Fire-suppression water may pose a significant risk of damage to mass timber construction, according to a new interim report published in December by Halliwell Fire Research on behalf of the Fire Protection Research Foundation (FPRF), the research affiliate of the US National Fire Protection Association. The first of three case studies, it examines how mass timber structures can be assessed and repaired following a fire event with sprinkler suppression.
Authored by Daniel Brandon, Jonas Niklewski, Konrad Wilkens and Panos Kotsovinos of Halliwell Fire Research in London, the report documents three full-scale fire tests of CLT mock-up structures in a sprinkler-protected room, assessing water absorption, drying methods and repair procedures after suppression. The research was conducted in partnership with Lund University and MSB, the Swedish Civil Contingencies Agency.
That work builds directly on a Phase 1 gap analysis completed in 2024 by Brandon and Joseph Su of the National Research Council Canada, which found that whilst prior research focused on how timber performs during a fire, the repair of mass timber after a fire event has not been well addressed. The Phase 2 program was commissioned to begin filling that gap — starting with the scenario most likely to occur in practice.
Reliability statistics of sprinklers indicate that the majority of fires in mass timber construction will be mitigated by automatic systems before causing significant structural damage. Water, however, presents a separate and underestimated problem. As the report notes: “timber is sensitive to water, especially when trapped in locations where it exposes the end-grain of mass timber” — and in all three fire tests, sprinklers discharged between 3,000 and 4,400 litres per test, with water penetrating structural connections in every specimen within approximately one hour of suppression.
Point-loaded column connections were consistently the wettest locations across all tests. Hidden voids within those connections trapped water in locations that standard surface moisture measurements could not detect — a finding with direct consequences for post-incident assessment in real buildings. Drying with the acoustic mat and floor screed left in place proved largely ineffective. Removal of those layers was necessary before moisture content could be brought to acceptable levels, and in the most stubborn connections, targeted airflow through drilled voids was required to achieve adequate drying.
It comes as Wood Central reported that Canada’s Mass Timber Demonstration Fire Test Program confirmed mass timber remained structurally stable after five large-scale fire tests of varying severity — improving understanding of how exposed mass timber performs during fire events, though stopping well short of what post-incident recertification would require.
Direct fire damage across the three tests was limited. Tests B and C produced only superficial discolouration with no charring. Test A — which placed the ignition source in the most onerous corner location — recorded a maximum char depth of 11mm. As the report’s conclusions note, “structural damage due to sprinkler discharge was limited or nearly absent.”
What follows suppression is where the complexity begins.
The research was funded by the Property Insurance Research Group (PIRG), whose members include AXA XL, CNA Insurance, FM, The Hartford, Travelers Insurance, Verisk and Zurich Insurance Group. Material contributions were provided by KLH, Wiehag, Rothoblaas and Dynea AS. The involvement of seven of the world’s largest insurers as project sponsors confirms that post-fire water damage in mass timber is no longer a theoretical concern — it is an active underwriting question.
Whilst this interim report provides preliminary repair recommendations for water damage, conclusions on fire-damage repair remain outstanding and will be included in the final report. Two further case studies are underway — examining the post-fire repair of structural columns and beam-column connections after unsuppressed fires, the scenarios that the sprinkler system cannot resolve. Repaired specimens from Phase 2 are currently in storage for 12 months before mechanical testing evaluates whether the repairs restored structural performance to original design standards.
Published as the second phase of the FPRF’s long-running Fire Safety Challenges of Tall Wood Buildings program, the interim report is the first to move the post-fire research agenda from identifying what is unknown to generating the data the industry will need to act on it.
