Wood rot from exposure to excess moisture is likely to have weakened horizontal beams supporting an apartment balcony that collapsed in California, killing six people, several independent experts believe.
But the consensus among structural engineers was that the balcony should otherwise have been sturdy enough, under normal circumstances, to safely bear the weight of the 13 people on the deck at the time.
All 13, mostly college students from Ireland working in the San Francisco Bay Area for the summer on temporary visas, fell to the street below when the fourth-floor balcony gave way during a birthday celebration on Tuesday (local time).
Splintered wooden support joists, which experts said were visibly decayed, were left protruding from where the platform detached from the building's outer wall as it crashed onto a vacant third-floor balcony just below.
Three men and three women in their early 20s, including an American friend of the Irish students, died in the collapse. Seven others were hospitalised.
The integrity of the stucco-over-wood frame construction at the Library Gardens apartment complex, near the University of California at Berkeley, immediately came under scrutiny as city inspectors began to examine the accident site.
Municipal officials declined to discuss the condition of the balcony's underlying structure or speculate on what caused the collapse.
But three Bay-area structural engineers and a veteran building inspector from New York who examined pictures from the site all agreed that wood rot from moisture seeping into the balcony's support beams was a likely factor in the failure of the deck.
"It appears that decay in the structural members probably played a role," said Derrick Hom, the Oakland-based president of the Structural Engineers Association of Northern California.
Hom and others said it would take a thorough physical inspection of the site to determine the source of water that led to decay of the wooden beams.
Alvin Ubell, founder and chief inspector of Accurate Building Inspectors in New York City, said internal moisture from condensation was the most likely culprit, citing high humidity in coastal regions like the Bay Area.
He and other experts agreed that defects in the design, installation and maintenance of waterproofing, flashing materials and ventilation could leave untreated wood framing especially vulnerable to decay.
Otherwise, they said the use of wooden timbers to support cantilevered platforms in such construction was commonplace.
Structural failures accounted for an estimated 5600 injuries from balcony-related falls from 1990 to 2006, according to data collected by the Center for Injury Research and Policy in Columbus, Ohio, and published in 2009 by the American Journal of Emergency Medicine.
As for questions of weight, Hom said the crowd on the deck may have put the balcony close to its maximum design capacity but the weight load "probably was not the critical deciding factor."
The deck below the fallen balcony was also to be removed after being tested and found to be unsound following the accident, a spokeswoman for Berkeley Mayor Tom Bates said.
