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Sauna Base Guide: Concrete, Slabs, Gravel or Timber? | KOVE

Sauna Base Guide: Concrete, Slabs, Gravel or Timber? | KOVE

Apr 13, 2026

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.

Key principle: The base itself should always be level. Drainage should happen around the base, not by sloping the sauna platform.
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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

Base Type Cost Difficulty Best For Notes
Paving Slabs ££ Medium Most homes Best balance of cost and stability
Concrete £££ Higher Permanent installs Strongest and lowest maintenance
Timber Frame ££–£££ Medium–High Raised or uneven ground Must be rigid and well supported
Gravel £ Low–Medium Drainage-focused setups Best when combined with slabs under load
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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
Important: If the timber base flexes, it is not suitable for a sauna.
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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.

Best setup: Gravel base with slabs under load 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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Short Version

Mark → dig → compact → level → install.

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