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Balcony design

Balcony design

Figure 1: Example of mixed-use podium wood construction utilizing structural concrete for the first floor and wood framing for the upper five floors. Photo: courtesy of Construction Science and Engineering, Inc.

IBC changes and wood durability considerations.

By Frank Woeste, Ph.D., P.E. and Don Bender, Ph.D., P.E.

Wood construction for mixed-use has always been popular in the western U.S. and is growing in popularity throughout the country due to favorable cost, availability, ease of construction, thermal performance, less embodied energy, and carbon sequestration. All building materials have advantages and disadvantages, and wood is no different. For example, wood is susceptible to deterioration from moisture exposure, but this risk can be mitigated through proper design and detailing.

Buildings such as the one shown in Figure 1 often include balconies as shown in the background. Cantilevered balconies have limited structural redundancy and have exposure to weather that requires special attention by design professionals, related contractors, permitting and inspection department, and maintenance by owners in-service. Figure 2 shows a different example of balcony framing for an apartment building constructed in 2018.

Balconies can add considerable value but 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 Jan. 30, 2017, and motivated changes to the 2018 IBC that address waterproofing measures and “special inspections” during construction.

Lessons learned from the Berkeley tragedy include:

  • Balconies have limited structural redundancy so special attention to design, construction, material selection, and inspection are critical.
  • Moisture usually finds a way into enclosed spaces, so there needs to be a way for moisture to exit the spaces.
  • Periodic inspections are needed to ensure the integrity of the balcony structure. Some means of access is needed to inspect the enclosed space (e.g., removable access panel).

The objectives of this article are to alert design and construction professionals about changes in the 2018 IBC that address occupant safety of wood-frame balconies, and to provide guidance on improving balcony safety through durable material selection and considerations for in-service inspections.

2018 IBC change: Impervious moisture barrier system

The new code requires an “impervious 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 (changes indicated in bold):

Figure 2: Example of balcony framing with untreated lumber with the framing enclosed. Note that the covering does not allow for ventilation of the enclosed space, nor does it have a removable access panel for periodic inspection. The guards are not yet installed, which also require careful consideration to carry guard loads given in IBC 1607.8 Loads on handrails, guards, grab bars and seats.

“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 impervious moisture barrier system to have positive drainage of water that infiltrates the floor topping. This change makes sense since hard surfaces can form cracks in-service and allow the passage of water by gravity and capillary action. Without the free drainage of water from the surface of the moisture barrier system, water can back up due to hydrostatic pressure even though the in-service (drainage) slope of a waterproofing element is positive.

Impervious moisture barrier system — The key word is “system,” as the system required per the 2018 IBC to have “shall provide positive drainage of water that infiltrates the moisture-permeable floor topping.” A 2016 publication by Joseph Lstiburek, Ph.D., P.Eng., reviewed the issues involved and presented excellent stepwise details of a system installation that incorporates a “drainage mat” above the “waterproof membrane.”

Lstiburek’s publication and details merit careful study by design and construction professionals as the integrity of the wood framing is conditioned on the proper installation of a waterproofing system that meets the requirements of IBC 2304.12.2.5.

Commentary on impervious moisture barriers — In our experience, it is entirely possible that moisture will find a way into the enclosed balcony space, as no barrier system is perfect. As such, it is important to provide a way for the moisture to exit the space. Next, we will cover another IBC change that calls for “free cross-ventilation.” Finally, given the limited structural redundancy of balconies, designers should consider using preservative-treated wood even for the case of impervious moisture barriers. Guidance on durable wood selection and fasteners is provided later in this article.

Durable wood options

Figure 3: Detailed intersection of a balcony edge perpendicular to a wall. Source: BSI-093: All Decked Out by Joseph Lstiburek (Building Science Corporation) https://buildingscience.com/documents/building-science-insights-newsletters/bsi-093-all-decked-out

Naturally durable wood versus preservative-treated (PT) wood — The IBC allows for both options, but practically speaking, PT wood is a better choice for balcony framing.

The IBC defines “naturally durable wood” as the heartwood of decay-resistant species except for the occasional piece with corner sapwood, provided 90 percent or more of the width of each side on which it occurs is heartwood. Decay-resistant species listed include redwood, cedar, black locust, and black walnut.

The reason for the 90 percent heartwood requirement is that sapwood of species listed is not decay resistant. From the 2010 USDA Wood Handbook, “Untreated sapwood of essentially all species has low resistance to decay and usually has a short service life under conditions favoring decay.” Visit www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf for more information on wood as an engineering material.

We have never seen black locust or black walnut used in building framing, so we examine the remaining choices. Redwood and cedar (with the stated heartwood requirements) generally have lower design values and are more expensive than other common framing lumber choices. These species are good choices for lower structural demand applications such as deck boards where the natural beauty of the wood is left exposed. However, for balcony framing, we favor PT lumber that has been treated and certified according to the specifications in IBC 2303.1.9 Preservative-treated wood.

