Hot Tub Waterproofing, Tile Adhesive and Grout: What to Use and How

Last updated: September 2026

One of the questions I get asked most about building a DIY hot tub is what to use for waterproofing, tile adhesive and grout. This guide covers the lot: preparing a block or poured concrete shell, the waterproof membrane, the flood test, then the adhesive and grout that go on top.

Before we get going, here’s the reassuring bit. Out of all the hot tubs and plunge pools I’ve helped people build (more than 1,500 now), very few leak from the structure. Nine times out of ten, a leak is in the plumbing, not the shell. Get the steps below right and your shell will be the last thing you worry about.

How the layers fit together

It helps to think of the waterproofing as a system, where each layer has one job:

Layer	Its job	Typical products
Structure (block, ICF or poured concrete)	Strength. Holds the weight of the water.	Core-filled block, ICF, concrete
Render or levelling coat	A flat, sound surface for the membrane and tiles	Sand and cement render, or a tile render such as Ardex AM 100
Waterproof membrane (2 coats)	The actual waterproofing	Ardex S 7 Plus (UK/EU), Laticrete Hydro Ban (USA)
Tile adhesive	Bonds the tiles, and has to cope with being under water	Pool-rated flexible adhesive, e.g. Ardex X 77
Tiles and grout	The finish. Protects everything underneath.	Mosaic pool tiles with epoxy grout

The important point is that the membrane does the waterproofing. As Weber puts it in its guide to tiling a pool, tiling “should not be viewed as a method of waterproofing the pool”. Don’t rely on the grout and tiles on their own. They’re the finish, and the membrane is what keeps the water in.

Preparing the surface

How you prepare depends on how you built the shell. I’ll cover the two most common: block built and poured concrete (including ICF, which ends up as a concrete wall behind foam).

Block walls: render first

Blocks are porous and the joints are never perfectly flat, so block walls get rendered before anything else. A scratch coat and a float coat of sand and cement render gives you a flat, sound surface. Aim for flat as well as waterproof. Tiles, especially mosaics, show every bump. Ardex’s guide to tiling swimming pools quotes the British Standard of no more than 3mm deviation under a 2m straightedge.

On my own tub, I added a waterproofing agent to the render mix. These additives are common in the UK and Europe (you’ll find them at any builders’ merchant) and less so in the USA. They make the render less absorbent, and I still think they’re a sensible belt-and-braces step. But they’re optional, and they’re no substitute for the membrane. If you use one, check with the membrane manufacturer that it’s compatible. The membrane needs to bond to the render, and it’s the membrane that matters.

If you’d rather not mix render yourself, rapid tile renders are made for this. Ardex’s pool system uses AM 100 on walls, which can be applied from 2mm to 30mm thick and tiled over in as little as 2 hours.

Poured concrete, shotcrete and ICF

You may not need to render a poured wall, but you do need to clean it. Laticrete’s Hydro Ban pool instructions say to remove “form release agents, laitance… dust, dirt, oil, paint, concrete sealers, and curing compounds or any other material that can act as a bond inhibitor”. In practice that means grinding or scrubbing off the weak, dusty top skin and any oily residue from the formwork, then filling any holes and making good. Fill coves and wall-to-floor corners smooth as well, so the membrane goes on in one continuous skin.

Let it cure

Concrete and render shrink as they cure. Tile over them too soon and that movement ends up in your tiles and grout. The British Standard for tiling pools (BS 5385-4) sets long waiting times for traditional materials. Modern rapid-set systems shorten them a lot. There’s a table further down.

Don’t use a PVA wash

On my own tub, after rendering I sealed the surface with a PVA wash (PVA glue and water, a couple of coats) before the membrane. It was cheap and easy, but I wouldn’t do that now, and I’d ask you not to either.

Here’s why. The two membranes I recommend both want to bond straight to clean, sound cement:

• Ardex S 7 Plus: the technical data sheet says it plainly: “Do not use any type of primer or diluted ARDEX S 7 PLUS W prior to applying the first coat.” The surface should be “mature, stable and free from all barriers to adhesion”.
• Laticrete Hydro Ban: the pool instructions treat sealers and curing compounds as contaminants that have to be removed. The Hydro Ban data sheet only calls for a primer on gypsum-based substrates, and even then it’s the manufacturer’s own sealer or a diluted coat of Hydro Ban itself. On hot, dry concrete, it says to dampen the surface and sweep off the excess, not to seal it.

A PVA coat sits between the cement and the membrane. It’s exactly the kind of “barrier to adhesion” both manufacturers warn about, and PVA isn’t designed to live permanently under water. It worked on my tub, but that’s not a risk worth taking, and if you use a product against its data sheet you’ve got no comeback with the manufacturer if it fails. It isn’t just theory, either. On John’s ICF build, he put a rendering primer on first and then S 7, and the membrane delaminated. He switched to a different product applied directly to the shell. It’s all in his ICF block hot tub case study. So the rule is simple: no PVA, no sealer, no primer unless the membrane’s own data sheet asks for it. Clean it, dampen it if the data sheet says so, then membrane it.

