How to Prepare a Base for a Hot Tub

How heavy is a hot tub?

Add up three things:

• Dry weight – the tub itself, usually 200–500kg for hard-shell tubs.
• Water – one litre weighs one kilogram, and most tubs hold 800–2,000 litres.
• People – allow around 80kg per person.

Example: a 4–5 seat tub weighing 400kg dry, holding 1,300 litres, with five people in it weighs 2,100kg. Spread over a 2.1 × 2.1m footprint (4.41m²), that's around 476kg per m². The Hot Tub Base Calculator does this for your tub.

Why decking usually isn't enough

Ordinary domestic floors and decks are typically designed for around 1.5kN/m², which is roughly 150kg per m². A filled hot tub can put three times that load on the same area. Standard garden decking is not designed for it. If you want a tub on or sunk into decking, the deck must be designed for the load by a structural engineer, with extra joists, posts and proper footings.

The best bases

Concrete slab (best option): a reinforced concrete slab at least 100mm thick on about 100mm of compacted MOT Type 1 is the gold standard. Make it level, extend it a little beyond the tub on all sides, and give it a very slight fall so water runs off rather than pooling underneath. Many manufacturers ask for this in their warranty terms.

Existing patio: a well-built patio – slabs on a full mortar bed over a compacted sub-base – can often take a hot tub, as long as it's level and firm. A patio laid on sand, on dot-and-dab mortar, or over an old garden with no sub-base may crack or sink. If you're not sure how it was built, ask a professional to check.

Compacted gravel base: a deep layer of compacted crushed stone inside a timber or edging frame can work for some tubs and drains well. Check your manufacturer allows it.

Inflatable hot tubs are lighter dry but still weigh around a tonne when filled. They need a flat, smooth surface free of anything sharp, and an insulated ground mat. Never put one on a slope, and never on a raised deck, balcony or roof without checking it can take the weight.

Other things to plan

• Electrics: plug-in tubs need a suitable outdoor socket. Hard-wired tubs need a dedicated circuit, and new circuits must be installed by a registered electrician under Building Regulations (Part P).
• Access: measure the route from the road to the base, including gates, side passages and steps. Hot tubs arrive on their side and need a lot of width.
• Cover lifter: leave space behind the tub for the cover to fold back.
• Drainage: plan where the water goes when you empty it.
• Neighbours: think about privacy, pump noise and steam. Most hot tubs don't need planning permission, but gazebos and permanent structures around them might – check with your local council.