How to Build a Concrete Slab or Base for a Hot Tub (Step by Step)

Last updated: September 2026

Whether you’re buying a hot tub or building one, it needs something solid to sit on. A hot tub full of water and people is one of the heaviest things you’ll ever put in your garden or backyard. If the base sinks, cracks or isn’t level, you’ll get a wonky waterline, a stressed shell and, on a DIY build, leaks.

The good news is that a concrete pad is a very achievable DIY job with a few basic tools, a plate compactor and a bit of planning. This guide covers pads for shop-bought (portable) hot tubs, bases for DIY tubs built above ground, and how that fits with the base for a fully in-ground tub.

Which kind of base do you need?

“Hot tub base” means different things depending on what’s going on top of it, so start here:

What sits on it	What the base has to do	Typical slab
Shop-bought (portable) acrylic hot tub	Carry the weight evenly and stay flat and level. The tub holds the water.	100–150mm (4–6in), reinforced
Swim spa	As above, but with a lot more weight. Follow the maker’s spec.	150mm (6in) or more
DIY tub built above ground (block or ICF walls)	It’s the floor of the tub and the footing for the walls, so it has to stay crack-free.	About 150mm (6in), reinforced, with starter bars for the walls
DIY in-ground tub	As above, at the bottom of a hole, plus a drain, a hydrostatic relief valve and insulation.	About 150mm (6in), reinforced. See my in-ground build guide

Most of this post is about the first three, which are all built the same way at ground level. If you’re going in the ground, the method is the same, but the build-up and extras are covered step by step in the in-ground guide, so I won’t repeat them here.

How heavy is a hot tub?

Water weighs 1kg per litre, so every 1,000 litres (264 US gallons) is a tonne before anyone gets in. As an example, Hot Spring’s pre-delivery guide lists its largest Landmark model at 6,217lb filled, about 115lb per square foot. Bullfrog Spas says a filled portable spa can weigh anywhere from 6,000lb to 20,000lb, not counting the people. That’s why a few paving slabs on sand won’t do.

Measuring up and choosing the spot

Measure the tub, then plan the pad around how you’ll actually use it:

• Add at least 100mm (4in) on every side. The concrete edge shouldn’t sit right at the tub’s footprint, and a small margin gives you some tolerance.
• Allow for steps and access panels. You need to be able to get to the equipment side for servicing.
• Plan where the cover goes. Hot tub covers are big, and it’s easy to forget you need somewhere to put one while you’re in the tub. If you’re using a cover lifter, check how much space it needs behind the tub.
• Think about the future. If you might upgrade to a bigger tub later, pour a bigger pad now.
• Check the route for the electrics before you pour concrete across it. There’s more in A guide to hot tub electrics.

Pick a spot that drains well, away from trees and big shrubs. Roots can lift and crack a slab over time, and falling leaves are a pain in a hot tub anyway. If you need privacy, see hot tub privacy ideas.

Hot tub size	Minimum pad (100mm/4in margin each side)	Concrete for a 150mm (6in) slab
2m x 2m (6ft 7in)	2.2m x 2.2m (7ft 3in)	About 0.73m³ (0.95yd³)
2.3m x 2.3m (7ft 7in)	2.5m x 2.5m (8ft 2in)	About 0.94m³ (1.2yd³)
8ft x 8ft (2.44m)	8ft 8in x 8ft 8in (2.64m)	About 1.05m³ (1.4yd³)
10ft x 10ft (3.05m)	10ft 8in x 10ft 8in (3.25m)	About 1.58m³ (2.1yd³)

Add around 10% to the concrete for waste and an uneven sub-base, and more if you’re making the pad bigger for steps.

How thick should a hot tub pad be?

For shop-bought tubs, US manufacturers usually say at least 4in. Bullfrog says at least 4 inches, and 6 inches or more for larger models like swim spas, and Hot Spring requires a reinforced 4in pad for its biggest models. UK installers tend to go thicker. British Concrete Polishing suggests 100mm reinforced for a 2-person tub, 125–150mm for a 4-person tub and 150mm minimum for 6+ seats.

