Suitable Foundations for Saunas and Hot Tubs

The foundation determines whether a sauna or hot tub stands level and undamaged over the long term. Depending on its dimensions, insulation, glazing and type of heater, an outdoor sauna weighs approximately 500 to 2,500 kg, from compact barrel saunas to large modular buildings with an entrance room. A hot tub filled with water and bathers weighs approximately 0.9 to 2.5 tonnes. This page explains which foundation to choose by weight, how to build it and what to prepare before delivery.

A poorly designed or badly executed foundation leads to uneven settlement of the ground, twisting of the timber load-bearing frame, sticking doors and windows, cracked glazing and, in extreme cases, permanent damage to the waterproofing and vapour-control layers of the building.

Who prepares the foundation

The foundation is prepared by the customer before delivery, usually with the help of a building contractor. The customer is responsible for its suitability and load-bearing capacity. If the foundation is not suitable on the day of delivery, the sauna or hot tub cannot be installed and installation is postponed until the foundation is ready. Details are set out in the General Terms and Conditions.

Overview: which foundation for which sauna weight

Besides the weight of the sauna, the choice of foundation also depends on the soil type, the slope of the site and the local frost depth. Where ground conditions are uncertain, have the foundation designed by a structural engineer or a building contractor.

What every foundation must achieve

Transfer the load into the ground

A sauna transfers permanent loads to the foundation, namely the weight of the timber frame, cladding, insulation, heater and sauna stones, together with variable loads from bathers, snow and wind. It does so across its full surface or through linear or point supports. The foundation must spread this load into the ground without exceeding the bearing capacity of the soil or deforming the building.

Respect the frost depth

Concrete pad and strip footings are founded below the local frost depth. In Central European conditions this is approximately:

• 80 cm in permeable sandy gravel soils,
• 100 cm in common loams,
• 120 cm in clay soils and at higher mountain altitudes.

If the footing is shallower and the soil is frost-susceptible, especially clay, water freezing in the pores of the soil lifts the ground. In spring, as it thaws, the ground settles back unevenly, which distorts the geometry of the sauna and causes cracks in its structure.

Keep the timber away from moisture

The timber structure of a sauna must not be in direct contact with the ground. The lower load-bearing frame must be permanently protected against three sources of moisture:

• Rain and splash water: the bottom edge of the structure must be at least 100 mm above the surrounding ground.
• Rising damp: a waterproofing layer, such as bitumen membrane or EPDM pads, is placed wherever concrete or steel meets timber.
• Moisture trapped under the sauna: a continuous ventilated air space must remain beneath the floor so that the substructure dries out naturally.

Foundation by sauna weight

Light saunas: 500 to 1,000 kg

This category includes small barrel saunas, compact Finnish saunas for 2 to 3 people without an entrance room, and freestanding outdoor infrared saunas. The load is usually carried by 2 to 3 transverse timber skids or a perimeter frame. A compacted gravel bed with concrete paving slabs beneath the load-bearing skids is sufficient. An equivalent alternative is 4 to 6 ground screws 500 to 600 mm long, which retain an adequate reserve even with the load from bathers and snow.

Medium-weight saunas: 1,000 to 2,000 kg

These are family Finnish saunas for 4 to 6 people with insulated walls, a small relaxation room and a glazed front. They need a stiffer foundation so that the building does not twist and the doors do not stick. A suitable solution is 6 to 9 adjustable ground screws with bayonet extensions driven to a depth of 700 to 850 mm. Each point then carries approximately 750 kg, which limits the risk of settlement even on sloping ground. An equivalent alternative is 300 × 300 mm concrete pad footings below frost depth with height-adjustable anchor brackets.

Heavy saunas: 2,000 to 2,500 kg

This category covers large modular sauna houses delivered in one piece, log buildings made of solid timber beams, and saunas with a shower, WC, heavy wood-burning heater and large areas of triple insulating glazing. They require a rigid and level foundation surface. A deviation in flatness of more than 5 mm over the full length creates internal stress in the glazed areas and can damage the roof waterproofing. The most suitable solution is a monolithic reinforced concrete slab 100 to 150 mm thick with steel reinforcing mesh on a compacted gravel sub-base. On broken or steeply sloping ground, it can be replaced by continuous reinforced concrete strip footings under all load-bearing external and internal walls.

