“The risk is that all the vaults up there fall. It is that simple,” says Philippe Villeneuve, chief architect of Notre-Dame, who led TIME on a tour of the fire-ravaged cathedral. “For the moment we do not know because no one has gone to see them, because you cannot go and see them.”

The man responsible for overseeing the reconstruction of Notre-Dame says the risks of a catastrophic collapse are small but that the true extent of the damage will not be known until at least the end of the year. Until then, it will remain a triage site.

Those assessing Notre-Dame’s damage are working to a tight deadline: President Emmanuel Macron has declared that the building should be rebuilt within five years. But Villeneuve says there remain some deeply worrying unknowns about what state Notre-Dame is in, especially as the building’s interior is now open to the elements. “We do not know if there are fissures or fractures,” he says.

Rows of chairs remain in place on July 4, unmoved since the April fire. | Patrick Zachmann—Magnum Photos for TIME

Netting was set in place to catch pieces of falling debris. | Patrick Zachmann—Magnum Photos for TIME

In early July, Villeneuve took TIME to the rooftop where the fire began in April 15, the first journalists to visit the spot. To reach the site, we mounted the ancient spiral stone steps leading to what remains of Notre-Dame’s roof, an area that has remained off-limits to outsiders since the fire began on the upper levels of the Parisian cathedral.

The scene atop the almost 900-year-old building is dismaying. The roof’s frame is now just a giant tangle of molten lead, twisted and bent like spaghetti in the fire. The roof itself is a gaping hole with nothing between it and the nave 226 feet below. Plastic sheeting and netting cover parts of the hole, while other parts remain exposed to rainfall and the high temperatures that France has experienced in recent weeks.

Two statues covered in cloth rest inside the cathedral. | Patrick Zachmann—Magnum Photos for TIME

It was up on roof level that a spark from an unknown source ignited a fire around sunset on April 15, whipping within minutes through the roof. The 300-foot wooden spire, mounted about 150 years ago, tilted in the blaze and then crashed in pieces through the roof, bringing down with it the vaulted oak ceiling that Medieval craftsman built by hand in the 13th century.

Now, for the next several months, the task will be dealing with the impact from shocks that occurred those first few hours the night of April 15—not only from the flames, but from the collapsing wood and stone, as well as the high-pressure water hoses pumped into the cathedral to put out the blaze; it took fire fighters nine hours to douse the fire.