Because of their limited structural redundancy and exposure to weather, cantilevered balconies require special attention by design professionals, contractors, permitting and inspection departments, and owners in-service. As shown in the photos, modern wood-framed, multi-family projects often include free-end cantilever balconies.
Balconies can add considerable value, but they require special attention to ensure public safety. A case in point—on June 16, 2015, a balcony on a wood-framed apartment building collapsed, causing six fatalities and injuring at least seven others, precipitating emergency changes to the 2016 California Building Code, effective January 30, 2017, and motivating changes to the 2018 International Building Code (IBC) to address waterproofing measures and “special inspections” during construction. In addition, in September 2018, California Gov. Jerry Brown signed a bill requiring periodic in-service inspections of apartment balconies, decks, outdoor stairs, and elevated walkways.
This article is intended to alert design and construction professionals on changes in the 2018 IBC that address occupant safety of wood-frame balconies, and to summarize “best practice” design recommendations for enhancing balcony safety in-service. Visit https://csengineermag.com/article/balcony-design/ for an in depth discussion of design options that include a moisture barrier system, specification of adequate treatment level (AWPA Use Category) for preservative-treated (PT) wood framing, ventilation of enclosed balcony framing, and inclusion of access panels to facilitate periodic inspections. Given the limited structural redundancy of a cantilever balcony system, a case is made for redundant moisture protection of the framing system by specifying both PT wood for the balcony framing and an imperious moisture barrier system to protect it.
2018 IBC change – Impervious moisture barrier system
The 2018 code requires an “imperious moisture barrier system” when the wood structural framing is “exposed to the weather, such as concrete or masonry slabs,” as an alternative to preservative-treated or naturally durable wood. 2018 IBC Section 2304.12.2.5 follows (change indicated by underline).
IBC 2304.12.2.5 Supporting members for permeable floors and roofs. Wood structural members that support moisture-permeable floors or roofs that are exposed to the weather, such as concrete or masonry slabs, shall be of naturally durable or preservative treated wood unless separated from such floors or roofs by an impervious moisture barrier. The impervious moisture barrier system protecting the structure supporting floors shall provide positive drainage of water that infiltrates the moisture-permeable floor topping.
Positive Drainage. One critically important element for protecting untreated structural framing is the requirement for the “imperious moisture barrier system” to have “positive drainage” of water that infiltrates the “floor topping.” This change makes sense because hard surfaces can form cracks in-service and allow the passage of water by gravity and capillary action. Without the use of an imperious moisture barrier system that includes a “drainage mat component,” water that infiltrates the moisture-permeable floor topping can “back up” above the lower surface of the waterproofing system, creating hydrostatic pressure and back flow/migration of water.
2018 IBC change – Enclosed balcony framing must be ventilated
The 2018 IBC has a new provision requiring ventilation of enclosed balcony framing as follows:
2304.12.2.6 Ventilation beneath balcony or elevated walking surfaces. Enclosed framing in exterior balconies and elevated walking surfaces that are exposed to rain, snow or drainage from irrigation shall be provided with openings that provide a net free cross-ventilation area not less than 1/150 of the area of each separate space.
This new code provision guards against the accumulation of water vapor (for any reason) through natural drying. Balcony ventilation openings should be visible to the inspecting building official and reported if they are not present or are deficient. In addition, we recommend that the ventilation covers, or some other access panel, be removable to allow for periodic in-service inspections.
We believe the impervious moisture barrier system option is a “best practice” for balconies when coupled with ventilation per 2018 IBC 2304.12.2.6 and Special Inspection of the “manufacturer’s installation instructions” specified by the design professional and contained in the construction documents. In addition, we recommend specifying:
- AWPA Use Category UC4A, UC4B, or UC4C PT wood (and appropriate fastener and connector protection based on selected Use Category) even though an impervious moisture barrier system is used, and
- Access panel(s) that facilitate periodic inspections.
This redundant protection against decay is appropriate given the limited structural redundancy of a cantilever balcony system, and the importance for life safety.
We view the new balcony code provisions to be an opportunity to proactively address the safety and reliability of balconies in-service. As such:
- In the interest of public safety, design professionals are encouraged to follow the new 2018 balcony provisions before they are officially adopted by the governing jurisdiction or state code.
- At a minimum, we believe that owners of new construction projections should be advised of the “balcony safety issue,” the new IBC provisions that address water-related issues, and the need for periodic inspections to ensure the balcony framing is being protected from moisture conditions that can compromise structural integrity.
About the Authors
Frank Woeste, P.E., Ph.D., is Professor Emeritus, Virginia Tech, and frequently consults with the building designers and code officials on various aspects of wood construction. Frank, along with his colleagues, continues to offer continuing education courses at Virginia Tech and approved by the ICC PPP. For a short course example, visit http://www.cpe.vt.edu/sdwnds/index.html.
Don Bender, P.E., Ph.D., is Weyerhaeuser Professor of Civil Engineering and Director of the Composite Materials & Engineering Center at WSU–Pullman. He is an expert in testing, design, and construction of timber structures. Don teaches university and outreach courses in structural engineering and is active in national building code and standards development.