What base does a hot tub need?

A hot tub is unlike anything else people put in a garden. The load is enormous relative to its footprint, it is permanent, and the tub itself is a rigid shell that will crack if it is unevenly supported. On top of that it needs a certified electrical supply, a way to drain several hundred litres, and access to the panel that everyone forgets about until something needs servicing.

Safety

What you'll need

• The manufacturer's filled weight and footprint figures
• Concrete, reinforcement mesh and formwork, or a designed alternative
• MOT type 1 for the sub-base
• Plate compactor, hired
• Long level and a straightedge
• A qualified electrician for the supply

Step by step

1. Get the real numbers from the manufacturer.Dry weight, water capacity, and the maximum number of occupants. Filled weight is dry weight plus roughly a kilogram per litre plus about 80kg per person. Divide by the footprint area for the load per square metre. It is usually a considerably larger number than people expect.
2. Compare that against what your existing surface was built for.An ordinary domestic patio is built for foot traffic and furniture. A hot tub load is closer to a vehicle load, concentrated and permanent. In most cases the honest answer is that the existing patio is not adequate and a purpose-built base is needed.
3. Build a level base, not a falling one.This is the difference between a hot tub base and every other outdoor surface. Paving is laid with a fall; a hot tub must sit dead level or the water line sits crooked and the shell is unevenly stressed. Build a level pad and deal with the drainage around its edges instead.
4. Excavate to firm ground and compact the sub-base.Strip topsoil and all soft material, then MOT type 1 in 75 to 100mm layers, each compacted with a plate. This does the structural work — a thick slab on soft ground still settles.
5. Cast a reinforced concrete pad.For a domestic tub, a reinforced slab typically around 100 to 150mm thick over a compacted sub-base is the usual specification, and heavier tubs need more. Get the thickness and the reinforcement confirmed against your tub's actual load rather than working from a general figure.
6. Make the pad bigger than the tub.At least 300mm beyond the tub's footprint on every side. You need somewhere to stand while getting in and out, somewhere to put a step, and access all round for servicing.
7. Plan the drainage around the pad, not through it.The pad is level, so water needs to leave at its edges — a gravel margin, a channel drain at the low side, or a fall on the surrounding paving. Also plan where several hundred litres will go when the tub is drained, which is a surprising volume to release into a garden or a drain.
8. Keep access to the equipment bay.One side of a hot tub has a removable panel with the pump, heater and controls behind it. It needs clear access, and a technician needs room to work. Building a tub into a surround with no access is the commonest regret.
9. Get the electrical supply designed and installed by an electrician.A hot tub needs a dedicated outdoor circuit with the correct protection, an accessible isolator, and appropriate cabling and enclosure ratings. This is notifiable work and it must be installed and certified by a qualified electrician. It is not a DIY item under any circumstances.
10. Check the delivery route before building anything.Hot tubs are usually craned or trolleyed in and they are large. Confirm the route, the gate widths, the crane access and any overhead cables with the supplier before the pad is poured, not after.

Common mistakes

• Putting a tub on an ordinary patio — Slabs on a spot bed over a light sub-base cannot take a concentrated permanent load. Slabs crack, the bed settles unevenly, the tub goes out of level and the shell is stressed.
• Building the base with a fall — Every instinct from paving says put a fall on it. A hot tub must be level. The drainage is dealt with at the edges.
• Setting the tub on a standard deck — Decks are designed for a distributed live load of people and furniture. A hot tub is several tonnes over a few square metres, and a standard frame will deflect badly or fail.
• Building a surround with no access to the equipment bay — Every hot tub needs servicing. A surround that has to be dismantled to reach the pump turns a routine repair into a construction job.
• Leaving the electrical supply as an afterthought — The cable route usually has to be dug in, and it is far easier before the base and the surrounding surfaces are built. Talk to the electrician at the planning stage.

Alternatives

• Reinforced concrete pad: The default and the most straightforward to specify. Level, continuous, permanent, and it will take any domestic tub with the right thickness and reinforcement. The main cost is the excavation and the concrete.
• A deck designed for the load: Possible and common, but the frame must be designed for the concentrated load — much closer joist centres, deeper sections, additional beams and additional footings directly under the tub. This is a designed structure, not a standard deck with a tub put on it, and it is worth having the design checked.
• Existing patio, if verified: Occasionally adequate — a patio laid on a full mortar bed over a thick, well-compacted sub-base may take a small tub. It has to be verified rather than assumed, by lifting a slab and looking at the build-up, and it must be genuinely level.
• Proprietary hot tub base panels: Interlocking recycled plastic panels sold specifically for tubs, laid on a compacted and levelled sub-base. Quick, no concrete and no curing time. Check the panel system's rated load against your tub's filled weight, which is the whole question.

Questions people ask