Fire reaction testing of wood façade materials is typically performed on new materials under controlled and repeatable conditions. While such testing provides valuable insight into initial performance, façade materials in practice are exposed for years to solar radiation, moisture cycles and temperature variations. Over the past years, a large number of wood species have been tested using identical SBI test configurations, allowing material-related behaviour to be distinguished from test-induced variability. Repetition tests show that results tend to cluster strongly within a given wood species, while pronounced differences occur between species, even within a relatively narrow density range. These observations indicate that fire behaviour is inherently wood-species dependent. Based on this, the working hypothesis was formulated that the effects of natural weathering may also be species dependent and potentially non-linear. This blog outlines the research approach used to investigate these mechanisms and explains why understanding surface evolution over time is essential when interpreting fire behaviour of wood façades.
Repeatability and material-specific fire behaviour
Over the past years, a large number of wood species have been tested for their fire reaction using identical SBI test configurations.
By deliberately keeping geometry, fixation, orientation and boundary conditions constant, it becomes possible to distinguish material-related behaviour from test-induced variability.
Repeated testing has revealed a consistent pattern: results tend to cluster strongly within a given wood species, while pronounced differences occur between species, even when their density lies within a relatively narrow range.
This indicates that fire reaction is not governed by density alone, but is inherently wood-species dependent.
From initial performance to performance over time
While initial fire performance of façade materials is relatively well documented, their behaviour over time is far less understood.
In practice, façade materials are exposed for years to solar radiation, moisture cycles, drying, and temperature fluctuations.
All of which may alter surface characteristics.
Given the clear wood-species dependency observed in initial testing, the working hypothesis was formulated that natural weathering effects may also be species dependent and potentially non-linear.
In other words, materials that perform similarly when new may evolve very differently over time.
Natural weathering under controlled conditions
To investigate this hypothesis, a selection of wood species was exposed to natural weathering according to EN 927-3, under fully identical conditions and exposure duration at an accredited fire testing laboratory.
This approach allows weathering effects to be studied without introducing uncontrolled variability.
After one year of exposure, the materials remain visually comparable at façade scale.
However, when observed in more detail, clear differences emerge at surface and micro-scale, including checking, surface roughening and fibre exposure.
These observations do not constitute conclusions, but they are consistent with the original hypothesis that surface evolution during weathering is not uniform across wood species.
Why surface evolution matters
Fire growth in system-level tests is strongly influenced by the effective burning surface, flame attachment and early heat release development.
Changes in surface roughness, cracking and fibre exposure may therefore play a role that is not captured by material chemistry alone.
Understanding whether and how such surface-driven effects translate into fire behaviour requires careful, stepwise investigation, combining repeatability, controlled exposure and subsequent fire testing.
Scope and intent of the research
This research focuses on understanding the mechanisms governing the ageing of façade materials, in order to better interpret fire behaviour over time.
It does not aim to evaluate individual products, specific applications or regulatory frameworks.
Subsequent fire testing after conditioning will provide further insight into how surface evolution and fire behaviour relate to each other, and how this relationship may differ between wood species.
Looking ahead
As the dataset expands, a more complete picture is emerging.
One that combines repeatability, material specificity and time-dependent behaviour.
This work is being conducted carefully, systematically and with restraint, as robust conclusions can only follow from robust data.