Climbers have been brought in to unfurl protective tarpaulins over Notre Dame to protect it from the rain after the Parisian cathedral was left badly damaged and open to the elements by a fire last week.

The blaze on 15 April felled the 850-year-old Gothic cathedral’s spire and destroyed two-thirds of its vaulted roof, leaving the building in a fragile condition.

With efforts continuing to shore up sections still at risk from collapse, experts faced a new challenge after several days of rain were forecast.

If left exposed, the surviving part of the roof and the vaulted ceilings could suffer further damage, prompting a race to erect a temporary protective cover.

Workers stand on top of Notre Dame on Tuesday as rainclouds gather. Photograph: Thibault Camus/AP

Forecasters expected the rain to begin on Tuesday night, with conditions deteriorating on Thursday.

“The biggest priority is to protect the cathedral from the coming rain,” the site’s chief architect, Philippe Villeneuve, told France’s BFMTV, saying he wanted to speed up efforts to erect the tarpaulins before the rain.

“The beams are in place; the tarpaulins have arrived; the climbers and the scaffolders, who will put it up, are ready,” he said.

Christophe Villemain, a specialist in restoring historic buildings, told the channel the rain could potentially cause further sections of the roof to collapse. “The rain risks falling on the vaulted ceiling and filling up what we call its haunches, or hollow sections, and that would put the arches at risk of collapse,” he said.

The covering will only be temporary, with plans to replace it with a more sturdy protective “umbrella” that would remain in place throughout the work to restore Notre Dame, which took 200 years to build. The French president, Emmanuel Macron, has said he wants it rebuilt within five years.

Before the umbrella can be set up, scaffolding that was erected for renovation work before the fire must first be removed, in an operation that is likely to take up to a month, Villemain said.