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WHY DO ROOFS OF HOUSES FLY AWAY DURING STORMS?

When wind flows with a high velocity above the roof of a house if causes a lowering of pressure above the roof. However, pressure below the roof is still atmospheric. If the wind blow fast enough, the internal air pressure(atmospheric) can blow the roof off. Let us calculate the upward force on the roof during wind storm. Air above the roof pushes down with a force F₁ = P₁A where P₁ is the air pressure above the roof and A is the area of the roof. The air inside the house pushes up with a force F = P_a A where Pa is the atmospheric pressure of the air inside the house. The net upward force is

         F_net = F - F₁ = P_a A - P₁A = A(P_a - P₁)

A small pressure difference over a large area can produce a large upward force.