Foundation types and how to build them

Reinforced concrete slab

A full-surface solution with high load-bearing capacity and dimensional stability. It suits the heaviest saunas, solid insulated buildings and locations where the sauna adjoins a hot tub or terrace. Build-up from the bottom:

1. non-woven separating geotextile weighing 200 to 300 g/m² on the original load-bearing soil,
2. compacted gravel bed of washed 16/32 mm aggregate, 150 to 250 mm thick, acting as a drainage and capillary-break layer,
3. separating PE membrane,
4. C20/25 concrete (or C25/30) 100 to 150 mm thick, reinforced with welded steel mesh of Ø 6 to 8 mm wire at 150 × 150 mm centres,
5. on the cured concrete, beneath the load-bearing timber beams, waterproofing of bitumen membrane or a waterproofing coating, plus EPDM pads.

The slab should not extend beyond the external perimeter of the sauna walls. A projecting edge forms a ledge where water running down the walls collects and soaks the lower timber plinth. If the slab does extend further, for example because of an adjoining terrace, its surface must fall away from the sauna at 1.5 to 2.5 %.

Concrete pad and strip footings

Pad footings provide point support and strip footings linear support. They are chosen when you do not want to concrete the whole area, want to keep the floor well ventilated from below and reduce the amount of wet construction work. Procedure:

1. Excavate to the frost depth of 80 to 120 cm.
2. Place 100 mm of 16/32 mm drainage gravel at the bottom and compact it thoroughly.
3. Pour C20/25 concrete, most often into permanent formwork made of concrete blocks or cardboard formwork tubes.
4. Set height-adjustable steel anchors (U-brackets or threaded rods) into the fresh concrete to hold the timber frame at least 100 mm above ground level.

Across the whole footprint of the sauna, remove the topsoil to a depth of 150 to 200 mm, lay a geotextile and cover it with washed 16/32 mm gravel. This prevents weeds, mud and standing water under the sauna. All footings must lie in a single horizontal plane with a tolerance of ±5 mm, and the diagonals must match to within 10 mm.

Ground screws

A dry method with no excavation or concreting. Hot-dip galvanised steel screws are driven into the original, undisturbed ground. As the screw is turned in, its thread compacts the surrounding soil, creating high shaft friction and capacity in both compression and pull-out. The capacity of each point depends on the driving depth and the soil type; as a guide:

• 500 mm screw: approximately 250 to 300 kg,
• 600 mm screw: approximately 350 kg,
• 600 mm screw with a 250 mm extension (850 mm in the ground): approximately 750 kg,
• 600 mm screw with a 500 mm extension (1,100 mm in the ground): up to 1,300 kg.

The advantages are immediate loading without waiting for concrete to cure, easy levelling on sloping ground and the option to dismantle and relocate the building later. Ground screws are not suitable for stony soil with large boulders, fresh uncompacted fill or permanently waterlogged clay without a load-bearing layer below.

Compacted gravel bed with concrete slabs or block paving

A permeable foundation for lighter and medium-weight saunas set on full-length timber skids. Procedure:

1. Remove the vegetation layer to a depth of 250 to 300 mm.
2. Lay a geotextile at the bottom of the excavation to stop the gravel mixing with the subsoil.
3. Fill with 16/32 mm crushed stone in 100 mm layers, compacting each layer with a plate compactor.
4. Spread 30 to 50 mm of fine 4/8 mm grit as a bedding layer.
5. Lay concrete paving slabs, for example 500 × 500 × 50 mm, or block paving to an accurate level.

Loosely spread, uncompacted gravel or slabs laid directly on the lawn without compacted sub-base layers are not a suitable foundation for a sauna.

