DIY Hot Tub Construction: Block, ICF or Poured Concrete?

Last updated: September 2026

When I set out to build my own hot tub, I couldn’t find the information I needed anywhere. I researched spas, hot tubs, swim spas and plunge pools, and most of what I found was either written for professional pool builders or trying to sell me a plastic shell. That’s why buildahottub.com exists. My mission has always been to be the most helpful resource on the internet for anyone who wants to build their own.

The good news is that DIY hot tub construction is nowhere near as difficult as it looks. You do need some basic DIY skills, and you need a plan, but I’ve seen time and again (just look at the case studies on this site) that ordinary people can build a tub that looks and feels like one costing several times as much. This guide covers the construction methods that are actually within reach of a DIYer, and the ones that aren’t.

Why build your own?

Mostly, the cost. The prices commercial pool builders charge can be eye-watering. Two of my case-study customers, Ray and Kyle, were quoted huge sums. Ray’s builders wanted around $45,000, and he built his for around $12,000. Kyle, because of his rural location, was quoted more than $100,000. Yes, you read that correctly.

A few years ago, most of the people I helped built their whole project for under $10,000. Prices have gone up since, and the final figure depends on size, finishes, heating and how much you do yourself, so treat that as a ballpark. If budget is the big driver, see What is the cheapest hot tub to build?

That’s my own tub above. I absolutely love it, even if I’d build it a little differently today.

Gunite and shotcrete: not DIY methods

If you’ve watched pool builds on YouTube, you’ll have seen concrete being sprayed at high pressure onto a cage of rebar, then trimmed and troweled in place. That’s shotcrete. “Gunite” is the older name for the dry-mix version, where the water is added at the nozzle. The American Shotcrete Association explains that shotcrete is the umbrella term for both the dry-mix and wet-mix processes.

Either way, it’s not a DIY method. Think about plastering. A DIY plastering job might be okay, but it’s never as good as when a pro does it. Spraying a pool shell is the same, only with a compressor, a pump or gun, and a crew who do it every day. Leave the kit aside and it’s still the skill that makes the difference. I’ve written more on this in 2 alternatives to gunite or shotcrete.

The three DIY construction methods compared

There are three methods open to DIYers: block, poured concrete and insulated concrete forms (ICF), which is the method I recommend to most people.

Method	Best for	Watch out for
Concrete block (CMU / cinder block)	Square and rectangular tubs. Cheap, familiar and easy to buy anywhere.	Slow if you’ve never laid blocks. Needs rebar in the cores, separate insulation, and render before the membrane.
ICF (insulated concrete forms)	Most DIY builds. Light, quick, easy to cut for plumbing, and insulated in one go.	Needs bracing and a careful concrete pour. Curves are possible but need planning.
Poured concrete in timber forms	Curves, unusual shapes and plumbing cast inside the walls.	The most formwork and bracing. A blow-out mid-pour is a very bad day.
Gunite / shotcrete	Professional pool builders.	Not a DIY method. Hire a specialist crew if you want it.

There’s a deeper comparison in Hot tub wall construction: what are the options?

Step 1: Plan it properly

The planning has to be one of the most important parts. Without a good plan, things go wrong in one of two ways. Either your hot tub doesn’t work properly, or it costs you a small fortune to get someone in to “fix” what you’ve built. Neither is a good option.

Before you start, you want to know:

• The internal size, water depth, seat heights and wall method. Together these decide how big and deep the hole is.
• The plumbing layout: main drain, skimmer, jets, returns and the route to the control room. See my DIY hot tub plumbing guide.
• Where the power comes from, so you can lay ducts before the concrete goes in.
• Permissions. In England a garden hot tub typically doesn’t need planning permission unless you’re in a listed building or conservation area, says the Planning Portal, but the electrics must meet Building Regulations. In the USA, many areas want a permit for an in-ground spa, so check with your local building department.

This is what my DIY hot tub plans are for. They’re the same process I use to design tubs for customers: CAD drawings with every dimension, the plumbing layout, a shopping list and a video course. If you want something bespoke, I can design it for you and guide you through the build.

Step 2: Clear the site and dig

Clear the area of trees, shrubs and roots. The last thing you want is roots coming up under your tub. Before you dig, check for buried services. In the UK use the free LinesearchbeforeUdig service, and in the USA call 811 a few business days ahead.

If you’re going in-ground or partly in-ground, hire a mini or micro digger if it’ll fit. I couldn’t get one into my garden, so I dug mine by hand. It was well worth it in the long run, but it was back-breaking. Remember that the hole is deeper than the water. You have to allow for the sub-base, slab, insulation, render, membrane, tiles and freeboard. My in-ground hot tub build guide has a full layer-by-layer depth table and my own digging story.

