The first thermally modified board I handled surprised me. It was light — noticeably lighter than untreated pine — and it had that toasted brown colour all the way through, not just on the surface. Someone at the yard said "it's baked wood." That's crude, but honestly, it's not far off.
So what is it, really?
ThermoWood is ordinary timber — usually pine, spruce or ash — that has been heated to around 180–215°C in a chamber with almost no oxygen. No chemicals go in. Nothing is added at all. The heat alone changes the wood.
Why no oxygen? Because at those temperatures, wood in normal air would simply catch fire. The process uses steam to keep the atmosphere inert while the heat does its work.
What the heat actually changes
Wood rots because fungi eat it. What they mostly eat are the hemicelluloses — the sugary parts of the wood. The high heat breaks these down. Less sugar means less food for fungus, which means the wood lasts far longer outdoors.
The same heat also makes the wood absorb much less water. And wood that doesn't drink water doesn't swell in the monsoon and shrink in the dry season the way untreated timber does. Anyone who has seen a door jam every July knows exactly what problem this solves.
There's a price for all this: the wood becomes a bit more brittle and loses some bending strength. That's why you'll see thermally modified timber on decks, cladding and saunas, but not holding up a roof.
Thermo-S and Thermo-D — the two grades
You'll see these two names on every thermally modified timber quote, so it's worth knowing what they mean.
Thermo-S (around 190°C treatment) is the milder bake. The wood keeps more of its strength, changes colour less, and suits indoor work — panelling, furniture, doors.
Thermo-D (around 212°C) is the full treatment. Deeper brown, maximum rot resistance, rated durability class 2 under the European EN 350 standard — good enough for decking and cladding that faces rain for decades. This is the grade most of the export trade means when they just say “ThermoWood”.
The D stands for durability, the S for stability. Simple as that.
Why the market keeps moving toward it
Three reasons, and they're all practical, not fashionable.
First, tropical hardwood is getting harder and costlier to buy legally, and buyers increasingly ask where wood came from. ThermoWood is made from fast-growing plantation species, so the supply question is easy to answer.
Second, it replaces chemically treated timber in many jobs. Some markets have restricted the old copper-based treatments, and nobody loves handling chemically soaked wood anyway.
Third, the stability. Fit a thermally modified deck with the correct gaps and it stays flat. Untreated softwood in the same spot will cup and move within a couple of seasons. If you want to work out those gaps properly, our decking gap calculator does the arithmetic for you.
Looking after it
Left alone outdoors, thermally modified timber fades from brown to a silver grey. Some people pay extra for that look; others hate it. If you want to keep the brown, you oil it once or twice a year with a UV-protective oil. That's the whole maintenance story.
One tip from experience: use stainless steel screws. The treatment makes the wood slightly acidic, and ordinary screws can stain it black around the head.
Questions I get asked about it
Can I use it in a bathroom or sauna? Yes — saunas are actually one of its oldest uses. The low moisture uptake is exactly what you want in a wet room. Just ventilate like you would with any wood.
Can termites eat it? They find it much less appetising than untreated wood, but thermally modified timber is not termite-proof and no honest seller will tell you it is. For ground contact in termite country, you still need proper detailing or a different material.
Can I cut and drill it normally? Yes, with ordinary tools. It cuts cleaner than untreated softwood, actually, because it's drier. Just pre-drill near board ends — the extra brittleness shows up there first.
Is it worth it?
For outdoor work where you'd otherwise buy teak or chemically treated pine — yes, in most cases. You pay more than plain softwood and much less than good teak, and you get predictable, stable boards with paperwork that stands up to scrutiny. For structural work, no. Buy it for what it's good at.
If you're comparing costs, our ThermoWood calculators cover mass loss, durability class and coverage — all free.