Architects working on preserving Notre-Dame are rushing to cover the cathedral before rain can cause further damage, reports the BBC.
Rain is forecast for the French capital on Wednesday, with further showers predicted for Thursday.
The cathedral's vault, which partly collapsed in the fire, is already partly waterlogged after fire-fighting efforts.
Architects fear that heavy rainfall could result in further collapse of the 800-year-old cathedral.
The chief architect of Notre-Dame, Philippe Villeneuve, told French broadcaster BFMTV that erecting an emergency tarpaulin was "the highest priority".
"The beams are there, the tarpaulin is arriving. The climbers, since it will be climbers who will do that, and the scaffolders, are ready," he said.
There are already plans to erect a large, purpose-built "umbrella" on the roof of the landmark, which will have its own peak and protect the structure while reconstruction takes place.
But the umbrella is not ready - and the threat of impending rain is too serious to wait.