Mass Timber – Exposed Wood in Type IV-B Construction

There are several reasons developers and architects are contemplating using Mass Timber for their next project – most notably sustainability, speed and ease of construction and reduced weight of materials. An additional benefit remaining at the forefront of Mass Timber is the exciting aesthetic that comes with exposed wood beams, columns, ceilings, and walls. Given this element, understanding the allowances and limitations of the new Type IV-A, IV-B, and IV-C construction types in the 2021 IBC is important, with each type carrying a prescriptive requirement related to the amount of exposed wood.

Type IV-C construction provides the most opportunity to expose the Mass Timber elements in the future code, is allowed to have all walls, ceilings, and structure be 100% exposed, provided that the required fire-resistance ratings for the building are achieved. At the other end of the spectrum, Type IV-A construction is required to have all wood concealed behind gypsum wallboard, as a trade-off for superior building heights (which can reach up to 18 stories for most occupancies).

In the middle, Type IV-B construction is allowed a partial exposure of floors and ceilings. The Type IV-B exposure allowance is approximately 30% across walls and ceilings, with specific regulations as follows:

  • Exposed ceilings, including attached beams, are limited to a maximum area equal to 20 percent of the floor area in any dwelling unit or fire area.
  • Exposed walls, including attached columns, are limited to a maximum area equal to 40 percent of the floor area in any dwelling unit or fire area.
  • Where both ceilings and walls have exposed portions, the exposure is required to comply with the equation shown below. This equation requires the ratio of total exposed ceiling area (Utc) to allowable exposed ceiling area (Uac) plus the ratio of total exposed wall area (Utw) to allowable exposed wall area (Uaw) to be less than one. (If this approach seems familiar, it is the same concept that is used for separated mixed-use occupancy area calculations in Chapter 5.)

The exposed ceiling area is required to include beams, and the exposed wall area is required to include columns. However, beams and columns that are not an integral part of floor/ceiling or wall assemblies, respectively, do not count toward these calculations, and are permitted to be exposed.

It should be noted that the 2021 IBC will not be applicable in Massachusetts until the 10th edition of 780 CMR is adopted, which, as of this writing (December 2021), is still likely over a year away. However, with this coming trend, it is important to understand the new 2021 IBC requirements related to Mass Timber construction, especially as they may be able to be used in the meantime to justify building code variances to the 9th edition of 780 CMR.

If you are considering Mass Timber for your next project, please reach out to info@crcfire.com.

Photo credit: Jones Architecture and DCAMM