Cinder Block Hot Tub – A DIYers’ Guide

Can I build a hot tub from Cinder Blocks?

Cinder block hot tub? You absolutely can build a DIY Hot Tub from cinder blocks. In this article I am going to look at some of the important points you need to think about when you embark on your hot tub project.

For me, I think that building a hot tub out of cinder blocks is the easiest way of doing things. It is also the way that I built my own DIY hot tub. Methods that you might have seen on YouTube involving the likes of shotcrete and Gunite are not methods that DIYers such as us can do. These are highly skilled trades, kind of like plastering.

However, the cinder block method of building a DIY hot tub is most definitely something you can do yourself. Even, if like me, you had never laid block before. There is so much information available on YouTube and I have a course that you can take too that will help you with all elements of building a cinder block hot tub.

That said, this article will cover the main points so let’s jump in and get started.

Planning my Cinder Block Hot Tub

Without a plan, you are going to fail. This is certainly something that you cannot just go into blindly. You need to sit down and have a clear plan for what goes where and exactly how your are going to build your cinder block hot tub.

How big is it going to be? This will really be dictated by the available space that you have in your back yard as well as how many people you plan to have in the tub.

To give you some rough ideas, a 6’ x 6’ (1.8m x 1.8m) is probably suitable for 4 people comfortably. Whereas, a 10’ x 10’ (3.05m x 3.05m) which happily have 10 of you in the tub together.

Planning the size of your cinder block hot tub was also dictate how much water it is going to hold. This, in turn will dictate what kind of heater you need. Or, at the very least will give you an idea of how much power you are going to need to heat your tub up to temperature.

The size will also dictate what size of filter you need – again, in most cases just to give you an idea a 50sqft cartridge filter is sufficient for a cinder block built hot tub.

The Layout – Walls and Seats

Part of the planning stage for building your hot tub is deciding where the walls and the seats are going to be. Will you have seating all the way around. What shape will your hot tub be?

Traditional square or rectangle or are you going for a more advanced layout with curves. One of the things to remember is that building curves out of blocks is a different level of skill to just laying blocks in straight lines.

As I said, when I embarked on my own DIY Hot Tub project, I had never laid blocks. A few YouTube videos later and I was well on my way. I am pretty sure that today, I still would not be able to lay a curved structure in blocks – I just don’t have the skills of a professional bricklayer. So keep that in mind.

With your seating, have a think about the height and depth of the seating. What you can do here is that if you are looking to build a large tub but want to reduce the running costs, make your seats a little larger so that they decrease the volume of water.

This was what I did myself as when I initially sketched up my own tub before modifications it was going to need 12KW of electric to heat. That was going to cost me an absolute fortune.

By modifying the design and reducing the volume, I got the volume down to a level where a standard 3KW heater can be used. In the USA, this is a 5.5KW standard as this tends to be the smallest available in the States.

Hot Tub Base for my Cinder Block tub

Once you have cleared the area for your cinder block tub, you are going to need to build a base or a pad for it to sit on. Firstly, you should have 10cm or 4” of crushed rock that you will then pound down flat.

On top of that, you are going to then create a wooden frame to house your pad. Again, this should be a minimum of 6” thick. Using a spirit level, you will make sure your wooden mould is perfectly level.

In that way, when you pour in your concrete, you can then just smooth off to the level of the wood with a straight edge and you know it will be perfectly level.

You are going to want to put some metal rebar in there too. This normally comes in sheets and something with a 10cm or 4” centre will be fine. You also want to make sure that this is lifted off the ground, either with holders or just some rocks. The rebar should fall as close to the center as you can make it.

Rebar Structure

Into your centre blocks, especially if they are the hollow ones, you want to be putting some #3 10mm / 0.4” thick rebar to give your walls some strength.

Once in place, you can then pour concrete into these walls which will hold the rebar in place but also provide some serious strength for your cinder block walls.

Waterproofing

Waterproofing your cinder block structure is an integral part of the build. What I suggest is that you skim over the surface first with a waterproof render mix. Simply add your waterproofing agent into your render mix. This reduces the amount of air that is in the mix which causes the concrete to be porous.

You can pick up your waterproofing additive from any good hardware store.

Your final layer of tiles and grout will provide the ultimate waterproofing measures on your tub.

Insulation

Insulating your cinder block hot tub is key. If you don’t, it is going to cost you a lot to run. The simplest way of adding insulation is using the structured insulation boards which are foil backed. These you can get in 4” / 10cm thicknesses which are ideal for going in-between the plumbing.

With a cinder block hot tub, you are either going to want this to be in ground in which case you can backfill the walls with cement when you are finished with the plumbing. Alternatively, you will want to erect some form of a second skin or wall to cover your plumbing. That way, you can finish it off with tiles or cladding on the outside.

Cinder Block Hot Tub Plumbing

The plumbing for your cinder block hot tub is no different from the plumbing for any other block or concrete hot tub.

You need to think about where you will house your heater, pump, blower and filter. This is what I like to call the control room.

You need to decide from the start if you are going to have your control room above or below the water level. This then dictates the type of pump you need.

You will need to work out how far your control room is going to be from the tub. This can dramatically decrease the flow rates coming from your pump so you need to make sure that you have the correct calculations in place.

Next, how many jets are you going to have. If you are a regular on my YouTube channel or on this blog, you will have seen that I talk a lot about 16 jets being the maximum in most cases that you can have on a single pump.

If you want more than this, then we need to have a look at the plumbing design and have a multiple pump setup.

You blower, where is that going to be located?

In order to attach your jets, you are going to need to core some holes for the 2.5” pipe to come in from the Gunite bodies.

I don’t want to go into too much detail in this post, but I do have detailed plumbing posts on this blog. There are quite a few things to consider and think about with the plumbing but it is certainly an area that I can help you with.

Can I help you with your Cinder Block Hot Tub?

I can certainly help you with your cinder block hot tub. Getting the design right from the start is key to any successful project and this will be no different.

The key here as I see it is the plumbing layouts and working out exactly how you want it to function.

Everything is possible, we just must plan and design for it accordingly. Making dramatic changes later on is always difficult and this should be avoided.

Please do get in touch if you would like my help with the design.

Want to learn how to build your own DIY Hot Tub – https://buildahottub.com/diy-hot-tub-course/ 

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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 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 1000 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