Fibreglass Hot Tub Refurbishment: Replacing a Fibreglass Tub with Poured Concrete (Case Study)

Last updated: September 2026

Not every DIY hot tub project I work on is a new design and build. I do a lot of “rescue projects”, where a tub or pool is partly built or partly designed and the owner needs help to finish it off. I also help people with refurbishments, getting tired or non-working tubs back in action.

This project is a hybrid of the two. Mark came to me wanting to remove his fibreglass hot tub, which had seen better days, and replace it with a poured concrete tub while keeping most of the plumbing he already had. Let’s see how he got on.

The background

The background to this project is pretty straightforward. Mark has a beautiful indoor pool and hot tub at his home. Both were 20 years old, and while the pool was in good shape, the hot tub had definitely seen better days.

What he wanted to do was rip it out and replace it with a poured concrete tub. Pretty straightforward, right? Let’s see if that was the case.

If you’re weighing up whether to repair or replace an old tub, my guide to restoring or repairing a hot tub or spa covers the options.

The plan

Mark had a very clear idea of the size and dimensions he wanted, so the new tub would fit the void left by the old one. What he needed my help with was the plumbing layout.

One of the problems with the old tub was that, as it was a spillover tub, the water would drain back into the pool whenever the pumps stopped. Somebody had forgotten either a Hartford loop or a one-way check valve on the lines, and that was going to be pretty easy to fix. If you’re planning a tub that spills into a pool, have a read of adding a spillover hot tub and how to size a pool overflow or spillover.

Mark sent me pictures of the existing pipework, and he’d already labelled exactly what each pipe did. Most of the kit on his pool pad was working, so the plan was to reuse it. We needed a new jet pump, but the pipe runs to and from the tub had all been working fine, so we kept them.

Mark also wanted to do away with the dipping pool at the end of the overflow and replace it with a slide.

With the plumbing design done and the parts ordered, it was time for Mark to get to work.

Let the work commence

The first job was to remove the existing fibreglass hot tub. This wasn’t too much of a problem, and it came out of its housing quite easily.

With the tub out, the hole wasn’t deep enough, so Mark had to dig down about another foot (30cm) to give himself enough space for a poured base and formed seating.

The base

Next, the damp proof membrane went in to keep dirty water out, and then it was time to plan the base. The rebar went in as you can see below. (There’s more on steel in my guide to rebar.)

You can clearly see the existing pipes that we were going to tap into. Mark had labelled them ready for us to use.

With the base in place, the next stage was to pour the seats. Rather than show you lots of individual pictures, here’s a montage of this stage. There was quite a lot of formwork to do before he could pour, and Mark already had the plumbing in place.

Pipework

A quick note on the plumbing. As I mentioned, the old tub was draining back into the pool when it wasn’t in use, so we put a couple of Hartford loops in place to make sure that didn’t happen again.

You can also see in the picture above that Mark insulated the backs of the seats before he formed them up and poured. My DIY hot tub insulation guide explains why that’s worth doing.

The pour

Below is the main pour. Again, there’s plenty of bracing on the forms, because there’s quite a bit of weight to hold back with poured concrete.

With the seats all formed up, Mark turned his attention to the slide.

The slide

Here’s the pour for the slide. Check out the lilo in the pool being used to hold it all up. Genius!

Waterproofing and tiles

Once the concrete had cured, Mark moved on to the waterproofing coat and the tiles. He did run into a snag: he didn’t use a cementitious waterproofing coat first, so he had to pull his tiles back and redo them. It looks good now, though.

This is one of the most common mistakes I see. Concrete on its own isn’t waterproof, so the tub needs a proper waterproofing layer under the tile adhesive. I cover the options in waterproofing, grout and tile adhesive and choosing hot tub tiles.

Job done

And here it is: the finished tub, all working fantastically, and Mark is really pleased with the end result. The slide is getting plenty of use too. Really nice job, Mark!

Replacing or adding a tub next to a pool?

Every pool and tub combination is different, so this kind of job really benefits from a proper plumbing design before anything gets ripped out. If you’re adding a tub to an existing pool, start with Can I add a hot tub to my existing swimming pool?

Once you know roughly what you want, sketch it in the free Tub & Pool Designer Pro software, and I can turn it into architectural CAD and a 3D plumbing design. If you’re starting from scratch with a standard size, my DIY hot tub plans give you the CAD drawings, plumbing layout, parts list and video course.

What do you think of Mark’s refurb (and that slide)? 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