Timber cladding has a fire performance problem. Not an insurmountable one — but a real one that trips up projects when it’s left too late in the specification process. For anyone working in sustainable timber cladding right now, understanding exactly where the regulatory lines sit in 2026 is the difference between a smooth building control submission and an expensive redesign.
Here’s the landscape (the regulatory one, not the poetic kind): post-Grenfell changes to Approved Document B drew hard lines based on building height. Above 18 metres, external cladding must hit at least Euroclass A2 or B as part of a complete tested wall assembly. Between 11 and 18 metres, Euroclass B or Class 0 is typically required — depending on use and risk assessment. Untreated timber usually lands at Euroclass D or E. That’s not good enough for most higher-risk applications.
The fix? Fire retardant pressure impregnation. It can push treated timber up to Euroclass B — making it a genuinely viable option across a wide range of building types without abandoning the sustainability brief.
One thing worth being precise about: reaction-to-fire and fire resistance aren’t the same thing. External cladding specs address how a material behaves when exposed to flame. Structural fire resistance is handled separately through structural design. Both are required. Both are assessed differently. Mixing them up is a common source of confusion on site.
And here’s where specification timing matters. Engaging a specialist fire rated timber cladding service UK at RIBA Stage 2 or 3 — not Stage 5 — means full system documentation is ready for building control from the outset. Boards pre-treated to Euroclass B, with the cavity design, substrate, and cavity barriers all factored in. The fire classification must apply to the complete wall assembly, not just the board in isolation.
The Thermally Modified Option
For projects below the height thresholds where chemical-free specification is a priority, thermally modified timber makes a strong case.
The process is straightforward: heat and steam only. No preservatives, no biocides, no chemical treatments. The cell structure of the timber is permanently altered — improving dimensional stability, reducing moisture absorption, and building in natural resistance to biological decay. At end of life, it can be recycled or composted. No hazardous substances during installation, maintenance, or disposal.
That last point matters more than it sounds. Whole-life carbon assessments increasingly scrutinise what happens to materials at the end of a building’s life. A product that just… goes away cleanly is genuinely useful.
ThermoWood cladding for sustainable buildings — sourced from FSC and PEFC-certified Nordic forests — achieves Durability Class 2 (BS EN 350) without any chemical intervention and carries a BRE-accredited 30-year service life for external cladding. Profiles include shadow gap, tongue and groove, rainscreen, and triple shadow gap, all available from UK stock.
Here’s where it gets practically useful: thermally modified timber is also compatible with fire retardant treatment. So on projects where both the sustainability brief and the fire performance brief apply, the same material can address both. One specification. One supplier conversation. That’s a meaningful advantage when you’re managing a complex low carbon building programme.
Bringing It Together
The instinct to treat fire performance and sustainability as competing priorities is understandable — but it’s usually wrong. For buildings below 11 metres, standard thermally modified timber delivers a clean, low carbon cladding solution with a well-evidenced service life. For taller buildings, fire retardant treated thermally modified timber provides a route to Euroclass B without surrendering environmental credentials.
The question most projects should be asking isn’t which priority wins. It’s whether the right specialist was brought into the conversation early enough to make both work.
