In short
A filled hot tub with people in it can weigh two to three tonnes, so it should sit on a concrete or slab base, not on ordinary deck joists. The usual answer is to build the deck around the tub, either flush with its top or part-way up its side, leaving access panels for servicing and a dedicated supply fitted by a registered electrician.
On this page
A hot tub on a deck looks great in the brochure. In real life, the weight, the water and the electrics all need planning before the first board goes down. Get those right and a composite deck is one of the best surrounds you can give a tub.
How heavy is a hot tub?
Much heavier than most people expect. A typical four to six seat tub weighs:
| Typical weight | |
|---|---|
| Empty | 250 to 450 kg |
| Filled with water | 1,300 to 2,000 kg |
| Filled, with five adults | 1,700 to 2,400 kg |
That load sits on a footprint of around four to five square metres. An ordinary garden deck is built for people, furniture and the odd barbecue, not two tonnes in one spot. Putting a tub straight onto standard joists risks sagging boards, cracked joists and a tub that is no longer level.
Three ways to combine a tub and a deck
1. Tub on its own base, deck built around it
The most common approach. The tub sits on a reinforced concrete slab or a solid paved base at ground level, and the deck is built around it, finishing somewhere up its side. The deck carries people; the slab carries the tub. It also leaves the sides easy to reach for servicing.
2. Tub sunk into the deck
Here the deck is built at, or just below, the height of the tub’s rim, so you step straight in. It looks the most finished and makes getting in and out easy. The tub still sits on its own slab, which ends up lower than the deck. Leave a removable section of decking over the equipment side so the pump and heater can be reached.
3. Tub on a reinforced deck
Possible, but only with a frame designed for the load: closer joists, extra beams, more posts and proper footings beneath. It costs more than a slab and is usually only worth it when the tub has to sit high on a sloping garden where a slab is impractical. Get the frame checked by someone who can do the calculation.
Base and drainage
Whatever the layout, the base under the tub should be:
- Level. Most makers allow only a few millimetres of fall across the base.
- Strong. A reinforced concrete slab of around 100 to 150 mm thick is typical, or a manufacturer-approved base system.
- Drained. Tubs spill water every time someone gets out, and need emptying a few times a year. The water needs somewhere to go other than under the deck frame. A gravel margin and a planned route to a drain help.
Why composite suits a hot tub
The area around a tub is wet most of the time. Timber in that spot stays damp, grows algae and needs constant care. Capped composite boards cope far better: they do not soak up water, are easy to wipe clean and do not splinter under bare feet. A textured board gives better grip than a smooth one, which matters when people are stepping out wet in the dark.
Leave the recommended expansion gaps around the tub. Composite moves a little with temperature, and boards butted tight against a tub cabinet can buckle in summer.
Electrics
A hot tub needs a dedicated supply, usually a 32 amp circuit with its own RCD protection and an isolator switch close by. That is notifiable work in a home under the rules on electrical safety in dwellings, so it should be done by a registered electrician who can certify it.
Plan the cable route before the deck goes in. It is far easier to run a duct under the deck frame on day one than to lift boards later. Plug-and-play tubs that run from a normal socket still need that socket outdoors and RCD protected, not on an extension lead.
Layout tips
- Steps. Getting into a tub that sits 900 mm above the deck needs a step or two. Wide, shallow steps are much safer when wet than a small plastic tub step.
- Somewhere to put things. Leave room for towels, drinks and the cover. A tub cover lifter needs clear space behind the tub to fold down.
- Privacy. A slatted screen on the neighbours’ side does more for comfort than anything else. Keep the total height within the limits for garden structures.
- Shelter. In an exposed garden, a screen against the prevailing wind keeps the heat in and the running costs down. On a hillside site, that is often the west side.
- Light. Low step lights and a light near the controls make evening use safer.
Planning
A tub sitting on a ground-level slab does not normally need planning permission, and nor does a low deck around it, as long as no part of the platform is more than 30 cm above the ground and the half-garden limit is respected. A raised deck with a tub on a sloping garden is more likely to need an application, mainly because of overlooking. Listed buildings and some conservation areas are stricter.
What hot tub decking costs
The deck itself costs the same as any other composite deck: typically £100 to £200 per square metre fitted. On top of that, allow for:
- A concrete base for the tub, often several hundred pounds depending on size and access.
- Extra framing, steps and access panels around the tub.
- The electrical supply, which varies with the distance from the consumer unit.
As a rough guide, a 20 m² composite deck around a sunken tub on level ground often comes to £4,000 to £6,000 before the tub and electrics. Our composite decking cost calculator is a good place to start with the deck size.
If you are thinking about a family deck with a tub, our page on decking for family gardens covers sizes and layouts that leave room for everything else.