Fire resistance of cross-laminated timber: Recent research findings
Cross-laminated timber (CLT) is made from timber boards bonded at right angles to form stiff walls, floors and roofs. Its fire resistance is not determined by wood alone. Charring, heat transfer, adhesive lines, fasteners, cavities and compartmentation all influence how long an assembly can support load and contain a fire.
Recent studies have moved beyond small material samples towards full-scale furnace tests, instrumented connections and numerical simulation. At the XIII DBMC in São Paulo, this work connects laboratory findings with the harder question of whether complete timber buildings can maintain structural stability under real fire conditions.
For Australian readers, these findings are increasingly relevant. Melbourne’s expanding medium-rise construction, Sydney’s mixed-use development and the country’s growing use of prefabricated timber make the connection between research and building approval especially practical.
Evidence on charring and structural behaviour
Current evidence confirms that a surface char layer can slow heat penetration into CLT, but its protective value is conditional. Char depth and stability vary with heat flux, moisture, species, grain direction, panel layup and the behaviour of adhesive lines. A single rate is therefore difficult to justify without identifying the exposure and protection condition.
Full-scale tests have also shown that structural failure need not begin with the panel face. Heat can damage steel connections, joints or concealed cavities first, while local char loss can expose fresh timber. These observations support designing CLT as a load-bearing system with vulnerable details, rather than judging it from an isolated board.
What fire tests now reveal
Modern furnace testing tracks flame spread, insulation, deflection, temperature distribution, char depth and residual load capacity. Floor assemblies also receive attention for hidden voids and service penetrations, which can create paths for heat and flame even when the main panel remains intact.
A useful comparison separates the main assembly conditions:
| Assembly condition | Fire behaviour | Design evidence |
|---|---|---|
| Unprotected panel | Surface heat produces char, while local damage may reduce the remaining section. | Representative temperature and char-depth data. |
| Protected panel | Encapsulation slows heating and can extend structural endurance. | Furnace data for the proposed panel and protection build-up. |
| Connections and joints | Steel fittings, timber bearing zones and fasteners can weaken before the panel fails. | Tests or validated analysis of critical details. |
| Compartments and services | Open cavities, penetrations and inadequate seals can spread heat and flame. | System testing that includes joints, seals and services. |
Each condition needs evidence tied to the actual design. Protected results cannot be applied to exposed surfaces, and a successful panel test cannot validate a vulnerable connection. Full assembly data therefore gives designers more defensible inputs for charring, connection design and compartmentation.
Linking test data with design models
Numerical models are increasingly calibrated against furnace tests instead of relying only on simplified time-temperature curves. Coupled heat-transfer and mechanical models can estimate char depth, residual strength and deformation, while moisture migration is being included to better represent real timber behaviour.
In Australia, that evidence supports alternative solutions under the National Construction Code (NCC). AS 1720.1 remains central to timber structural design, while AS 5131 provides the framework for fire-resistance testing and classification. Neither removes the need to verify CLT-specific assumptions; generic solid-timber charring rates and untested protection details are inadequate substitutes.
Structural fire resistance must also be separated from reaction to fire. Low flame spread does not establish capacity under a fully developed compartment fire, and surviving load-bearing panels do not prove that rooms are separated or escape routes remain usable.
Practical measures for Australian timber construction
Australian conditions make the translation of research into practice essential. The Fort House project in Melbourne demonstrates commercial acceptance of CLT, although much of the supply remains imported; Sydney’s mixed-use development is increasing demand for repeatable fire engineering. Traceable information on species, adhesives, moisture content and protection therefore matters throughout the supply chain.
Bushfire-prone suburbs in Melbourne, Sydney and Perth introduce a second exposure problem. Balconies, screens, decks and courtyards can contribute to ember pathways, and the Bushfire Attack Level (BAL) system for conventional houses cannot certify a multi-storey CLT building. Common outdoor entertaining also makes combustible screens and decks near occupied buildings relevant to external fire assessment. Site-specific exposure must therefore sit alongside internal room-fire design.
Practical recommendations for project teams
- Base charring assumptions on the actual panel layup, exposure and protection rather than an unqualified generic rate.
- Protect steel connections, timber bearing zones and fasteners for the required period of fire resistance.
- Test complete wall, floor and ceiling assemblies, including joints, penetrations, cavities and firestopping.
- Verify that render, intumescent coatings and insulation remain compatible with the timber, adhesive lines and expected moisture conditions.
- Coordinate detection, sprinklers and compartmentation with the structural fire strategy instead of assuming one measure compensates for another.
- Retain test reports, product specifications and installation records as evidence for NCC approval and future condition assessments.
Confidence through verified assembly design
Recent findings point to a consistent principle: CLT fire performance depends on the interaction of layers, connections, cavities and adjoining construction. Protected charring can preserve capacity, but reliable results require evidence for the complete assembly and its most vulnerable details.
The São Paulo conference provides a useful international setting for comparing test methods, especially as Australia develops local guidance for mass timber.
The practical takeaway is to base every CLT project on verified, assembly-specific evidence, reviewing charring assumptions, connection protection, firestopping and documentation together before construction begins.