How to Convert Wind Speed to Roof Uplift
One of the most dangerous events that take place in a hurricane, tornado or straight line wind storm is the uplift and removal of a roof. The physics behind this refers to the velocity pressure of the wind or the pressure induced by wind speed. Pressure is a force per unit area so a pressure of 10 pounds per square foot (psf) becomes a very large force when there is a high surface area (measured in square feet). For instance, a roof lip with a square foot area of 100 square feet will experience a force of 1000 pounds (10 psf x 100 square feet). The force required to lift a roof is extremely variable and depends on the direction of wind, materials of construction and roof supports.
Determine the calculated wind speed pressure based on the dynamic pressure formula: P = 1/2 ρ x v^2 where P is pressure in pounds per square foot, ρ is the air density in pounds per cubic foot (lbs/ft3) and v is the air velocity in feet per second (fps). Assume there is a 100 mph wind (146.67 fps) and air has a density of 0.075 lbs/ft3. This is calculated to be 806.7 pounds per square foot.
Determine the maximum designed force a roof can withstand before failure. For instance, assume a residential roof has an exposed surface area of 125 square feet and can withstand 110,000 pounds of force applied to it.
Calculate the maximum force that will be applied to the roof by multiplying the velocity pressure by the exposed roof surface area. This is 806.7 x 125, or approximately 101,000 pounds of force. This means that a roof designed for 110,000 pounds of force should withstand the wind speed.
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