Step 3: The base (the same for every method)

All the methods start with a solid, level, reinforced concrete base. Here’s the short version of what I’d do. The full detail is in the build guide.

1. Sub-base. 100–150mm (4–6in) of crushed rock (Type 1 or MOT in the UK), compacted with a plate compactor.
2. Sand blinding and a DPM. A proper 1200-gauge polythene damp-proof membrane, lapped and taped, and turned up the sides.
3. Insulation. Compression-rated rigid board under the slab if the tub is in-ground.
4. Rebar or mesh on spacers, so the steel ends up in the middle of the slab and not on the bottom. My rebar guide suggests around 12in (300mm) centres for most tubs. Leave starter bars sticking up where the walls go.
5. Everything that goes through the floor, before you pour. That means the main drain, a hydrostatic relief valve, and sleeves for any pipes or cables.
6. Pour the slab in one go. Use ready-mix C25 or C30 concrete, about 150mm (6in) thick for an in-ground tub, and keep it damp while it cures.

Don’t treat it like a patio pad. A simple pad in a timber frame is fine for a shop-bought tub sitting on top. A built tub is different, because the slab is the floor of a vessel holding a few tonnes of water. So it needs the DPM, the drain and relief valve cast in, and starter bars to tie the walls in.

Method 1: Block-built DIY hot tub

Having completed lots of projects with customers, as well as my own tub, I still think block is one of the easiest methods for a square or rectangular tub. It’s how I built mine, and I’d never laid a block before. I cover the details in Cinder block hot tub: a DIYer’s guide, but here’s the process.

Outer walls

Lay the outer walls around the perimeter first. It’s the same whether you’re building above or below ground. For the structural walls, use hollow blocks (8in CMU in the USA, or 215mm hollow blocks in the UK). Drop vertical rebar into the cores, tie it to the starter bars from the slab, and fill the cores with concrete. Don’t use 4in blocks for these: there isn’t really room for rebar and concrete in them, so save the thinner blocks for seat fronts and non-structural skins.

Block is cheap, durable, available everywhere, and easy to cut and core for plumbing. Blocks have good thermal mass, but don’t count on that to keep the water warm. Thermal mass stores heat, but it isn’t insulation. A block wall against cold ground will happily soak heat out of your water, so it needs insulating. See DIY hot tub insulation.

Seats, steps and plumbing

Once the outer walls are up, build the retaining walls for the seats and steps. Then add the plumbing. You’ll core holes in the blocks so your gunite jet bodies can go into the blockwork. In the build below, the lower jets come in from the outside wall and run all the way through the seats into the tub.

With the plumbing in place (and pressure tested before anything gets buried), and some insulation too, you can backfill the seats and steps with concrete.

Then do the same on the seat backs: core the holes, fit the gunite bodies for the jets and connect up the pipework.

If your tub is partly above ground, you’ll need a second skin of block or brick outside to hide the pipework, which you can then clad or render. On a fully in-ground tub, you generally don’t need that.

Here’s a finished block-built tub. It has all the look and feel of a tub costing up to five times as much.

Method 2: ICF (my pick today)

If I built my tub again, I’d use ICF without hesitation. ICF blocks are hollow polystyrene forms that dry-stack like Lego for adults. You put rebar inside and fill them with concrete, and you end up with a reinforced concrete wall with insulation already on both faces. They’re light enough for one person to handle, easy to cut with a saw, and simple to run plumbing through.

The two things to get right are bracing (the forms need to be held straight and plumb while you pour) and the pour itself. Fill in lifts, don’t dump a full height of concrete in one spot, and follow the manufacturer’s guidance. There’s more in What are the best ICF blocks for building a hot tub? and What I’d change if I built my DIY hot tub again.

Method 3: Poured concrete DIY hot tub

Pouring concrete walls is a bit more involved than block. The big difference is the preparation of the rebar cage, because the steel is what gives the concrete its strength and stops it cracking.

The rebar cage

In most cases people use #3 rebar, which is 3/8in (about 10mm) thick, and build what looks like a wire frame of the tub. The rebar isn’t just in the base. It’s in the walls, the seats and the steps too. Typical residential pool practice is a grid at around 12in (300mm) centres, with tighter spacing at stress points, and with about 3in of concrete cover between the steel and the earth. Your plans or engineer set the final spec.

Formwork and plumbing

Next comes the form (or mould). This is timber or ply on each side of the rebar, so the steel ends up in the centre of the wall. Start with the inner form. The big difference from a block tub is that most of the plumbing is cast inside the walls, so you tie the pipes and jet bodies to the rebar before you close up the outer form. If the plumbing goes in the wall, allow for a wall of at least 10in (250mm) thick to fit the gunite bodies.