The same goes for “concrete and masonry sealants”. You don’t need any of them. The membrane is the sealer.

The waterproof membrane

These are paint-on (or trowel-on) products, applied in two coats. Both of the products below are designed for pools and hot tubs, and both are tile-ready.

UK and Europe: Ardex S 7 Plus

Ardex S 7 Plus is a two-part flexible cementitious coating made for swimming pools. You mix it and brush or trowel it on. From the data sheet and product page:

• Two coats, no primer.
• About 6.25m² (67ft²) per 15kg bag with two coats.
• Around 60 minutes pot life once mixed.
• Tiles can go on about 4 hours after the second coat, once it’s dry. The product page also lists 8 hours in its technical data, so allow longer if it’s cool.
• Resists water pressure up to 5 bar.

Ardex makes reinforcing tapes for the weak spots: SK-12 for junctions, SK-90 for internal corners, SK-270 for external corners and SK-M collars for pipe outlets. Use them. Corners and penetrations are where membranes fail.

USA: Laticrete Hydro Ban

Hydro Ban is a ready-to-use liquid membrane. You paint on one coat, let it dry, then paint the second at right angles to the first. It goes on light green and dries to a dark olive green, which makes it easy to see where you’ve missed. It isn’t the cheapest, but it’s very good, and Laticrete lists “swimming pools, fountains and water features” and “spas and hot tubs” among its uses. From the data sheets:

• Two coats are required. Laticrete says three or more aren’t required or recommended.
• Each wet coat is 15–22 mils (0.4–0.6mm) thick.
• No fabric is needed for general use, although it’s available for cracks and joints.
• It bonds directly to PVC and ABS plumbing fittings, which makes sealing round jets and drains easier.
• Flood test once the second coat has turned uniformly olive green, plus 2 hours at 21°C (70°F) or 24 hours in cooler weather (10–21°C, 50–69°F).
• In a pool, it has to be covered with tile or another hard-wearing finish within 30 days.

One thing a membrane can’t do

Laticrete is clear that Hydro Ban is designed to hold water in from the “positive side (top) only”, and it isn’t recommended where there’s negative hydrostatic pressure (groundwater pushing from behind). That’s one of the reasons I always fit a hydrostatic relief valve in the floor of an in-ground tub. See Why do I need a hydrostatic release valve? and How to install one.

Sealing around jets, the skimmer and drains

This is another question that comes up a lot, so here’s how I approach it.

The jet wall niche (above, top left) is only there to give the jet something to butt up to. The jet screws in, and it doesn’t seal to the niche at all. The idea is that the niche sits flush with the finished surface. Most people, me included, set it so it will be level with the face of the tiles, leaving room for the render, membrane, adhesive and tile.

On my build, I packed plenty of mortar with waterproofing additive around the niche, as you can see above. Where the membrane meets a fitting:

• With Hydro Ban, you can take the membrane straight onto the PVC fitting. Laticrete says it bonds directly to PVC and ABS, so brush a generous coat onto and around it.
• With Ardex S 7 Plus, use Ardex’s pipe collars (SK-M) around outlets so the membrane isn’t relying on a bare joint.
• Epoxy putty (products like JB WaterWeld or Jenolite’s epoxy putty stick, made for underwater repairs) is handy for packing a gap around a fitting before the membrane goes on. I’d use it as a filler under the membrane, not instead of it.

The same goes for the inward suction fittings, the skimmer and the main drain. For more on getting the fittings in right in the first place, see my guides to installing gunite jet bodies, installing a skimmer and installing a main drain.

Flood test before you tile

Once the membrane has cured, fill the shell with water, mark the level and leave it for a few days. Check the level each day (allowing for evaporation and rain). If it drops, find the leak and fix it now, while it’s a lick of membrane rather than a row of tiles. It’s also a good moment to pressure test your plumbing. If you do find a problem later, my guide Help! I’ve a leak will help you track it down.

Tile adhesive and grout

It must be swimming pool grade

This is the most important point. The adhesive and grout need to be swimming pool grade. Not wet room grade, not bathroom grade. Swimming pool grade. They’ll be under warm, chemically treated water all year round, and bathroom products aren’t made for that.

Do I need a separate grout and adhesive?

Not necessarily. On my own hot tub I used a swimming pool grade grout and adhesive in one, a generic product from my local hardware store. It’s since been discontinued, but these products do still exist, though. In the USA, Ardex’s pool system includes Ardex WA, a 100% solids epoxy sold as a “grout and adhesive”, which Ardex specifies for fixing submerged glass tile as well as grouting.

