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How to Convert Wind Speed to Roof Uplift

Brian Baer

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.

Roof uplift is a function of wind speed pressure.

Step 1

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.

Step 2

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.

Step 3

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.