What base does a garden sauna need?

The base is the part of a garden sauna project that gets left until last and should be settled first. A 3-5 person cabin weighs about 600kg empty and considerably more with people and water in it, and that load sits on a small footprint. Put it on the wrong ground and the doors stop closing within a winter.

The three bases that work

Lawn, bare soil, loose gravel and standard patio slabs laid on sand all fail, either immediately or over the first wet winter. The failure is rarely dramatic. The cabin settles unevenly, the frame racks by a few millimetres, and the door that closed perfectly in September binds in February.

Concrete slab

The default, and the right answer if the cabin is staying put. Excavate to about 150mm, lay 100mm of compacted MOT type 1 hardcore, then 100mm of concrete over a damp-proof membrane, with mesh reinforcement if the cabin is large. Finish it level, and make it slightly larger than the cabin footprint so rain running off the walls lands on concrete rather than washing out the edge.

The constraint is time. Concrete needs to cure before it takes the load, which in practice means pouring it a couple of weeks ahead of a delivery date. A made-to-order cabin arriving with nowhere to stand is an expensive storage problem, so book the groundwork before you confirm the delivery slot.

Paving on a proper sub-base

Cheaper, more DIY-friendly, and perfectly adequate when it is built properly. The mistake people make is treating this as a patio: the slabs are not the base, the compacted sub-base underneath them is. Excavate, lay and compact at least 100mm of type 1, then bed the slabs on mortar rather than sand so they cannot migrate.

Sand-bedded slabs move. Under 600kg concentrated on a few bearers, they move quickly, and the cabin follows them. If you are doing one thing to the specification, make it the sub-base depth.

Reinforced deck

The answer for sloping ground, where levelling by excavation would mean a retaining wall. A deck built for a sauna is not a garden deck: joists want to be at closer centres than normal, sized for the point loads under the cabin bearers, and supported on concrete pads rather than on soil.

Leave the underside ventilated. A sealed void under a timber cabin traps moisture and rots the deck from below, which is the same principle that makes airflow around the cladding matter for keeping the cabin weathertight in a damp climate.

Drainage, and the detail everyone forgets

Water leaves a sauna in two ways: down through the floor when it is washed out, and off the roof onto the ground around it. Both need somewhere to go.

• Fall the base by about 1 in 80 away from the door so washing water runs out rather than pooling
• Extend the base 50mm to 100mm beyond the cabin on all sides so roof runoff lands on hard standing
• Keep the finished level above the surrounding ground, so the cabin is never sitting in a puddle
• Do not butt the base against a house wall below the damp-proof course

Sequence the whole job

1. Confirm the cabin footprint and weight, then size the base 50mm to 100mm larger all round
2. Check the position against the height limits, since where it sits decides whether it needs permission
3. Book an electrician registered under a competent person scheme to check the consumer unit and agree the cable route
4. Build the base, and lay the supply cable duct while the ground is open
5. Let concrete cure, then take delivery

Step four is the one that saves the most money. Digging the trench for the supply while the base excavation is already open avoids a second visit and a second dig, and that trench is usually the largest single line in the quote for the electrical work a garden sauna needs. The cable itself is notifiable work under Approved Document P, so the duct can be yours but the connection cannot. Position also decides the planning question, since the height limits in the Planning Portal's outbuilding rules turn on how close the cabin sits to a boundary, which is what whether the building needs permission works through.

Base cost scales with footprint, so this is another reason to settle how many people the cabin actually seats before ordering. The weights quoted on our outdoor sauna rankings are the figures to design the base against.