How to Build a Base for an Outdoor Sauna

A properly built base is one of the most important parts of any sauna installation. If the base is not level, solid, and well-drained, it can lead to issues with alignment, stability, and long-term performance.

This guide covers the four most common sauna base options: paving slabs, concrete, timber frame, and gravel — along with when to use each and how to build them properly.

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General Rules for Any Sauna Base

• Make the base at least 100–150mm larger than the sauna footprint
• Ensure the surface is flat and level in all directions
• Use a solid, compacted foundation
• Allow for drainage around the base
• Plan access and electrical routing early

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Sauna Base Comparison

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Option 1: Paving Slab Base

A slab base is the most common and practical option for outdoor saunas.

Steps

1. Mark out area larger than sauna
2. Excavate 150–250mm
3. Remove soft soil and roots
4. Add hardcore in layers and compact
5. Check level carefully
6. Add thin sand bedding layer
7. Lay slabs and level
8. Check stability

Advice:

• Keep bedding layer thin
• Use large slabs where possible
• Fix any movement before installation

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Option 2: Concrete Base

The most durable and permanent solution.

Steps

1. Mark out base area
2. Excavate
3. Add compacted hardcore
4. Build shuttering
5. Add reinforcement if needed
6. Pour concrete
7. Screed level
8. Allow to cure fully

Advice:

• Do not rush curing
• Ensure edges are square and level
• Plan drainage around slab

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Option 3: Timber Frame Base

A timber base works well where a raised platform is required or the ground is uneven.

Steps

1. Install solid support points (pads or ground screws)
2. Build outer frame
3. Add joists at regular spacing
4. Level entire structure
5. Add bracing
6. Install decking surface
7. Check for flex

Advice:

• Use pressure-treated timber only
• Ensure no movement or flex
• Allow airflow underneath

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Option 4: Gravel Base

A gravel base is a simple, cost-effective option that provides excellent drainage.

Steps

1. Mark out base area
2. Excavate 150–250mm
3. Install edging
4. Lay membrane
5. Add and compact sub-base
6. Check level
7. Add top gravel layer

Advice:

• Use angular gravel, not rounded
• Compact properly
• Keep top layer thin

For best results, place slabs or pads under sauna support points.

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Which Base Should You Choose?

• Best overall: Paving slabs
• Best long-term: Concrete
• Best for raised installs: Timber frame
• Best for drainage: Gravel (with supports)

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Common Mistakes to Avoid

• Base too small
• Not compacting sub-base
• Using soft sand to level
• Building directly on grass
• Ignoring drainage
• Not checking level properly

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Base Considerations for Australian Conditions

Reactive clay soils, common around Melbourne, Adelaide and parts of Sydney, move as they wet and dry, so a properly prepared concrete slab or engineered footings are the safest option. On sandy coastal blocks, compact the base well and allow for drainage.

In termite-prone areas, keep timber off the ground and leave room for inspection. Check with your local council whether your sauna needs approval; many small outbuildings are exempt, but the rules vary by state and by block.

Short Version

Mark → dig → compact → level → install.

If the base is right, the sauna build becomes much easier.