Pressure test the plumbing now, while you can still get at it. Once the concrete goes in, there’s no second chance.

Bracing and pouring

Wet concrete puts a huge amount of pressure on the forms, so brace them with as much structural timber as you can. If you’re not sure you have enough, add more. The last thing you want is the forms flexing and leaving you with a wall that isn’t flat and vertical.

Order ready-mix and pour in one go. You’ll get a far more uniform wall than mixing by hand, and you avoid cold joints, which are weak spots where leaks start. Pour in layers and use a poker vibrator (easy to hire) to get rid of voids, especially around pipes and jet bodies.

Striking the forms

This is where it gets exciting, because you can finally see what you’ve made. After the concrete has cured for the right time (usually several days), remove the forms. There are normally a few areas that need rubbing down or patching before you move on to waterproofing.

What customers who poured their walls told me

Customers who’ve poured their walls have been really positive. They liked the consistent, professional-looking walls, being able to build insulation in during the pour, and how neatly the plumbing could be cast into the structure.

Here’s a finished poured concrete tub:

Waterproofing and tiling (all methods)

I used to rely on a waterproofing additive in the render or concrete, then tiles and grout. Integral waterproofers reduce how much water the concrete absorbs, but tiles and cement grout aren’t waterproof, and additives on their own aren’t a waterproofing system. Here’s what I’d do instead:

1. Waterproof render on block walls (a scratch coat and a float coat) for a flat, sound surface. On poured or ICF walls, patch and smooth the concrete.
2. A pool-rated waterproofing membrane in two coats. In the UK and Europe, Ardex S 7 Plus is made for swimming pools. In the USA, Laticrete Hydro Ban is approved for in-ground pools. Hydro Ban only holds water in from the inside, not out from behind, so the hydrostatic relief valve matters.
3. A flood test before you tile. Fill it, mark the level and leave it. Fixing a leak now is far easier than behind finished tiles.
4. Tiles on a flexible, pool-grade adhesive. They look so much better than a painted finish. They cost more, but they’re worth saving up for. I’d now choose mosaic pool tiles every time.

See Waterproofing, grout and tile adhesive for the detail.

Common construction mistakes

• Digging to the water depth instead of the water depth plus the build-up, so the seats end up too shallow.
• Using a mortar mix for the slab. A 4:1 sand and cement mix is a mortar or screed, not structural concrete. I did this on my own build, and I wouldn’t again.
• Forgetting the hydrostatic relief valve or the sleeves before the pour.
• Not pressure testing the plumbing before it’s buried in concrete.
• Skipping insulation, or relying on tiles and grout to keep the water in.
• A control room that’s too small. Mine was, and I’ve had to rebuild it since.

There’s more in Avoid these 7 DIY hot tub mistakes.

So which method should you choose?

For a simple square or rectangular tub on a tight budget, block is perfectly good. It’s what I did. If you want curves, or plumbing cast inside the walls, pour it. For most people building today, though, I’d go for ICF, because you get structure and insulation in one easy step. All three are perfectly achievable for a DIYer, and I’ll leave it to you to decide which suits you and your garden or backyard best.

Planning your own build?

If one of the standard sizes works for you, my hot tub plans give you the dimensions, plumbing and shopping list in CAD, plus the video course. They come in sizes from a compact 7ft x 7ft or 8ft x 8ft up to a 12ft x 12ft, with metric versions too.

If your space is an odd shape, try the free Tub & Pool Designer Pro software, and I can then turn your design into architectural CAD and a 3D plumbing design. For parts, visit parts4tubs.co.uk in the UK or parts4tubs.com in the USA.

FAQs

What is the easiest way to build a DIY hot tub?

For a square or rectangular tub, concrete block or ICF on a reinforced slab. Both use materials you can buy locally and skills you can learn. I’d pick ICF because it’s quicker and already insulated.

Can I use gunite or shotcrete for a DIY hot tub?

Not realistically as a DIY job. It needs specialist equipment and a skilled crew. You can hire a crew to shoot the shell and do everything else yourself, but most DIYers use block, ICF or poured walls instead.

How thick should hot tub walls be?

For block, use 8in (200–215mm) hollow blocks with rebar and filled cores. For poured walls with plumbing inside, allow at least 10in (250mm). ICF thickness depends on the block system. Your plans or engineer set the final figure.

Do I need rebar in a DIY hot tub?

Yes. A grid in the slab, tied to vertical bars in the walls, turns the floor and walls into one structure and resists cracking.

Is waterproof render enough to make a hot tub watertight?

Not on its own. Use render as a base, then a pool-rated membrane in two coats, then flood test before tiling.

Which method are you going for? Let me know in the comments below. I read them all.

Cheers,
Andi

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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.

Fast forward to 2025, I've helped over 1400+ 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