Mass Timber – Concealed Spaces

With the adoption of the 10th edition of the Massachusetts State Building Code (MSBC) and an anticipated increase in projects utilizing Mass Timber, it is important to understand the new 2021 IBC requirements related to this type of construction. This Insight will focus on new code provisions related to allowance and protection of concealed spaces in Type IV construction.

In the 9th Edition of the MSBC (based on the 2015 IBC), buildings constructed of Type IV construction are prohibited from having any concealed spaces at all. All other construction types are allowed to contain concealed spaces, which are otherwise required to be protected via MSBC Section 718. The prohibition of concealed spaces in Type IV construction is unrealistic for modern buildings, almost all of which have concealed space in some locations.

In the 2021 IBC, however, concealed spaces are prescriptively permitted in Type IV construction. For the new Type IV-A, IV-B, and IV-C tall mass timber construction types, concealed spaces are allowed, but must comply with the following provisions:

  • Such spaces must not contain any combustibles other than electrical, mechanical, fire protection, or plumbing materials and equipment permitted in plenums in accordance with International Mechanical Code
  • Concealed spaces within any combustible construction within Type IV-A and IV-B construction must be protected with noncombustible protection with a minimum assigned time of 80 minutes
  • Concealed spaces within any combustible construction within Type IV-C construction must be protected with noncombustible protection with a minimum assigned time of 40 minutes

For the legacy Type IV-HT construction type in the 2021 IBC, concealed spaces must not contain any combustibles as described above, and must be protected with one or more of the following:

  • Sprinkler protection in accordance with NFPA 13
  • Be filled with noncombustible insulation
  • Surfaces within concealed spaces must be fully sheathed with not less than 5/8-inch Type X gypsum board

If concealed spaces are within any interior walls/partitions that possess a 1-hour or greater fire-resistance rating, no additional protection is required.

These new updates allow for more flexibility in the design of Type IV buildings and will hopefully help owners, developers, and architects to further consider Mass Timber for building projects. If you require assistance with your Mass Timber project, please reach out to info@crcfire.com.