A block of King Street remained closed Thursday morning, two days after a large slab of marble tumbled off Canada's tallest office building onto a roof below.

The 115-kilogram panel fell off the side of First Canadian Place and plummeted more than 50 storeys before crashing on a third-floor mezzanine roof.

The bare spot on the side of First Canadian Place is barely visible from the street after a slab of marble fell off Tuesday evening.
(CBC)

No one was injured, but as a precaution, several streets in the heart of Toronto's financial district were shut down to traffic and pedestrians as the rest of the building was inspected.

The only street still closed Thursday was King between Bay and York, in front of First Canadian Place.

The city has issued a work order against owners of the more than three-decade-old skyscraper.

"We've deemed this building to be unsafe at the moment," said Jim Laughlin, deputy chief building official for the City of Toronto.

Building regularly inspected

Engineers are in the process of searching for cracks and bowing in all the 2.5-centimetre-thick marble panels that line the sides of the building, a process that could take days. 

The marble slats are held in place with steel brackets then sealed along the edges with caulking.

Passersby peer up at the empty square from an area not cordoned off by police.
(CBC)

Laughlin said the owner conducts biannual building inspections and regularly replaces problem pieces.

"A situation like this is very uncommon. I've been with the city 30 years and this is the first incident that I'm aware of where a piece of marble falling from the 54th storey of a building has ever occurred," Laughlin said.

Laughlin said he's not worried a similar problem could affect other buildings in the city since the First Canadian Place is the only one that uses marble to such an extent, with the heavy stone covering its entire 70 storeys.

Should marble be used in Toronto?

Architects have raised concerns about whether marble is suitable for use in Toronto's climate.

Moisture easily penetrates the heavy, porous stone, causing disintegration, warping and discolouration, said architect Harry Stinson.

"The engineering in this city is actually pretty good, and nobody would deliberately cut a corner on it, but it's a massive, massive building and it’s been there for many years," said Stinson. "Things are going to happen."