The reduced pressure is lower inside the building, which is increased by air flowing through the building openings. In this condition, fast-moving exterior air flows over and around the building, creating a reduced pressure, or suction. This is typically created by openings in the building (windows, doors, overhead doors) at the windward side of the building. Interior pressurization can also contribute to wind uplift damage to a roof system. Wind, especially at high velocity, creates a vacuum or negative pressure, lifting the membrane and roof insulation material loose from points of attachment. The parapet wall eliminates the air from flowing over the building, prevents the formation of vortices and reduces wind uplift. Research also indicates that the addition of parapet walls with heights of two feet or more can minimize perimeter damage. Proper perimeter edge design and attachment calculations can eliminate these concerns. As air mass from the wind transcends up the building’s perimeter, it forms vortices and creates uplift damage throughout the roof as it flows over and around the building. Wind uplift is severe at the roof perimeter, primarily at corners, where it exceeds the normal static pressure against the wall. The most common form of roof failure from wind effects occurs at perimeter edges. Wind failures of roofs generally occur in one of four forms: vortices, interior pressurization, kinetic energy and punctures from wind driven projectiles.

Wind damage to buildings can range from complete demolition of the structure to total or partial roof displacement (blow-off) to damage to penetrations (skylights, mechanical equipment, vents, etc.), doors and windows. In order to properly design for wind attachment, a basic understanding of the effects of winds on buildings is required.

(Photo courtesy of CRS Inc.) The Effects of High-Velocity Winds Through Biloxi, Miss., held up rather well. This section of the Edgewater Mall’s roof assembly, photographed after Katrina went

Metal roof attachment for sheds