34 csengineermag.com august 2019
When looking to maximize the code's current allowances in terms of building size for mass timber structures, considering the differences
between Type III-A and IV construction is important. For example:
• Type IV does not allow concealed spaces in floor or roof assemblies (e.g., dropped ceilings, soffits, chases, etc.), but 1-hour
fire resistance-rated interior partitions are permitted. All other construction types including III-A allow concealed spaces. Note that
requirements for sprinklers and draft stopping/fire blocking apply within concealed spaces per IBC Section 718 and the applicable
NFPA sprinkler standard.
• Except for exterior bearing walls, Type IV does not require demonstration of fire-resistance ratings for structural elements. This is a
requirement for all other construction types including III-A (but only when a fire-resistance rating is required).
• Type IV construction allows the use of CLT in exterior walls; Type III does not.
Table 5 illustrates these differences and others for a group B occupancy building.
