Mass Timber Firestopping
Pt. 1 - Floor Penetrations
A common question from design teams is how to deal with fire stopping in mass timber buildings. Because mass timber is relatively new in North America, there weren't many tested assemblies proving that standard firestopping would work with a wood slab. Companies like Hilti Group are leaning in hard though, and have tested a number of assemblies for many firestop conditions. You can find a great document about slab penetrations here Hilti Firestop in Wood Construction.
More tests are being performed all the time, increasing the library of tested assemblies for design teams to work with. Let me know if you have any creative firestop stories from your mass timber project!
Photo Credit: Hilti Group
Pt. 2 - Floor to Wall Joints
As we see more mass timber based curtain wall facade systems, the question of how to seal the slab edge to facade panel for smoke and fire is increasingly coming up. Similar to the slab penetrations, companies like Hilti Group and ROCKWOOL Group have prioritized finding a tested solution, with and without an edge beam. There are now tested assemblies up to a 2-hr rating, which rely on a combination of packed mineral wool safing topped with an edge seal product. You can find a couple of examples here, with an edge beam Hilti Perimeter Fire Barrier System - Beam and without Hilti Perimeter Fire Barrier - No Beam.
Take a look at Hilti's mass timber tested assemblies here for more information! Hilti Firestop Support and Solutions While these are great steps forward, they do require a minimum thickness of mass timber panel, which may not be required for the structural design. If you have a variation you want to use, you can reach out to Hilti's team directly to get an engineering judgement from them!
Photo Credit: Hilti Group
Pt. 3 - Panel to Panel CLT Floor Joints
Someone recently asked me about the panel to panel joints between CLT panels in fire rated floor assemblies. They asked if there was anything in the crack to stop fire from going up to the next level. What I found most interesting is that the person asking was a developer, not a code official or engineer, and this questions popped into their mind while we were touring a completed mass timber multifamily project here in the Vancouver area. They wanted to understand any potential cost or risk that they hadn't thought about before.
Luckily, many fire tests have been completed with a panel to panel joint, both in half-lab and topside plywood spline configurations, with successful ratings up to 3 hours. Find an inventory of fire resistance-tested mass timber assemblies and penetrations compiled by WoodWorks here Inventory of Fire Resistance-Tested Mass Timber Assemblies and Penetrations.
Typical panel to panel connections do not have firestop or insulation stuffed into the gap, because it is usually very small, usually around 1/16" to 1/4", and this is important for expansion/contraction over time. This small space, combined with the spline at the top, is too small for significant fire and smoke to get into the space, and prevents the cool air above from drawing the heat and smoke up through the gap. The fire tests prove this out, and coupled with additional measures like a topping slab or layers of drywall on top, this is reinforced.
If the gap is large enough, then firestop measures DO need to be accounted for, like in this Firestopping Test Witness Report for Nordic Structures by GHL Consultants Ltd. CLT Firestopping Testing Report. There is a provision in the 2021 IBC that allows control joints up to 5/8" and tested to ASTM E119 or UL 263 to be installed without fire-resistant joint systems. Be sure to read the fire test reports for the manufacture you are using to verify!
Image Credit: WoodWorks