The Sauna Co.Design yoursSheet 404 · Guide
Sauna cladding in Gloucestershire: charred larch, cedar or thermowood
Three claddings, three ways of getting old. Each starts a different colour and, left alone, ends somewhere between silver-grey and black. This sheet sets out what each timber is, how it moves and weathers through a wet Gloucestershire winter, and what it asks of you afterwards. It gives no prices, because timber prices change with the merchant and the month.
404.1 · Outside and in
The cladding is the outside. The inside is a separate choice.
Cladding is the skin of the building. It takes the rain, the sun and the frost, and it is what the garden sees. The inside of a sauna has a different job: its lining and benches have to stay comfortable to touch at sauna temperatures, so they are usually a low-density timber such as aspen, alder or cedar, left unfinished. This sheet is about the outside.
404.2 · Cedar
Western red cedar, durable without treatment
Western red cedar is a softwood from the Pacific coast of North America, also grown in Britain. It is light, stable and durable without treatment, because of natural oils in the heartwood. Imported cedar is rated durable under BS EN 350, the European standard for natural durability. UK-grown cedar has more knots and is rated moderately durable.
New, it is a warm mix of pinks and browns. Left untreated it weathers to silver-grey, unevenly at first and fastest on the walls that take the weather. The same oils react with iron, so cedar is fixed with stainless steel; ordinary steel fixings leave a black stain round every nail.
To keep the first colour it needs a UV-protective oil or stain, renewed from time to time. Left to silver, it needs nothing.
404.3 · Thermowood
Pine or spruce, changed by heat
Thermowood is ordinary softwood, usually Nordic pine or spruce, heated to over 200°C with steam and no chemicals. The heat changes the wood itself. It takes up less water, so it moves less with the seasons and resists decay far better than the same timber untreated. It comes out of the kiln an even, toasted brown all the way through.
The trade-off is strength. The process makes the wood more brittle, so it is cut and fixed with care. It is for cladding and decking, not for the frame.
Lunawood rates its Thermo-D grade durable, class 2 under BS EN 350, for use above ground and out in the weather, and asks for stainless fixings and pre-drilled holes near the ends of boards.
Left untreated it silvers like cedar, and more evenly than most. An oil holds the brown for longer.
404.4 · Charred larch
Larch, charred: shou sugi ban
Charring is a Japanese way of finishing timber, called yakisugi in Japan and usually shou sugi ban here. The face of each board is burned until it carbonises, then cooled, and sometimes brushed and oiled. In Britain it is most often done on larch, a resinous softwood grown here and also imported from Siberia.
The carbon layer is what you see and what weathers. A deep char stays black and asks little, though when new it can mark hands and clothes. A brushed board shows the grain in black and bronze, and usually takes an oil to stay that way. Charred larch does not silver the way the other two do, because it is already dark; it loses depth slowly as the char wears.
Charring does not stop larch moving. It can twist, split at the ends and bleed resin in warm weather, and the char moves with it. Boards are chosen and fixed with that in mind.
The Scottish Executive's 2002 guide Timber Cladding in Scotland, working from the 1994 standards, groups European, Japanese and hybrid larch as moderately to slightly durable, a class or more below imported cedar. The guide does not cover Siberian larch.
404.5 · The weather
What a wet Gloucestershire winter does to each
The prevailing wind here, as across most of Britain, comes from the south-west, and the walls facing it take most of the rain. Those walls change colour first. A sauna turned to a south-westerly view can be two colours for a while, which usually evens out as the other walls catch up.
