The base under a hot tub has three jobs. It has to carry well over a tonne without sinking, it has to be level so the water and the seams sit evenly, and it should stop the cold ground from stealing your heat. Here is how the common options compare.
How much weight are we talking about?
Take the Arebos Barcelona 6: 910 litres of water (910 kg), 25 kg of tub and six adults at 80 kg each. That is 910 + 25 + 480 = 1,415 kg on a 1.85 m square. Most 6-person tubs in our data land between 1,400 and 1,700 kg filled. Our decking weight guide works through six models.
The options side by side
| Base | Strength | Level | Heat loss | Rough effort |
|---|---|---|---|---|
| Concrete slab | Best | Best, if laid well | High without a mat | A builder's job or a weekend for a keen DIYer |
| Paving slabs | Very good | Good, if laid on a proper bed | High without a mat | DIY-friendly |
| Compacted gravel with a mat | Good | Fair | Fair | DIY-friendly |
| Decking | Only if designed for the load | Good | Low | Needs an engineer or builder |
| Interlocking pads or tiles alone | Poor on soft ground | Copies the ground under it | Low | Very easy |
| Lawn | Poor | Poor | High | Do not use |
Concrete slab
A concrete slab is the gold standard. It does not move, it does not rot and it carries any tub in our data. A builder usually lays around 100 mm of concrete on a compacted hardcore base, with a very slight fall so rain runs off.
- Size it right. Leave room to walk round the tub, and if your tub has a separate pump box, room for that too. Check the outside size on the product page: the MSpa Tuscany is 193 cm across, the Wave Garda 6 is 180 cm square.
- Make it level. A slight fall for drainage is fine. A slope you can see is not.
- Add a mat on top. Bare concrete is cold and rough on the liner.
Paving slabs
Paving works almost as well as a slab, as long as the slabs sit on a full mortar bed or a well-compacted sub-base. Slabs laid on spots of mortar or straight onto soil can crack or sink under a filled tub. If your existing patio is sound and level, it is usually fine.
Check for rocking slabs before you fill. One loose slab under the tub becomes a hollow later.
Gravel
Compacted gravel drains well and costs little. It only works if you compact it properly and lay a firm mat or board over it, because loose stones push into the floor of the liner. Use a frame to stop it spreading. Fine for an inflatable, less good for a heavy frame tub.
Decking
A deck looks great and is warmer underfoot. But a filled tub puts down roughly three times the load a normal domestic floor is designed for, and many garden decks are built lighter than that. Only use decking that a builder or structural engineer has designed or checked for your tub's filled weight. See our decking weight guide.
A good compromise: build the deck around the tub and stand the tub on a slab in the gap.
Pads, mats and interlocking tiles
These are not a base on their own. They go on top of a hard base and do two jobs:
- Protect the liner from grit and sharp edges.
- Cut heat loss through the floor.
Some tubs come with one. The Arebos Barcelona 6 includes a thermal cover and floor mat, and the Wave Atlantic 6 EnergySave bundle includes a floor mat. On soft ground, tiles simply copy the dip underneath them, so do not use them to fix a bad surface.
Why not the lawn?
Grass dies under the tub, the soil goes soft when it rains and one side sinks before the other. A tub that sits lopsided strains its seams and the water line sits higher on one side. It also loses heat straight into the wet ground.
Frame and foam tubs need a better base
Heavier tubs need a firmer base. The MSpa Oslo weighs 118 kg empty and needs a two-person delivery. The Lay-Z-Spa Stockholm weighs 74 kg empty on a 164 cm square. A rigid frame does not flex to follow a dip the way an inflatable does, so a frame tub on uneven ground puts stress on the panels. Give rigid-frame and foam-wall tubs a slab or well-laid paving.
Before you fill
- Check the level with a long spirit level, both ways.
- Clear the area of stones and sharp bits.
- Plan the power: an outdoor socket with RCD protection within reach of the lead. See the 13 A plug guide.
- Plan the drain: where the water goes when you empty it.
- Check the hose reaches.
Quick answers
Do I need planning permission for a slab? A small patio slab in a back garden does not normally need permission. Listed buildings and conservation areas are different. See where to put a hot tub.
Can I put a tub on artificial grass? Only if a hard, level base sits underneath it, and then you might as well lay the tub on the base with a mat.
Does the base affect running costs? Yes. A mat on a hard base keeps far more heat in than bare concrete. See using a hot tub in winter.
Picking a tub to match your space? Read the hot tub buying guide.
