Sauna Heater Stones: Types, Placement & Maintenance
The Right Stones: Olivine Diabase and Peridotite Only
Only two types of rock are suitable: olivine diabase and peridotite. This is non-negotiable.
Olivine Diabase (Recommended)
Dark volcanic rock. Excellent thermal density, heat retention, and durability. Standard choice globally.
Peridotite (Alternative)
Green-toned igneous rock. Equally durable, similar thermal properties. Less commonly available.
Why River Rocks and Granite FAIL
Do not use river rocks, creek stones, or landscape rocks. They will fail catastrophically within weeks to months.
River Rocks
- • Porous composition with air pockets and moisture
- • Moisture turns to steam, explodes rock from inside (spalling)
- • Mixed minerals expand at different rates, causing cracks
- • Safety hazard: fragments can fly into sauna space
Granite
- • Quartz content creates stress under thermal cycling
- • Not engineered for rapid 200°F+ temperature swings
- • Cracks visibly within weeks of sauna use
- • Failure is permanent and accelerating
Stone Size, Shape & Placement
Ideal Stone Size
2-4 inches diameter. Smaller stones heat unevenly. Larger stones heat slower to core and crack more easily.
Shape
Irregular (natural chunks) pack more densely than round stones. Avoid smooth, polished stones.
Placement
Loosely pack, not tightly. Allow hot air to circulate around and between stones. Height: typically 12-18" above heater element.
Stone Thermal Properties and Löyly
Stones create thermal capacitance — they absorb heat, then release it slowly when water is poured.
- • Without adequate stones: Water vaporizes instantly, creating harsh steam, uncomfortable temperature spike
- • With proper stones: Water absorbs gradually, vaporizing slowly, creating soft rolling steam (good löyly)
- • More stones = better löyly: Premium saunas hold 200-300+ lbs of rocks
Annual Maintenance & Replacement
What to Inspect
- • Visible cracks (indicate stress and failure)
- • Spalling (chunks broken away)
- • Discoloration or whitening
- • Any river rocks or smooth pebbles (replace immediately)
Replacement
Replace any stone showing cracks or spalling. Allow sauna to cool, remove failed stones, inspect remaining stones, add new olivine diabase stones.
Cost: $1-2/lb. A typical sauna uses 100-150 lbs, so full replacement: $100-300.
Loss Rate
With proper stones, you might lose 5-10% of mass annually in a frequently-used sauna. This is normal.
Summary
- • Use ONLY olivine diabase or peridotite
- • NEVER use river rocks, granite, or landscape stones
- • Select 2-4 inch stones, irregular shape, loosely packed
- • Inspect annually for cracks or spalling
- • Replace any failed stones (normal maintenance)
- • More stones = better löyly