Mass timber construction is no longer a fledgling concept in the U.S. – it’s a movement. Proponents of mass timber got a boost when the 2015 International Building Code (IBC) was introduced, which specifically allowed for cross-laminated timber (CLT) as a building material. Recently-approved changes to the 2021 IBC will allow for even taller designs.
Mass timber is a category of framing typically characterized by the use of large solid wood panels for wall, floor and roof construction. Products in the mass timber family include CLT, nail-laminated timber (NLT), dowel-laminated timber (DLT), glue-laminated timber (glulam, or GLT when used as panels), and structural composites such as laminated veneer lumber (LVL) and laminated strand lumber (LSL). These products, combined with a heightened awareness of wood’s carbon benefits, have focused attention on the possibility of “tall wood” buildings, either made entirely from wood products or a combination of wood and other materials.
In North America, mass timber projects have typically been located in the Pacific northwest or Canada. However, the potential for mass timber construction on the east coast is not lost on suppliers, where in 2018, two mass timber manufacturers announced plans to build production facilities in Maine. The industry appears to be capitalizing on the positive reviews of New England’s jewel mass timber building – the Olver Design Building at UMass Amherst.
2015 IBC Allowances
Massachusetts’ state building code (780 CMR 9th edition) is based on the 2015 IBC, which allows mass timber buildings, including cross-laminated timber. As a result, several of these buildings are currently on the drawing boards, several of which Code Red Consultants is involved. For states that have not yet adopted the 2015 IBC, like New Hampshire and Rhode Island, variances may be sought to utilize the 2015 provisions. The 2015 IBC allows for mass timber buildings to be up to 85 ft in height, with the following allowable number of stories for major occupancy types:
| Assembly (Group A-3) |
4 stories |
| Residential (Group R-2) |
5 stories |
| Business (Group B) |
6 stories |
2021 IBC Allowances
The 2021 IBC will feature three new construction types to support tall mass timber buildings, as follows:
| IV-A |
18 stories; gypsum wallboard on all mass timber elements |
| IV-B |
12 stories; limited area of exposed mass timber walls and ceilings allowed |
| IV-C |
9 stories; all exposed mass timber |
For these new construction types, all mass timber elements must have a 2-hr fire-resistance rating. Although the 2021 IBC will not be available or adopted for a few years, this is a significant development and roadmap for the industry.
“Mass timber has been capturing the imagination of architects and developers, and the ICC result means they can now turn sketches into reality,” said American Wood Council (AWC) president Robert Glowinski. “ICC’s rigorous study, testing and voting process now recognizes a strong, low-carbon alternative to traditional tall building materials used by the building and construction industry.”
Making Mass Timber Projects Work
While CLT is prescriptively allowed, the technology and code provisions are new enough that many building and fire officials are asking for continuing education during the design process as to the code approvals path and the structural and fire safety compliance. Additionally, there may be opportunities to negotiate the earlier use of 2021 IBC provisions for taller mass timber buildings once the edition is published in its final form.
Please email our office if we can be of assistance with continuing education, AHJ negotiations, or general questions.