PT wood — The proper and adequate specification of PT wood requires a knowledge and use of the 2018 IBC, Chapter 35 Referenced Standard for PT Wood: AWPA U1—16: USE CATEGORY SYSTEM: User Specification for Treated Wood. As discussed next, a PT wood specification such as “all balcony framing lumber shall be PT wood” by a project designer is extremely vague and not sufficient for balcony framing that is critical for life safety.

Referring to AWPA U1-16, Table 2.1 Service Conditions for Use Category Designations review of the table for “Above Ground” and “Ground Contact” yields 10 Use Categories and Service Conditions. In Table 1, the Use Categories are tabulated with reference to whether or not the Service Condition applies to “critical components” or could involve a “difficult replacement.”   

Table 1: AWPA U1-16 Use Categories that make reference to Service Conditions, “Non-critical components,” “Critical components,” and “Difficult replacement.”

Our interpretation of Table 1 leads to the conclusion that while a balcony is clearly “Above Ground” based on the elevation of the balcony framing, the application of PT structural wood-framing per the AWPA U1-16 Standard is “Ground Contact” UC4A, 4B, or 4C. The choice of UC4A, 4B, or 4C is the responsibility of the design professional and should be clearly stated in the construction documents to enable the general and framing contractors to use the proper preservative treatment.

The most recent version of the code-referenced AWPA U1-18 Standard Excerpt can be downloaded at www.awpa.com/standards/U1excerpt.pdf. 

PT structural composite lumber — Structural composite lumber (SCL), which includes laminated veneer lumber (LVL), parallel strand lumber (PSL), laminated strand lumber (LSL), and oriented strand lumber (OSL), are engineered wood composites with excellent engineering properties. However, other than Parallam Plus PSL, we are not aware of any SCL that is treated to decay protection levels above AWPA Use Category UC2.

Fasteners in PT wood — Some of the chemical formulations in wood preservatives can accelerate corrosion of fasteners and flashing. IBC 2304.5 specifies requirements for zinc-coated and stainless-steel fasteners and connectors in contact with PT wood. Simpson Strong-Tie provides excellent information about corrosion risks and solutions

at their site (www.strongtie.com/products/product-use-information/corrosion-information).

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 addition to the code guards against the accumulation of water vapor (for any reason) through natural drying. Balcony ventilation openings should be visible to an inspector, and the inspector should report 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 inspections.

A special case: Open framed cantilevered balconies

Figure 4. Balcony with cantilevered joists extending through a masonry wall. The deck surface is pervious.

PT wood framing for balconies that rely solely on cantilever beams for structural support as depicted in Figure 4 is not recommended due to several structural and in-service issues. We are not aware of any method to inspect or determine the structural integrity of the cantilevered joist section embedded in the wall. It should be noted that even “early decay,” not visible or detectable by physical means, significantly reduces the strength properties of wood. In addition, the water trapping joints/surfaces created by the entry of the framing into the wall cavity creates a decay hazard as it is very difficult to prevent the movement of moisture into the contact areas between the joists and masonry wall.       

Summary recommendations

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 using UC4A, UC4B, or UC4C PT wood (and appropriately protected fasteners) even though an impervious moisture barrier system is used, and we recommend access panels 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.

In the words of the late Professor Stan Suddarth (Purdue University), the focus of  the impervious moisture barrier system option is to protect wood framing from decay by the most fundamental way: “Keep wood dry. Don’t let wood get wet. Keep water away from wood.” If installed properly, the life of the wood framing is only limited by the service life of the impervious moisture barrier system.

We recognize that Special Inspections of the manufacturer’s installation instructions are not part of the new IBC, only given in the 2018 IBC Chapter 1 Scope and Administration, and thus may not be adopted by states and local jurisdictions when the IBC is officially adopted. The inspection provisions follow:

“[A]107.2.5 Exterior balconies and elevated walking surfaces. Where balconies or other elevated walking surfaces are exposed to water from direct or blowing rain, snow, or irrigation, and the structural framing is protected by an impervious moisture barrier, the construction documents shall include details for all elements of the impervious moisture barrier system. The construction documents shall include manufacturer’s installation instructions.”

“[A]110.3.6 Weather-exposed balcony and walking surface waterproofing. Where balconies or other elevated walking surfaces are exposed to water from direct or blowing rain, snow, or irrigation, and the structural framing is protected by an impervious moisture barrier, all elements of the impervious moisture barrier system shall not be concealed until inspected and approved.

Exception: Where special inspections are provided in accordance with Section 1705.1.1, Item 3.”


We view the new balcony code provisions to be an opportunity to proactively address the safety and reliability of balconies in-service. In the interest of public safety, design professionals are encouraged to adopt the new 2018 balcony provisions before they are 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 provsions 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.

Frank Woeste, Ph.D., P.E., is Professor Emeritus, Virginia Tech, and frequently consults with the public, design professionals, contractors, and building code officials on various aspects of engineered wood construction and residential construction, including decks and balconies. Along with his colleagues, Woeste continues to offer continuing education programs at Virginia Tech annually. Contact him at fwoeste@vt.edu. 

Don Bender, Ph.D., P.E., 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. Bender teaches university and outreach courses in structural engineering and is active in national building code and standards development. Contact him at bender@wsu.edu.