Sydney school fire 'suspicious': police

A suspicious fire at a Sydney public school overnight has caused a roof to cave in on a building.

A week before students are set to return for the new school year, a suspicious fire is being investigated at Chatswood Public School that started at about 10pm on Sunday, NSW police said.

It took five fire crews and more than two hours to battle the blaze at the isolated building, which was well alight when they arrived, with the task made more difficult by a partial collapse in the roof, a NSW Fire and Rescue spokesman said on Monday.