Adjustable pedestals and EPDM pads

These are used mainly when a sauna is installed on an existing hard surface, such as a concrete terrace, stone paving or a balcony structure. Adjustable pedestals are plastic supports with stepless height adjustment and a capacity often exceeding 1,000 kg each. They allow slopes and unevenness to be corrected to the millimetre and leave a continuous gap under the base frame for water to drain away. EPDM pads 3 to 8 mm thick are placed between the concrete or steel base and the timber beam. They interrupt rising damp and reduce sound transmission.

On a balcony, roof or timber deck, the load-bearing capacity of the structure itself is decisive. This must always be assessed by a structural engineer.

Drainage and ventilation under the sauna

• The surrounding ground should fall away from the sauna at 1.5 to 2.5 % so that rainwater does not collect at the foundations.
• Leave a clear air gap of at least 100 mm above ground level beneath the sauna floor.
• If you cover the base of the sauna with a perimeter plinth, fit ventilation grilles so that air flows continuously under the sauna.

Electrical supply for the sauna

Electric sauna heaters rated at 6 to 15 kW require a dedicated three-phase supply. The usual standard is a 5 × 6 mm² copper cable (for example NYY-J or CYKY-J), or 5 × 4 mm² on shorter runs.

• Run the cable in a protective duct at least 700 mm below ground, in a sand bed and with warning tape.
• Leave a cable reserve of at least 3 m at the location of the future sauna distribution board.
• The supply and its protection are installed by a qualified electrician according to the output of the selected heater.

Circuit protection, RCD and cable size according to the heater output and length of run are explained on the page Electrical Connection for Saunas and Hot Tubs.

Connecting the sauna to the prepared supply and starting the heater are part of commissioning, which we carry out free of charge. Details are on the Delivery and Installation page.

Water and drainage

If the sauna includes a shower, steam generator or WC, bring the water supply and waste pipes to the site before the foundation is completed.

• Lay the water supply pipe below frost depth.
• On the uninsulated section between the ground and the sauna floor, insulate the pipe and add a self-regulating heating cable, or design it with a fall so that it can be drained for winter.
• Lay DN 50 or DN 110 waste pipes at a fall of at least 2 % to the sewer or an approved cesspit.

Foundation for hot tubs

A hot tub stands on a timber base that must rest on the foundation across its entire load-bearing surface. A tub filled with water and bathers loads the foundation with approximately 370 to 610 kg per m² of footprint, and the total weight can exceed 2,000 kg.

• On open ground, a concrete slab at least 10 cm thick with steel reinforcement, or paving on a compacted gravel bed that is level across its whole surface, is suitable. Grass and bare soil are not suitable, as they settle unevenly under load.
• On a timber deck, balcony or roof, the load-bearing structure is decisive. We recommend a structural assessment.
• Level and fall: the tub must stand level, and the surrounding area should fall away from it.
• Water: prepare a water supply and a place for the water to drain to when the tub is emptied, for example a connection to the sewer.
• Electricity: for electric heating, hydromassage and filtration, the supply is usually 230 V/16 A or 400 V/3 × 16 A with a Type A residual current device, depending on the model. A tub with a wood-burning heater only, without hydromassage or lighting, does not need an electrical connection.
• Wood-burning heater: the surface under the heater and in front of its door must be non-combustible, with clear access to the door.

Checking the foundation before delivery

Prefabricated saunas and hot tubs are often lifted onto the foundation by a truck-mounted crane or a hydraulic loader crane. Before delivery, therefore, check the foundation:

• the deviation in level across the entire foundation surface is no more than ±5 mm,
• the difference between the diagonals is no more than 10 mm,
• the layout of the footings or ground screws matches the dimensions of the base frame of the specific sauna, which you should confirm with us before work begins,
• the electrical supply and, where applicable, the water and waste connections are brought to the installation point.

Moving the sauna or hot tub from the gate to the installation point, and the equipment required, are described on the Delivery and Installation page.

Any questions?

If you are not sure which foundation to choose for a particular model and your site, please contact us before work begins. We will advise you based on the weight and dimensions of the selected sauna or hot tub. Contact us.