My advice is simple. The extra 50mm (2in) of concrete costs very little compared with the tub, so if in doubt, go for 150mm (6in). I’d definitely go for 150mm if:

• the tub seats five or more, or it’s a swim spa (check the maker’s spec)
• the ground is soft, clay or made-up ground
• you get hard frosts
• you’re building a DIY tub on it, because then the slab is the floor of your tub and the footing for the walls

Step by step: building a concrete hot tub base

1. Clear and dig out

Mark out the pad with pegs and string, a little bigger than the finished slab. Strip off the turf and topsoil, and remove any roots. Dig down so that the sub-base plus the slab finishes where you want it. That’s usually flush with, or slightly above, the surrounding ground. For a 150mm slab on 150mm of sub-base, that’s a dig of about 300mm (12in). If you find soft spots, dig them out and fill with more sub-base.

2. Lay and compact the sub-base

Spread 100–150mm (4–6in) of Type 1 (MOT) sub-base, or crushed stone/gravel base in the USA. Compact it in layers with a plate compactor (“wacker plate”). This layer spreads the load and stops the slab settling unevenly, so don’t skimp on the compacting. On soft or clay ground, a geotextile membrane under the sub-base helps stop it sinking into the soil.

3. Build the formwork

Build a timber frame (formwork) to the finished size and depth of the slab, and screw the corners together. Stake it firmly from the outside so it doesn’t bow when the concrete goes in. Check the diagonals: if they match, it’s square. Then level the top edges carefully, because they’re your screed rails. For a shop-bought tub, you want the finished slab flat and level. Bullfrog allows a maximum of ½in (12.7mm) of slope over an 8ft (2.44m) run, and says not to use shims to level the spa.

4. Add sand blinding and a DPM

Lay a thin layer of sand blinding over the sub-base to stop sharp stones puncturing the membrane. Then lay a damp-proof membrane (DPM). In the UK that’s usually 1200-gauge polythene, lapped at the joints and turned up the sides of the formwork. The DPM stops ground moisture coming up into the slab, and it stops the sub-base sucking water out of the fresh concrete, which would weaken it. It won’t make the concrete “waterproof”, and a pad under a shop-bought tub doesn’t need to be.

5. Insulation: do you need it?

For a shop-bought tub, usually not. The tub has its own insulated base and cabinet. For a DIY tub built on the slab, yes. The slab is the floor of your tub, and heat will pour into cold ground all year. Lay compression-rated rigid board (XPS or high-density EPS) on top of the sub-base and DPM, directly under the concrete. Don’t put it under the gravel, where it gets crushed and wet. There’s more in DIY hot tub insulation.

6. Reinforce it

Use steel mesh sheets (A142 is the common UK choice, and A193 is heavier) or a rebar grid at around 300mm (12in) centres. Rock Solid Concrete’s hot tub spec, for example, puts A142 mesh on 50mm spacers, with 300mm overlaps wired together. The key is to sit it on spacers (chairs) so it ends up inside the concrete. Mesh lying flat on the membrane does very little.

• Building a DIY tub on the slab? Leave vertical starter bars sticking up where the walls will go, so the walls tie into the floor. There’s more in my rebar guide.
• In the USA, Bullfrog and Hot Spring both say the rebar or mesh should be attached to a bond wire. Whether your inspector wants the pad bonded depends on your installation and local code, so ask your electrician before you pour. See Ground bonding a hot tub.

7. Pour the concrete

For anything bigger than a small pad, I’d order ready-mix and pour the whole slab in one go. That gives you a consistent, properly specified mix and no cold joints. In the UK, ask for C25 or C30 (some suppliers sell a C28/35 hot tub mix). In the USA, 4,000psi concrete is the equivalent. Air-entrained concrete is worth asking for if you get hard freezes.

If you mix it yourself, use a mixer and a proper concrete mix with aggregate. A mix of 1 part cement to 1.5 parts sand to 3 parts aggregate is the nominal ratio usually quoted for C20–C25. Don’t use a sand and cement mortar mix (like 1:1.5 or 4:1), because that has no aggregate and isn’t structural concrete. Add water gradually. You want it workable, not soupy, because extra water weakens the concrete. Wear gloves, eye protection and a dust mask, as wet cement can burn skin.