For most builds, though, you’ll use a separate adhesive and grout. Here’s what the manufacturers’ current pool systems specify:

	UK and Europe (Ardex UK)	USA (Ardex Americas)
Membrane	S 7 Plus	8+9 (3 coats) or SK 175 sheet. Laticrete Hydro Ban is the other common choice.
Adhesive	X 77 flexible tile adhesive	X 77 (X 78 for large-format floor tiles). WA epoxy for submerged glass tile.
Grout	EG 8 Plus epoxy, or Ardex-Flex FS/FL cementitious grouts	WA epoxy, or FL flexible sanded grout

Sources: Ardex UK pool guide and Ardex Americas pool brochure. I’m not affiliated with Ardex, I just think they make good products. In the USA there’s no shortage of choice, with pool-rated adhesives and grouts from Laticrete, Mapei, Custom Building Products and others. Just make sure the data sheet says it’s suitable for submerged pools.

Why I’d choose epoxy grout

Cement-based grout is fine in a well-balanced pool, but hot tub water is warm, heavily sanitised and often soft. As the Tile & Stone Journal explains, “aggressive water dissolves calcium salts in cement-based grouts, leading to erosion and loss of bond”, and where calcium levels are below 250mg per litre, “epoxy grout is strongly advised”. Epoxy is harder to work with (you have a short window to clean it off the face of the tiles), but it’s impervious and it doesn’t wash out. For a hot tub, I’d use it every time.

Whatever grout you use, keep your water balanced. For a tiled tub, that means calcium hardness of around 200–400 ppm and pH 7.2–7.6. All the ranges are in my hot tub chemical levels guide.

Which tiles?

Having seen so many customer builds, I’d go for mosaic pool tiles over big tiles every time. They follow curves and seats, they cope with movement better, and they look fantastic. I’ve covered tile choice, porcelain vs ceramic vs glass, and slip resistance for steps in Hot tub tiles: everything you need to know.

How long to wait before filling

This is where people get impatient, me included. Here’s what the traditional British Standard intervals look like, compared with a modern rapid-set system:

Stage	Traditional materials (BS 5385-4, via Weber)	Rapid-set pool system (per maker)
Concrete shell to render	6 weeks	Follow the render’s data sheet
Render to tiling	3 weeks	Ardex AM 100: from 2 hours. S 7 Plus on top: ~4 hours after the second coat.
Tiling to grouting	3 days	Per adhesive data sheet
Grouting to filling	3 weeks	Ardex FL: submerged after 72 hours. Ardex WA epoxy: after 7 days.

Mixing and matching brands means the longest wait wins, so stick to one manufacturer’s system if you can. When you do fill, Weber’s pool guidance suggests filling at no more than 750mm a day and warming the water gently (they quote 0.25°C an hour). That’s written for full-size pools. On a hot tub, I’d fill it in one go, check for leaks, and then bring it up to temperature steadily over a day or so on the first fill rather than blasting it. My guide on how to fill a hot tub the correct way covers the rest.

Common mistakes

• Sealing, priming or PVA-ing the surface before a membrane that says not to.
• Skipping the corner tapes and collars. The flat walls rarely leak. Corners and penetrations do.
• Too thin, or only one coat. Two full coats, at right angles to each other, at the thickness on the data sheet.
• Tiling before a flood test.
• Using bathroom adhesive or grout. It’ll look fine for a season, then start to fail.
• Filling too soon, before the grout has cured.
• Running soft or low-pH water, which eats cement-based grout.
• Ignoring groundwater. Membranes hold water in. They don’t hold groundwater out. Fit a hydrostatic relief valve.

Could I just use a liner instead?

You can. It’s a different approach with its own pros and cons, and I’ve covered it in Can I use a liner on my DIY hot tub? If you want to see a tiled finish done properly from start to finish, Patrick’s build is a great example: waterproofing and tiling his basement hot tub and cold plunge.

Planning your own build?

Waterproofing is only as good as the shell under it. My in-ground build guide covers the dig, the slab and the walls. My DIY hot tub plans give you CAD drawings with every dimension, including where the niches, skimmer and drains sit, plus the plumbing layout, a full parts list and the video course.

FAQs

Can I use PVA before a tanking membrane in a hot tub?

No, not before Ardex S 7 Plus or Laticrete Hydro Ban. Ardex says not to use any primer, and Laticrete says the surface must be free of sealers and bond inhibitors. Clean the surface, dampen it if the data sheet says so, and apply the membrane straight on.

How many coats of waterproofing does a hot tub need?

Two, at the thickness the manufacturer specifies, with reinforcing tape or collars at corners and fittings. Laticrete says three or more coats of Hydro Ban aren’t required or recommended.

Is tile grout waterproof?

Not on its own. Cement grout is porous, and even epoxy grout isn’t a substitute for a membrane. The membrane keeps the water in. The grout and tiles protect it.

What grout should I use in a hot tub?

A swimming pool rated grout, ideally epoxy. Warm, soft or chemically treated water can erode cement-based grout, and epoxy resists that far better.

How long after grouting can I fill my hot tub?

It depends on the products. It can be 72 hours for a rapid-set cementitious pool grout, about 7 days for some epoxies, and up to 3 weeks with traditional materials under BS 5385-4. Always follow the data sheet for the products you’ve used.

Got a question about waterproofing or tiling your own build? Drop it in the comments below. I read them all.

Cheers,
Andi

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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