There are a number of ways in which you can construct your walls for a DIY Hot Tub. The designs that are available in my store are all valid of each of the types of construction I describe below. In this article I am going to focus on the different types of wall construction you can opt for. Before we go any further, I must state I am not a structural engineer and this article has been written from experience rather than qualification.
Pouring a form is where you are creating a mould out of wood and then filling the mould with concrete to create your walls and seats. This is a popular way of constructing a hot tub. If you do opt for this method, make sure that you brace the walls correctly as the last thing that you want is to have curved walls when you are done!
There is a huge amount of weight and pressure that is created when you pour the concrete so you need a lot of bracing in order for this to work. The end result is a perfectly cast hot tub shape. This is a great way of doing things as you are embedding all your plumbing in advance so fitting the plumbing itself is easier.
You also get a good seal around all of the plumbing components so there are less chances of leaks when you use this method.
Concrete Block Cavity
One of the challenges of building a DIY Hot Tub or Plunge Pool is getting the correct strength in the walls.
Concrete blocks are arguable the easiest way of building your hot tub. However, if you do a concrete block construction, you are going to want to have a cavity. This cavity will house not only the plumbing, but also the insulation and a rebar construction for added strength.
The downside to this method is that the final construction is quite wide, you can see the measurements below.
With a 4” block, 6” cavity and then another 4” block, you are at 14” as a minimum. With a. cap stone on this, this can look really good. Inside of the cavity you are going to house your plumbing, your insulation and your rebar. As shown below.
To complete the construction you will then fill the cavity with concrete and make sure it is well vibrated down.
CMU blocks are the hollow concrete building blocks you commonly see in construction. They are cheap to buy and do a really good job of creating a strong structure for the hot tub.
You can see on the cross section drawing below that the metal rebar ties into the poured concrete base and also the walls too.
You should be rebaring every other hole so that all the blocks are tied into each other.
Once you have finished the structure, then you need to fill with concrete. The diagram also shows horizontal ties – these are optional.
Insulating concrete form or insulated concrete form (ICF) is a system of formwork for reinforced concrete usually made with a rigid thermal insulation that stays in place as a permanent interior and exterior substrate for walls, floors, and roofs.
The forms are interlocking modular units that are dry-stacked (without mortar) and filled with concrete. The units lock together somewhat like Lego bricks and create a form for the structural walls or floors of a building. ICF construction has become commonplace for both low rise commercial and high performance residential construction as more stringent energy efficiency and natural disaster resistant building codes are adopted.
This is a quick and easy way of creating your hot tub or plunge pool. The structure and the strength are already there for you. The main downside is the cost. This is an expensive way of doing things.
In ground Cavity
I’ve done a number of projects of late where there has already been a hole in the ground that the customer wanted to convert into a hot tub. This is totally fine of course. Perhaps there has been a sunken plastic shell tub that you are replacing – this is when this kind of a method would come to light.
Firstly, you need to remember that there will be plumbing and the minimum size for the Gunite Body fixtures would be 6” (150mm) cavity. You would then be able to fit insulation around the pipes and have a rebar structure inside of the cavity which you would then fill with concrete for the final stage.
The drawing below shows what the construction would look like. You can see that there is a 6” cavity with a 4” block. The cavity will be filled with concrete once the plumbing is in place.
Which Method Do I opt for?
Ultimately, this is down to personal preference, material availability, skill set and budget. All of these methods I have highlighted above produce a fantastic end result. The hot tub that you are dreaming of! However, each has their challenges so ultimately, you will pick the method that best suits you, your location, skillset and budget.
Happy Hot Tubbin’
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Thanks - Andi
Hi, Andi here. I own Buildahottub.com 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