Mel’s In-Ground Hot Tub and Oak Pavilion: DIY Case Study

Last updated: September 2026

When Mel got in touch in July 2021, his goal was pretty straightforward. He sent me a picture of an oak-framed pavilion with an in-ground hot tub, and said, more or less, “That’s what I want.” Oak pavilion, in-ground hot tub, view of the lake. Perfect.

Mel had already designed the pavilion himself. My job was the hot tub. Here’s how his build went, including the bits that didn’t go to plan, because that’s where the useful lessons are.

This was the inspiration picture Mel sent me at the start:

Mel had already built the pavilion before any work started on the hot tub. If you’re thinking about a roof over your own tub, it’s one of the best ways to get shelter and privacy at the same time. I’ve covered the options in Hot tub privacy ideas and Hot tub deck ideas.

The challenges

There were two things that made Mel’s build a bit different from the norm.

• The control room was a good 40ft (about 12m) away from the tub. That’s a long way for the water to travel there and back, so the pipe sizes and pump choice have to allow for it. The fix is usually to go up a pipe size on the long runs, because bigger pipe means much less friction. There’s more in Hot tub control room position and my DIY hot tub plumbing guide.
• He wanted a “Kyle Lounger”. If you’ve followed the blog or the channel, you’ll know I designed a contoured lounger seat for Kyle’s build, and it’s taken his name ever since. If you’d like one, see How to build a concrete lounger, or my Kyle 15ft x 8ft dual lounger plans, which have two built in.

The view

The difference for Mel was that he wasn’t going to put jets in the lounger. It was there as a comfy place to sit and look out over his lake. Not a bad view at all. What a place to relax.

Because Mel wanted a contoured seat, he decided to form his hot tub. That means building a timber mould, bracing it well, then filling it with concrete. It’s a good way of doing things and a real alternative to a block-built tub, especially for curved shapes. For how it compares with block, ICF and shotcrete, see Hot tub wall construction: what are the options?

Forming the hot tub

As you can see from the photos below, the initial forming went without a hitch.

You can see that the plumbing and the skimmer were already in place before the pour. That’s normal when you form a hot tub, and it’s exactly how you should do it. Once the concrete’s in, you can’t add anything through the wall without cutting into it. There’s more on the skimmer in How to install a skimmer on your DIY hot tub, and for the groundwork underneath (dig depth, sub-base, slab and drains), see How to build a DIY in-ground hot tub.

A tip for the light cable

Mel also ran a cable for his hot tub light, in conduit. If you’re doing the same, one of the easiest ways to make sure you don’t lose water through the conduit is to run it up and over the waterline before it heads off to the control room. Then, even if the light doesn’t seal properly for whatever reason, gravity stops the water getting out of the tub. It works on the same principle as the Hartford loop on a blower line.

It’s also the same thinking behind the US electrical code. For line-voltage wet-niche pool lights, NEC 680.24 puts the junction box at least 8in above the maximum water level and 4in above the deck. Rules for low-voltage lights and for the UK differ, so check with your electrician or inspector. For choosing and fitting a light, see Underwater hot tub lighting made simple.

The control room

At the same time, Mel was working on his control room. As you can see below, he’s running a spa pack with an external heater, a blower and a UV system for sanitation.

I’m sure Mel won’t mind me saying that the control room is where he had most of his problems. That’s normal, if I’m honest. It’s the part of the build where most people struggle.

The UV system

We wanted the UV to run only when the pump was running and water was passing through it, so we wired it off the pump. It draws very little current, so that’s not a problem. The plan was to wire it off the low speed, but it ended up connected to the high speed. That was a simple fix once Mel worked out what was happening.

One thing worth knowing: UV kills bugs as the water passes through the unit, but it doesn’t leave anything in the water. You still need a residual sanitiser such as chlorine or bromine, just less of it. See Hot tub chemical levels.

Configuring the spa pack

Configuring the spa pack was another small hurdle. Whatever brand you buy, the spa pack has to be set up for your equipment (heater, pumps, blower, light) before it will behave properly. This is one of the areas where I can add a lot of value to a project, and I’ve got plenty of documentation and videos on it. Start with The importance of configuring a spa pack and, for Balboa packs, Configuring a Balboa spa pack. For general control room layout, see Hot tub control room design considerations.

