When the wind blows at a high speed, the roof of  a house is blown off because :

• The pressure above a roof decreases when swift winds blowing over it. 
• Pressure will decrease more rapidly as speed increases. In comparison to the pressure above the roof, the air pressure below the roof is higher. 
• The ceiling is forced upward by this pressure difference.
• Because of this, when high-speed winds start to blow, the roofs of the huts are blown off.

Hence the correct answer is Pressure above the roof decreases.