Berkeley balcony collapse: A failure at the intersection of building science and microbiology

I saw the tweet below from James Scott first thing this morning, linking to a NY Times article about the tragic balcony collapse in Berkeley, CA two days ago:

From the article:

The engineers said photographs taken by news organizations, as well as what they had observed, showed that seven joists, or wooden beams, extending from the stucco wall of the apartment, on which the balcony was built, had rotted through. They suggested that construction defects had allowed water to seep into the deck, rather than rolling off the edge of it, leading to the growth of fungus and long-term decay of the wood.

“The most likely scenario is water got into the joint between the balcony and the wall, permeated the joist, sat in there, and then eventually deteriorated the joist where it couldn’t handle any kind of a load,” said Gene St. Onge, a civil and structural engineer based in Oakland, Calif. “It’s fairly clear it’s dry-rotted. The timber had all turned to dust.”

Bernard R. Cuzzillo, a mechanical engineer based in Berkeley, said that from his observation, the joists were “completely rotten. Falling apart rotten. They were soaking in water most of the time they were there.”

The LA Times also has an article this morning, Berkeley balcony collapse puts new focus on wood dry rot, which includes more details on the construction of the building and its balconies and potential paths for failure. From the article:

Wood construction of the balconies is common for low-rise residential buildings. Most often, horizontal beams, or joists, that hold up the floor inside the apartment simply extend through the exterior wall to hold up the deck.

But wood rot, also known as dry rot, can make a beam that appears sturdy porous, enough to crumble on contact, due to the growth of a fungus that feeds on the wood.

There are a variety of ways water could have leaked into the wood joists. “It could’ve been that the door above the balcony didn’t have a proper waterproof seal below it,” St. Onge said. “It could’ve been water coming in from the roof down the interior of the wall. It’s a tricky matter and it takes some investigation to determine where it came from.”

Another possibility is the lack of flashing underneath the door – a piece of angled sheet metal that diverts water draining down the stucco away from the wall.

In general, before the stucco layer is applied during construction, wood balconies can be wrapped in waterproof membranes – like sticky asphalt – and then sealed to the waterproofing of the exterior wall.

Any gaps could allow water to seep in.

The quotes from the LA Times article likely get it right. There was most likely a large source of water in this building that was unable to drain properly, which would have wetted the balcony for a long period of time and allowed sustained fungal growth, which would have fed on the wood and led to rotting and structural failure. If this is indeed the case, this tragic and extremely unfortunate event clearly illustrates the importance of understanding intersections between building science and the microbiology of the built environment.

Water in buildings is something that building scientists, including engineers and architects, spend a lot of time thinking about. Water problems (and solutions) in buildings are complex in practice, yet relatively simple in concept. All buildings have to manage water somehow, and proper drainage is an obvious key element for doing so. But maintaining drainage planes — both during design and construction — is tough to do in practice. As I teach in my Building Enclosure Design course at IIT, the solutions are really more of an art than a science, requiring clear details from engineers and architects, but more importantly, requiring good judgment and understanding by trades and construction teams (as well as strong oversight).

Unfortunately, these kinds of wooden structure balcony failures are all too common. The Building Inspection website InspectAPedia has an entire page devoted to balcony and porch/deck problems: Porch & Deck Ledger Flashing Errors Cause Leaks & Rot. The American Society of Home Inspectors (ASHI) wrote about this common balcony failure back in 2007. A common element of these kinds of failures is typically improper flashing.

Flashing is simple in concept: flashing refers to impervious materials that are installed in critical areas of the building enclosure and designed to serve as a guide for directing water in the direction you would prefer it to go. Generally that direction is away from the building enclosure assembly and away from joints and penetrations through which water almost always finds its way. There are a number of details that have to be done correctly in order for flashing to work properly, including sloping in the correct direction (always slope down and out) and overlapping layers in the correct order (so top layers drain to the bottom layers). Also, individual areas for flashing don’t operate in a vacuum. The entire enclosure has to be treated as a weather resistant barrier system, as a water entry problem at the roofline can just as easily lead to a water/moisture/dampness/mold problem multiple stories below in the basement.

If you’re interested in learning more about moisture management in buildings, there are plenty of resources online. The Building Science Corporation (BSC) has a number of helpful materials on flashing and other strategies for water management. Their 2007 document, Water Management Details, is a great resource to start learning about house wraps, flashings, and windows. They also have a shorter digest from 2006 on draining planes. BSC also has some information on stucco wall construction, the same type of construction used in this Berkeley apartment building. Other trade magazines such as The Construction Specifier have published articles in recent years on waterproofing buildings and ensuring wood balcony durability, as well as GreenBuilder Magazine.

One thought on “Berkeley balcony collapse: A failure at the intersection of building science and microbiology

  1. Awful news. My condolences to the Berkeley society. What is important now is to do technical surveys in similar structures, open suspicious walls and floors in order to find the source and its scope, and remove it. Good luck with the remediation work. A major part of the building microbiome is often hidden in closed structures.

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Brent Stephens

Brent Stephens is an assistant professor in the Department of Civil, Architectural and Environmental Engineering at Illinois Institute of Technology in Chicago, IL. He and members of his Built Environment Research Group at IIT conduct energy and air quality research within the built environment, primarily with field measurements in and around buildings. Their work continues to advance building science methods for assessing energy efficiency, indoor air quality, and environmental exposures within buildings.