Bring on the tiles… well, sort of

With the control room “under control” (see what I did there?), it was time for Mel to turn his attention to plastering the tub. And this was the next problem. The formed concrete was too smooth. Yes, you read that correctly: too smooth for his plasterers to put the top coat on.

They spent two days chiselling and roughing up the surface before they’d apply the finish coat. I’m guessing they were worried it would lift off if it didn’t grip the surface properly. Mel had no choice but to swallow two extra days of labour.

With the surface well and truly roughed up, the plasterers were happy the finish would stick, and on it went.

What I’d tell you today

The plasterers were right to worry. Pool finishes need a sound, rough, clean surface to grip. Pool & Spa News describes bond failures where the substrate underneath was “as smooth as can be”. Smooth formwork gives you a glassy face that finishes don’t like. So if you’re forming your tub:

• Talk to whoever is doing the finish before you pour. Ask what surface they need, and whether they’ll use a bonding coat.
• Plan how you’ll create a key. The usual options are mechanical roughening (scabbling or grit blasting), acid etching, or a bonding coat, as the finish manufacturer specifies.
• Clean it properly afterwards. Dust and loose bits stop the finish bonding just as much as a smooth surface does.

If you’re tiling rather than plastering, the same applies: follow the membrane and adhesive manufacturer’s surface prep. See Waterproofing, grout and tile adhesive and Hot tub tiles.

Priming the pump

The last issue Mel had was priming the pump. The pipes and pump were below the waterline, but he was struggling to get it to prime. The best way is to work from the pump side: take the lid off the filter and, with a hosepipe, get as much water in as you can. You can do the same from the union at the top of the pump. With enough water in, after a few splutters the pump should prime itself, and that’s exactly what Mel did.

If you’re still stuck, it’s usually an air lock. See How to deal with a hot tub air lock. If your equipment has to sit above the waterline, read Using a priming circuit above ground and What are the self-priming hot tub pump options?

Fast forward: wow, what a build

Cast your eyes back to the inspiration picture at the top of the page. Mel has well and truly knocked it out of the park. This is a beautifully built tub in an idyllic spot, and I’m sure he’ll enjoy the tub and the view for many years to come.

“Thanks for all your patience, guidance and support. As a project kind of guy this was a challenge but well worth the journey. Many of my mistakes could have been avoided had I read your documents more closely. But then again who buys a tool and reads all the instructions first.” – Mel

What was my role?

• Designing the tub
• Specifying the control room
• Supplying the parts
• Supplying the custom hot tub cover (if you need one, see The ultimate guide to hot tub covers)
• Build consultancy, helping Mel throughout the process

What Mel’s build teaches us

• Start with a picture of what you want. A clear goal makes the design so much easier.
• A long run to the control room is fine, as long as the pipe sizes and pump are designed for it.
• Set the plumbing, skimmer and light conduit before you pour.
• Agree the surface finish with your plasterer or tiler before you form the tub. It would have saved Mel two days.
• Read the instructions. Mel said it himself, and he’s not the only one.

Planning your own build?

If a standard size suits your space, my DIY hot tub plans include the CAD drawings, plumbing layout, parts list and video course. If you want something like Mel’s, with a pavilion, a lounger or a long run to the control room, sketch it in the free Tub & Pool Designer Pro software. I can then turn it into architectural CAD and a 3D plumbing design, or support you through the whole build with my custom hot tub design consultancy. There’s more on how that works in Custom designed hot tub: the process explained.

FAQs

Can a hot tub control room be 40ft away from the tub?

Yes. Mel’s is. The longer the run, the more friction the pump has to overcome, so it’s usually worth going up a pipe size on long runs and sizing the pump to suit. See How to correctly size a hot tub pump.

Why won’t plaster stick to smooth concrete?

Pool finishes need a rough, clean surface to bond to. Smooth formed concrete doesn’t give them one, so it needs roughening, etching or a bonding coat first, as Mel’s plasterers found.

Is a formed concrete hot tub better than a block one?

Neither is “better”. Forming suits curves and contoured seats like Mel’s lounger. Block and ICF are easier for most DIYers with square designs. It comes down to your shape and skills.

How do I prime a hot tub pump that won’t prime?

Fill the pump and pipework with a hose through the filter lid or the pump union, then run it. A few splutters are normal. If it still won’t go, look for an air lock.

Planning something similar? Tell me about it 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