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BERKELEY, Calif. — An estimated 6,500 people have been rushed to emergency rooms with injuries from collapsing balconies and porches, while 29 — including six college students in Berkeley this week — have been killed since 2003.

One reason, experts say, is the structures are particularly vulnerable to dry rot.

"It's all about creating a safe structure that has endurance, that has a reasonable life expectancy," said David Helfant, who identified potential flaws in design and construction after an unofficial inspection of the Berkeley balcony that collapsed.

"When I see something like that in a town I work and live in, it's extremely depressing, it's upsetting," he said.

A Consumer Product Safety Commission analysis for The Associated Press estimated that 4,600 emergency room visits were associated with deck collapses in the past decade and another 1,900 with porch failures.

With millions of ER visits a year in the U.S., "the type of incident that happened in Berkeley appears to be rare," commission spokesman Alexander Filip said based on data collected from 100 hospitals to make the projections.

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One of the worst collapses occurred in 2003, when a porch fell in Chicago and killed 13 people. The commission identified just 10 fatalities that occurred since then — until Tuesday.

The Berkeley balcony snapped off the fifth floor of an apartment building, tossing 13 people to the street 50 feet below. Seven survivors are being treated in hospitals, while funerals are being planned for the six who died.

Damage is seen at the scene of a 4th-story apartment building balcony collapse in Berkeley, California June 16, 2015. ELIJAH NOUVELAGE / Reuters

Experts and city officials have said the 40-square-foot balcony might have snapped off because supports had dry rotted, a problem that structural engineers say can be prevented through proper design, construction and maintenance aimed at sealing out water.

Left unrepaired, dry rot can weaken balconies and create collapse hazards, say structural engineers.

"This cantilevered wood balcony appears to be severely rotted at its support," structural engineer Peter Curry said while looking at pictures of the Berkeley balcony. "That's usually a problem with the waterproofing. These should not break like this."

Dry rot occurs when water gets into poorly ventilated areas of buildings and a fungus starts to decay the timber. If left unchecked, wood can fall apart or turn to powder.

The Berkeley apartment complex was built by Segue Construction and completed in 2007. Company spokesman Sam Singer said the firm has never "had an incident like this in its history."

Segue did not respond to further requests for comment.

Attorneys Thomas Miller and his daughter Rachel Miller, who specialize in construction defect lawsuits, say water problems — mold and dry rot — are the biggest culprits in such legal actions.

"Nearly every case is because of water," Rachel Miller said. "Water gets into the building. You have to anticipate that's going to happen."