Sauna base guide: what your outdoor sauna sits on (and costs)

Nobody gets excited about the base — which is exactly why it’s the most common corner cut, and why “what’s it sitting on?” is the first question every fitter asks. Get it wrong and the symptoms arrive slowly: doors that won’t seal, stressed glass, damp creeping up the frame. Get it right and it’s invisible forever.

The engineering problem is short: 1,000–2,000 kg, small footprint, dead level, forever dry underneath.

The three options that work

1. Slabs on a compacted sub-base — £600–£1,200. Dig out, 100mm+ of MOT Type 1 compacted in layers, sharp sand, slabs laid level. The value answer for barrels and lighter cabins, and a realistic DIY weekend if you’re careful with levels. Weakness: only as good as the compaction — skipped groundwork shows up as settlement a year later.

2. Reinforced concrete pad — £1,200–£2,500. ~100mm of mesh-reinforced concrete on hardcore. The permanent answer for heavy framed cabins and craned-in pods, and the base bespoke builders default to. Needs formwork, a mixer and curing time — allow a week-plus before the sauna lands on it.

3. Ground screws — comparable money, one day. Steel screws driven to refusal with a frame on top. No spoil, no cure time, forgiving on slopes and soft ground, easily removed later — the reason garden-building installers have largely adopted them. On rocky Cornish-type ground, a survey-first installer matters.

Honourable mention — the existing patio: often fine for a small barrel if it’s genuinely level and properly laid. Decking: almost always no — see the FAQ.

The details that decide the sauna’s lifespan

• Level within a few millimetres. Rigid building, tight tolerances: doors, glass and barrel staves all punish an out-of-level base.
• Air underneath. The sauna sits on bearers or its cradle, clear of the surface, so the underside can dry. Timber flat on concrete rots invisibly from below.
• Water runs away. Ground falls away from the base, or a French drain takes the wet side. Water pooling under a warm building is the failure mode.
• Oversize slightly. A 300–400mm margin around the footprint gives you a maintenance strip, somewhere to stand, and dry access — worth it every time. If a cold plunge is part of the plan, size the base (and its drainage) for both now rather than twice.
• Think about services while the ground is open. The electrical supply wants a buried duct — digging once is cheaper than digging twice.

Getting it priced

The base is one of the four real installation costs, and quotes only compare if each names its groundworks. Where the whole project lands — cabin, base, electrics, delivery — is home sauna cost, or let the cost calculator build your number in a minute. Choosing what actually goes on the base starts at the outdoor sauna buyer’s guide.