Fill the form, work the concrete into the corners and around the mesh, then screed it off the top of the formwork with a straight length of timber, using a sawing motion. Finish with a float. A light brush finish gives some grip if people will walk on it barefoot.

8. Cure it, then strip the formwork

Concrete doesn’t “dry”, it cures, and it needs moisture to do it. Cover the slab with polythene or keep it damp for the first few days, and protect it from frost and hot sun. You can usually take the formwork off after a day or two. Rock Solid Concrete suggests at least 7 days before a tub is delivered, 14 days if it’s being craned in, and 28 days for full strength. If you’re building a DIY tub on the slab, you can start setting out walls sooner, but wait the full time before filling it with water.

This is the process Neil followed on his build. You can read about it in Neil’s case study.

What does it cost?

It depends on size and access, but for a DIY pad the main costs are the concrete, the sub-base, mesh, timber, membrane and a day’s hire of a plate compactor. As a guide to ready-mix prices, one UK supplier lists its C28/35 hot tub mix from £171 per m³. Most pads need about 1m³, but small loads, delivery and pumping can add to the price, so get a quote for your job.

Alternatives to a concrete slab

• Paving slabs on a compacted sub-base and a full mortar bed can work for smaller portable tubs if the maker approves it. Individual slabs can settle unevenly, though.
• A compacted gravel pad in a timber or steel edging is approved by some manufacturers. It’s cheap and drains well, but it’s not suitable for a DIY built tub.
• Plastic spa pads (interlocking bases) suit small tubs on firm, level ground. Check the maker’s load rating.
• A deck is fine if it’s designed for the load. Most standard decks aren’t. See hot tub deck ideas for how to check.
• Grass or soil: never. Bullfrog specifically advises against grass.

Common mistakes

• Not compacting the sub-base, so the slab settles and cracks.
• Mesh or rebar lying on the membrane instead of on spacers.
• Using a sand and cement mortar instead of concrete.
• Adding extra water to make it easier to work, which weakens the concrete.
• A pad that’s too small for the steps and cover, or that isn’t level.
• Letting it dry out in the sun, or filling the tub after a day or two.
• Pouring concrete over the route the electrics needed to take.

Building your own tub on it?

If the slab is the start of a DIY hot tub rather than a pad for a shop-bought one, my DIY hot tub plans give you the dimensions, slab and wall layout, plumbing diagrams and a full parts list, plus the video course. They work for above-ground and in-ground builds. Read Above ground hot tubs and Hot tub wall construction: what are the options? for the next steps.

If you’ve got an awkward space, try the free Tub & Pool Designer Pro software. I can then turn your design into architectural CAD and a 3D plumbing design.

FAQs

How thick should a concrete pad for a hot tub be?

At least 100mm (4in), reinforced, for a small shop-bought tub on good ground. For bigger tubs, swim spas, soft ground or a DIY tub built on the slab, go for 150mm (6in) or whatever the manufacturer specifies.

How big should a hot tub pad be?

At least 100mm (4in) bigger than the tub on every side. Add more for steps, access to the equipment side and somewhere to put the cover.

Does a hot tub pad need rebar or mesh?

Yes. Mesh or rebar on spacers stops cracks opening up. In the USA, ask your electrician whether it needs to be bonded.

How long should concrete cure before I put a hot tub on it?

Give it at least 7 days, keeping it damp, and longer if the tub is being craned in or the weather is cold. Concrete reaches its design strength at around 28 days.

Can I put a hot tub on paving slabs?

Sometimes, for a smaller tub, if the slabs are on a compacted sub-base with a full mortar bed and the manufacturer allows it. For a big tub, or anything you’ve built yourself, pour a slab.

Do I need to waterproof a hot tub concrete pad?

Not under a shop-bought tub, as the DPM underneath is enough. On a DIY tub, the waterproofing is done on the inside of the tub with render and a pool-rated membrane. See my waterproofing guide.

Got a question about your base? Drop it in the comments below. I read them all.

Cheers,
Andi

You can also find me on my socials;

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.

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