Terms You Should Know:

  • wind uplift: The upward-acting pressure on a roof’s components resulting from wind traveling across the roof.
  • windward: The side of the building that’s facing the wind.
  • leeward: The side of the building that’s sheltered from the wind; downwind.

Wind Forces

A building is an obstruction to wind flow and causes a change in direction of airflow and pressure over a building’s surface. Consequently, wind damage may occur indirectly from the impact of wind-borne debris or directly through the pressure it exerts.

Generally, the direction of airflow corresponds to pressure in the following ways:

  • Positive pressure occurs on windward surfaces.
  • Negative pressure occurs on leeward surfaces.
  • Negative pressure occurs at sharp discontinuities (eaves, corners, and rakes).

NOTE: The presence of openings to the building’s interior also affects the roof. As air infiltrates through openings and cracks, the internal air pressure increases. This ranges from slightly elevated air pressure caused by HVAC equipment, for example, to strongly elevated air pressure caused by strong thunderstorm winds entering window and door openings.