| Timber | What it is | New | Left alone | Movement | Fixings | Upkeep |
|---|---|---|---|---|---|---|
| Cedar | Western red cedar, a softwood with natural oils. Imported cedar is rated durable under BS EN 350, UK-grown moderately durable. | A warm mix of pinks and browns. | Silver-grey, unevenly at first, fastest on the weather side. | Movement class small, in the 2002 Scottish guide. | Stainless steel. Ordinary steel stains black round every nail. | A UV-protective oil or stain, renewed from time to time, to hold the colour. Nothing if left to silver. |
| Thermowood | Nordic pine or spruce heated to over 200°C with steam and no chemicals. Lunawood rates its Thermo-D grade durable, class 2. | An even, toasted brown all the way through. | Silvers, more evenly than most. | Lunawood says it retains its shape far better than untreated wood. | Pre-drilled holes and stainless fixings. Brittle at the fixings. | An oil holds the brown for longer. |
| Charred larch | Larch with its face burned to carbon: shou sugi ban. The 2002 Scottish guide groups European, Japanese and hybrid larch as moderately to slightly durable. | Black from a deep char, black and bronze when brushed. New char can mark hands and clothes. | Stays dark, losing depth slowly as the char wears. | It can twist, split at the ends and bleed resin, and the char moves with it. | Boards chosen and fixed to allow for the movement. | A deep char asks little. A brushed board wants oiling again in time. |
The site matters as much as the species. Boards that stay damp in shade, or are splashed from the ground, go green and grey faster whatever they are. The cladding should stop well clear of the ground and the base should drain, which is one reason the base is settled before the cladding is chosen.
404.6 · Fire
A charred board is still timber
Building regulations exempt a small detached building with no sleeping accommodation. Under Schedule 2, Class 6, one with a floor area of no more than 15 square metres is exempt, apart from its electrics and any water it takes from the house. Regulation 9(3) keeps Part P for a building powered from the house, and regulation 9(2) keeps G1 and parts of G3 for one whose water comes from it. Up to 30 square metres, a single-storey one is exempt if no point of it is less than a metre from the boundary, or if it is built substantially of non-combustible material. Each of our four sauna forms is under 15 square metres.
Nothing in that wording turns on how the boards are finished. A charred board is still timber, so charring changes how a sauna looks and weathers, not whether it is exempt. The stove and its flue are a separate matter, covered in the heater guide.
404.7 · Choosing
Charring is the premium finish. Choose by the garden first.
Charring is an extra process on top of the board, and our configurator treats it as a premium finish.
Choose by the garden first. A sauna that should sit dark against a hedge or a stone wall may want charred larch. One that should read as warm timber on a lawn may want cedar, oiled or left to silver. One near a boundary, where nobody will reach it with a brush and a tin of oil, may want thermowood.
If the address is listed or in a conservation area and the sauna needs planning permission for another reason, the council may have a view on materials too.
Planning permission for a garden sauna in Gloucestershire
404.8 · Questions
Questions people ask about sauna cladding
Does charred larch need maintenance?
Little, if the char is deep, though new char can mark hands and clothes. A brushed board, which shows the grain in black and bronze, usually takes an oil to stay that way and wants oiling again in time.
Can you use larch inside a sauna?
The usual choice inside is a low-density timber such as aspen, alder or cedar, left unfinished, because the lining and benches have to stay comfortable to touch at sauna temperatures. Larch can bleed resin in warm weather, and this sheet treats it as cladding.
Which sauna cladding moves the least?
No source we have read ranks the three. Lunawood says its thermowood retains its shape far better than untreated wood. The Scottish Executive's 2002 guide gives cedar and larch the same movement class, small. Larch can still twist, split at the ends and bleed resin, charred or not.
Does charring make a sauna fire-resistant?
Building regulations do not count it as fire resistance, and neither should you. A charred board is still timber, and the exemption for a small garden building turns on its floor area and its distance from the boundary, not on the finish of its boards.
Which cladding is least work to keep?
Any of the three can be left alone if you like how it ages: cedar and thermowood go silver, and a deep char stays dark. The work comes from holding a colour, with oil on cedar, thermowood or a brushed char.
Sheet 00 · Yours
Describe your garden
A few plain words is enough. The studio turns them into a first design, and we confirm it on a site visit before anything is built.
- Drawn for one garden and built once, to your space and the way you will use it.
- Designed and built from our Gloucestershire workshop. Installed across England and Wales.
- One team from the first sketch to the last screw, electrics included.