after finishing concrete has dried completely for 24 hours . It’s important because this step helps protect against water seeping through from below ground level as well where hot tub sits.

In cold areas, if this water freezes your base could crack!

This was the exact process that Neil followed – you can read all about his build here.

Looking for a DIY Hot Tub plan or design – check out the shop!

Can I Help You?

If I can help you in any way I would love to hear from you. You can get in touch using the form below.

Thanks - Andi


Hi, Andi here. I own and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Today, I've helped over 900 DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi

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