How to build a base for a garden sauna

What a garden sauna base has to do

A sauna base has 4 jobs. It carries the cabin, the heater, the stones and everyone inside without settling; it keeps the floor level so doors close and joints stay tight; it keeps the bottom timbers out of standing water; and it does all that without lifting the cabin over its planning height.

Makers put the requirement simply. Clearlight says to install its outdoor infrared cabins on a flat, level concrete or timber base, raised on a stable platform where needed so water can't pool beneath. Finnmark's EMBA can go on a concrete foundation, concrete blocks or ground screws.

A poor base can also cost you the warranty. Palmako's sauna assembly manual excludes complaints caused by the building subsiding on an incompetently made foundation, and tells owners in strong-wind or heavy-snow spots to anchor the building to the ground.

How much a garden sauna weighs

Weight sets how strong the base has to be, and it varies more than the footprint suggests. These are figures from makers' and sellers' own pages.

The weights come from the listings for the 2-person and 5-person Clearlight cabins, Finnmark's EMBA and the Layzee Living barrel. A traditional cabin adds its heater and stones: Harvia's 6kW Vega weighs 11.2kg and takes 20kg of stones.

Then add people. Take the 5-person infrared cabin with 5 adults at 80kg each: 476 + 400 = 876kg on 2.75m² of floor, or about 320kg per square metre. A base that can't take that evenly will show it first as a door that starts to stick.

Weight isn't always on the product page, so compare it across the cabins on our best outdoor saunas list and ask the seller for the built weight of anything that doesn't state it before you plan the base.

Concrete slab base for a garden sauna

A concrete slab is the default because it suits every cabin and every maker's instructions. The Planning Portal's guidance on outbuilding floors describes the usual build-up for a garage floor: hardcore, sand blinding, a damp-proof membrane and concrete, with reinforcing mesh as good practice against cracking. A sauna slab follows the same pattern.

The ground under it matters as much as the concrete. The Planning Portal's foundations guidance says topsoil is normally removed down to firm, undisturbed ground, that some clay soils shrink and swell with the seasons to a depth of about 0.75m, and that trees on clay can draw out moisture and move the ground further.

It also warns that load spreads down and out from a foundation at about 45 degrees, so a slab near a drain or sewer can crack it. If a drain runs under or beside your chosen spot, move the sauna or ask a groundworker to design round it.

Make the slab at least the size of the cabin's footprint so no edge of the floor frame hangs over, set it slightly above the surrounding ground, and give the surface a gentle fall away from the door so rain runs off rather than under.

Paving slabs on a compacted sub-base

Paving slabs bedded on a compacted hardcore sub-base give a flat, free-draining pad that many people can lay themselves. It works best for lighter cabins on firm ground, where the load is spread over a frame or bearers rather than concentrated on a few points.

The weak spots are settlement and level. Slabs laid on a thin or poorly compacted base can rock or sink at the corners, and under a cabin you won't see it happening until the door binds. Compact the sub-base in layers and check the finished surface with a long straight edge and a spirit level across both diagonals.

Check that your sauna maker accepts paving before you build it. Clearlight's outdoor specification names concrete or timber, and Finnmark's names concrete, concrete blocks or ground screws, so a paved pad is a judgement call to agree with the seller, not a default.

Ground screws for a garden sauna

Ground screws are steel piles twisted into the ground by machine, with a plate or bracket on top that carries a timber frame. They suit sloping gardens, tree roots and spots where a lorry of concrete can't reach, and Finnmark lists them as a suitable base for its EMBA cabin.

Each screw carries a share of the load, so the sum is simple. A cabin, heater and people adding up to 876kg on 6 screws puts about 146kg on each; on 9 screws, about 97kg. How much each screw can actually hold depends on its size and your soil, which is why ground screw installers size them for the site rather than from a catalogue.

Give the installer the total, not just the empty cabin weight. The frame on top of the screws then has to be built level and braced so it can't rack.

Timber frame on pads or blocks

A pressure-treated timber frame sitting on concrete pads or blocks is the lightest groundwork of the 4. It lifts the cabin clear of the ground, which helps air move underneath and water drain away, and Clearlight's specification allows a timber base raised on a stable platform for exactly that reason.

It needs the most care with level and spacing. Each pad takes a point load, so the pads need firm ground under them and enough of them that the frame doesn't sag between supports. Use timber rated for ground contact and keep it off the soil itself.

Watch the height. Every centimetre the frame adds counts towards the planning limits, which the next section covers, and a deck that carries the cabin plus a seating area counts as a raised platform in England if it's more than 0.3m high.

Which base suits your garden

Most gardens can take more than one of these. Choose on the ground, the weight and how permanent you want it to be.

Levelness, drainage and clearances

Level comes first. A cabin built on a base that's out of level twists as it's assembled, so doors drag and panels gap; check the base along each side and across both diagonals before the cabin arrives, not after.

Keep water moving away. Set the base slightly above the surrounding ground, give hard surfaces a fall away from the cabin, and don't let soil, bark or gravel build up against the bottom timbers. A cabin that sits in a puddle every winter rots from the bottom up.

Leave room around it. Finnmark asks for at least 300mm between the base and any wall or fence. Clearlight specifies 5cm at the sides and rear and 30cm above the roof, and says site conditions may need more space than that.

If the cable to the cabin is going underground, lay a duct before the slab is poured, and agree the route with the electrician who'll handle the sauna's electrical work. A length of duct costs little at this stage and saves breaking out the base later.

Delivery access for a garden sauna

Plan the route from the road to the base at the same time as the base. Finnmark's EMBA arrives flat-packed in a crate of 2.22m by 0.7m by 1.6m weighing 630kg plus the heater and stones, delivered by Moffett, a forklift that travels on the lorry.

A cabin delivered built is harder still. Layzee Living says its 200cm barrel, if delivered fully built, weighs upwards of 750kg and has to be off-loaded by the customer, which means a crane or a team and a clear route.

How the base affects planning height

In England, the government's permitted development guidance measures height from the ground immediately next to the building, and says ground level doesn't include anything laid on top, such as decking. On a slope it's measured from the highest point of the ground next to the building.

So the base counts. A 2.2m cabin on a 300mm timber frame within 2m of a boundary reaches 2.5m, the limit there, with nothing to spare. A slab set into the ground adds little or nothing, which is one reason it suits a cabin that's already close to the limit.

The rest of the planning rules for garden saunas change between England, Wales, Scotland and Northern Ireland, but every nation measures height, so settle the base before you settle the spot.