Tens of thousands of Montreal students won't be returning to their schools until at least Tuesday after Quebec's largest school board cancelled classes amid concerns over the dangers posed snow-covered roofs.

The Commission scolaire de Montreal took the extraordinary measure of closing more than 150 schools on Friday so that workers can clear snow from roofs, some covered with as much as eight to ten feet of snow.

The late-morning decision to send about 90,000 students home came after worrying signs of fissures and cracks were found in many of the schools.

Snow has been removed in the most concerning cases, board director Diane de Courcy told CTV News, but said the cautious move is to fully inspect every building.

The Commission scolaire de Montreal, the city's largest school board, operates elementary and secondary schools in the city.

Hundreds of shelters were set up in churches and synagogues near the schools, where children spent the day until their parents picked them up.

Meanwhile, the English Montreal School Board has stepped up inspections of its buildings' roofs, and additional crews will be working this weekend to ensure the roofs of all of the facilities are safe.

Education Minister Michelle Courchesne said her department asked school boards to assess roofs of their buildings after the roof of a warehouse suddenly collapsed Wednesday in Morin Heights -- about an hour from Montreal -- killing three women.

Since then, many institutions and buildings have been closed to allow snow to be cleared from roofs.

On Thursday, a giant shopping mall outside Montreal decided to close indefinitely after some cracks appeared in the building's structure.

With a report from CTV Montreal's Paul Karwatsky