Collapsed floor: water infiltration, a common problem that rarely leads to collapse

Water infiltration in buildings is common, "but to cause a collapse" is rarer, according to Marie-Jeanne Jouveau, heritage architect.

What happened ?

Paris firefighters were alerted shortly after midnight on Saturday night to the collapse of an apartment on the 5th floor of a building on rue Amelot, in the 11th arrondissement of the capital, onto the apartment on the floor below.

According to a guest, Fabrice, who spoke to the newspaper Le Parisien, the fifty or so people gathered to celebrate a birthday came together around midnight "to make the cake and sing a little song".

"And when we sang apparently, part of the floor collapsed. About 30 or 35 of us went down a floor" and found ourselves "a little bit on top of each other, a little bit tangled up," he added.

Motionless while waiting for help, the guests were afraid that the collapse would worsen, "like a house of cards," according to Fabrice.

According to information from the Paris prosecutor's office, one person was hospitalized in critical condition and 14 others were in serious but stable condition on Sunday.

"An investigation into the causes of the injuries has been opened," the same source said.

What are the causes of the collapse?

Antoine Cardon, a security architect at the Paris police headquarters, noted "that a floor had been weakened as a result of water seeping in from a balcony," he told Franceinfo.

"Following the deterioration of a floor, and a party that took place in the accommodation, there was a chain reaction of collapses," he explained.

For heritage architect Marie-Jeanne Jouveau, the fact that an infiltration could have such serious consequences, even leading to the collapse of a floor, "is not necessarily very common".

"However, it is extremely common for a weakening of the building to come from an infiltration," she explains to AFP.

Water leaks are often found at the thresholds of French doors, in bathrooms, or in the roof. When water penetrates a floor, it "rots the wood, which reduces its strength, and if there isn't enough sound wood left, it breaks," explains Marie-Jeanne Jouvin. If the beams are metal, "corrosion will weaken them."

In both cases, "a lot of water" and infiltration "over a very long time" are needed to weaken the structure to the point of risking collapse, notes the architect, manager of the Capla architecture agency.

"It is rare for the floor to give way suddenly," adds the architect, who believes that the excess weight of the people present probably caused the structure to break.

What about the rest of the building?

The Paris police headquarters indicated that "the fourth floor was shored up and the entire structure was inspected. The building's structure was not damaged by the floor collapse." The entire building was evacuated as a precaution while the building's condition was assessed.

"If the leak is linked to a balcony, it is not necessarily worrying for the rest of the building," notes Marie-Jeanne Jouveau.

The condominium manager told the press that they had not received any warnings about the condition of the building.

"It is rare that there is no sign" of an infiltration, stresses the heritage architect, who calls for "being attentive and reporting" any traces of moisture, stains, areas where water regularly appears and monitoring the condition of bathroom joints.

"Sometimes, additional layers are added, including insulation work, and this will hide the problems," worries Marie-Jeanne Jouvin, who would like architects to be consulted systematically to prevent unsuitable work from being carried out on old buildings.