Want to build in wood? Consider covering at least 80 % of mass timber surfaces

New study finds that a fire in a mass timber compartment is less likely to auto-extinguish when the exposed surface is more than 20 % of the total surface area.

If you build your room, apartment, or other compartment in mass timber such as CLT, there is a possibility that a fire will continue to burn after the room’s contents has been consumed. The likelihood of continued burning increases as more timber is exposed within the compartment and if more than 20 % of the surface area is exposed, chances are that the room will continue to burn rather than the fire will die out.

This is one of the key findings of new research by DBI and The University of Edinburgh. Ahmed Ali Awadallah along with Dr Rory Hadden and Dr Angus Law have compiled data from 100 structural mass timber compartment fire experiments and summarized them in a new paper “Meta-analysis of temperature, heat release rate, delamination and auto-extinction of timber compartments”.

100 fire tests made comparable
“Many researchers have been conducting tests on mass timber and our idea was to bring this together in one paper, to see what we could learn. Some experiments were very small, others were large apartments – all had varying degrees of exposed wood. We tried to understand how best to compare each experiment – and what general lessons could be drawn.” explains Ahmed Ali Awadallah, research consultant at DBI, while underscoring the limitations that inherently lie in concluding from many different experimental setups.

Through statistical analysis of the data, the team calculated the likelihood of auto-extinction of a fire in a mass timber compartment. They found that 20% exposed timber represented a “magic number” above which continued burning appeared to be much more likely.

High wood exposure rates are not recommended
“I can understand the desire to showcase the sustainable materials that a building is made of, but from a fire safety perspective, the findings suggest that high exposure ratios are more likely to lead to fires that will continue to burn until structural collapse or fire service intervention,” says Ahmed Ali Awadallah.

Types of glue influence fire safety
Examining the data further, Ahmed Ali Awadallah and his co-authors found a strong link between auto-extinction and delamination of the timber. If the mass timber delaminated during the fire test, it would sustain a fire in nearly 8 out of 10 experiments. On the other hand, 93 % of the fire tests autoextinguished if the timber mass did not delaminate.

“We also saw differences according to which glue that was used. PUR HBS adhesive delaminated in 73 % of the cases, PUR HBX delaminated in 33 % of the cases,” explains Ahmed Ali Awadallah.

The paper has been published in June 2024 issue of the Fire Safety Journal. It is open access, and you can read it here