Black marks or dark staining on the interior wood of a sauna are very common and can occur in both indoor and outdoor saunas, whether they are domestic or commercial.

Unlike many other timber products, the wood inside a sauna is left untreated for health and safety reasons. Wood treatments and sealants can release harmful fumes when exposed to the high temperatures inside a sauna, so untreated timber is the safest option.

Over time, some woods naturally develop dark mineral or tannin stains, particularly in areas exposed to moisture. These marks are usually part of the wood itself and are not mould. They are most commonly seen near the sauna door, where moisture from outside is most likely to enter.

In most cases, the marks can be removed with light sanding once the wood is completely dry.

To help dry the timber, we recommend running the sauna using dry heat (for example, the infrared setting if your sauna has one) while using no water on the sauna stones. This allows any moisture in the wood to evaporate fully. Once the wood has dried, lightly sand the affected area to remove the staining.

For best results, carry out this process during a period of drier weather, as this will help the timber dry more effectively.