Thermowood Decking: What to Buy and How to Install It Right

A terrace deck is the harshest place you can put wood. Unlike a facade, which only takes rain, a deck faces direct sun, standing water, winter frost, and foot traffic all at once. It's true that thermowood decking holds up here — but only if the installation is done right.

Why Thermowood and Not Regular Wood

In thermal modification, wood is heated to roughly 180–215°C in the absence of oxygen. Two results come out of that process that matter directly for decking: the wood's affinity for water drops, so it swells and shrinks less and the joints open and close less often; and the sugars that fungi feed on are destroyed, so decay sets in much later. All of this happens without adding any chemicals to the wood.

Thermowood isn't maintenance-free material; it's material that needs less maintenance, and more predictable maintenance.

Four Factors That Decide How Long a Deck Lasts

• A ventilated, sloped substructure. Wood should never sit flush on a flat surface; air needs to move underneath it and water needs a way out. A deck where water pools underneath will ruin any wood, no exception.
• Expansion gaps between boards. Thermowood moves less than regular wood, so the gap can be narrower — but never zero. A zero gap traps water and forces boards to press against each other.
• Stainless steel screws with pre-drilled holes. Thermowood is more brittle than natural wood and will split without pre-drilling. Galvanized screws also stain on contact and blacken the wood around the hole.
• Sealing every cut edge. Every cut made on site opens up a raw surface that soaks up water; it needs to be sealed right there, on the spot.

The Sun Fades the Color, and That's Not a Flaw

Unfinished thermowood turns silvery-gray under UV exposure. That color shift isn't structural damage, and plenty of European projects accept it on purpose. If you want to keep that warm first-day color, you'll need an exterior-grade finish and periodic reapplication. Make that call on day one, because that's exactly where your maintenance cost gets decided — the same logic we laid out in the real cost of a wood facade.

Yearly Upkeep in Three Steps

First, a simple wash with water and a soft brush at the start of the warm season, to clear out the dirt and leaves that collect in the gaps and hold moisture. Second, check the screws and tighten any that have loosened. Third, if you've applied a colored finish, reapply it on the schedule the manufacturer specifies. These three steps alone prevent most premature failures.