Question

why are roof blown off when there is a strong wind

Answer 1

the higher velocity of the air outside causes the air pressure above the roof to decrease and when it has decreased to a certain extent such that the air pressure above the roof is lesser than the air pressure beneath the roof and due to some kind of osmosis, the air particles move from the area of higher pressure (beneath the roof) to the area of low pressure. In this process, the roof is blown away.