CLT and fire safety under the Dutch Bbl
"Doesn't a timber building burn faster?" is the question every architect gets in every timber project. This is the technical answer, with the numbers you can use in your substantiation.
Solid wood burns predictably
CLT chars at the surface at roughly 0.65 mm per minute (Eurocode 5). The charring layer insulates the underlying wood, which retains its load-bearing capacity — exactly that predictable behaviour makes fire resistance calculable. Standard ZTC build-ups achieve EI 30 to EI 120, depending on layer thickness and cladding.
Reaction to fire: untreated CLT classifies as D-s2,d0; encapsulated (gypsum or fibre board) up to A2-s1,d0.
The WBDBO thresholds per building height
WBDBO 30–60 min
Achievable without special measures, with standard build-ups.
Encapsulation or sprinklers
Cladding of the CLT or a sprinkler system; the trade-off is made per project.
Sprinklers almost always
In line with common practice for high-rise, regardless of construction method.
The functional fire-safety requirements of the Dutch Bbl are equivalent for timber construction and traditional building systems — the Bbl sets performance requirements, not material requirements.
Exposed or encapsulated — the design choice
Exposed CLT
The wood remains visible; the fire resistance comes from the charring layer. Aesthetically maximal, requires thicker panels.
Encapsulated CLT
Clad with gypsum or fibre board: a higher fire class with thinner panels. Hybrid combinations per room are possible — visible where it counts, clad where it must be.
Need specifications per build-up?
EI values, layer build-ups and test documentation per build-up are supplied on request for your project — including the connection to your fire-safety concept.