Sauna guide
Thermally modified timber for saunas and steam rooms
Thermal modification suits a sauna because it makes timber more stable, less resinous and more comfortable to touch at high temperatures. Whether you are building a garden sauna, a spa or a home steam room, this guide covers what to choose and how to install it.
Why thermally modified timber works in a sauna
- Stability: low movement through repeated heating and cooling cycles keeps joints tight and reduces warping.
- Low resin: heat-treated pine and spruce release far less resin, so surfaces stay clean and comfortable.
- Comfortable surfaces: lower thermal conductivity means boards do not feel as hot to the touch as untreated timber at the same air temperature.
- No chemicals: nothing is added to the wood that could off-gas when heated.
- Appearance: warm, even brown tones suit both traditional and contemporary interiors.
Best species for each part of the sauna
| Component | Recommended species | Why |
|---|---|---|
| Benches and backrests | Thermally modified aspen, alder or ash | Pale, smooth, low resin and comfortable to sit on |
| Wall and ceiling lining | Thermally modified aspen, alder, pine or spruce | Stable, attractive and practical across a wide price range |
| Door frames and trims | Thermally modified ash or pine | Strong enough for hinges and handles |
| Exterior of a garden sauna | Thermally modified pine, spruce, ash (Class D) | Durable, weather resistant cladding |
If you need a species that is not listed, ask: we can often source it for larger projects.
Profiles for walls, ceilings and benches
- Tongue and groove lining: the standard choice for walls and ceilings, installed horizontally or vertically.
- Bench boards: planed, with eased and rounded edges for comfort, in thicknesses suited to the span.
- Backrests and headrests: smooth profiles that avoid sharp corners.
- Duckboards and floor slats: for wet areas, with gaps for drainage and drying.
Installation advice
- Build a vapour-controlled, insulated frame suited to the sauna type, following the sauna heater manufacturer instructions.
- Fix lining boards with stainless steel nails or hidden fixings, which will not mark the wood or become uncomfortably hot.
- Leave a ventilation gap behind and above the lining and allow for air movement as the sauna warms and dries.
- Keep combustible materials clear of the stove by the distances in the stove manufacturer instructions and building regulations.
- Use only sauna-safe protective products inside. Do not use paint, varnish or ordinary stain on surfaces that get hot.
Care and cleaning
- Ventilate after use so the timber dries fully between sessions.
- Wipe benches with a damp cloth and a mild, sauna-safe cleaner. Avoid harsh chemicals.
- Sand lightly to remove marks or sweat stains, then wipe clean.
- Use a sauna-safe protective wax or oil on benches only if recommended by the manufacturer.
- Towel on benches protects the timber and is more hygienic.
Garden saunas: clad the outside too
Many customers build a garden sauna, a cabin or a log-effect building and clad it in thermally modified timber, so the outside matches the interior and weathers well. See our cladding guide for detailing. If you also need a fire-rated exterior, ask about fire retardant treated cladding.
Shop and specify
Frequently asked questions
Talk to our timber team
Tell us about your project and we will recommend the right species, profile, finish and fire performance. Free samples are available, and we can quote for bespoke milling.