Roof Wind Pressure Zones

Roof wind pressure zones are the areas into which design standards divide a roof to account for the fact that wind suction is not uniform. When wind separates from the sharp edge of a building, it forms swirling vortices that create very strong suction along the edges and especially at the corners. The middle of the roof, called the field, sees much lower pressures.

Typical zones

In the US standard ASCE 7 (Minimum Design Loads for Buildings and Other Structures), components and cladding on roofs are assigned to numbered zones:

• Field (Zone 1, and Zone 1' on some low-slope roofs): the interior area with the lowest pressures.
• Edge or perimeter (Zone 2): a strip along eaves, rakes and ridges.
• Corner (Zone 3): the areas where two edges meet, with the highest pressures.

Zone widths are calculated from the building's dimensions and mean roof height. Hip and gable roofs have their own zone layouts and coefficients that depend on slope.

Changes in ASCE 7-16

The 2016 edition of ASCE 7 revised the roof zones significantly. For flat roofs, corner zones became L-shaped with widths based on roof height, and an extra edge zone was added; gable and hip roofs also gained additional zones. The largest negative pressure coefficients increased on most zones, in some cases roughly doubling design pressures, although lower mapped wind speeds in parts of the Midwest and West Coast and new elevation and exposure adjustments partly offset the increase. Later editions (ASCE 7-22) continued to refine the method.

Practical effects

Because corner and edge pressures are higher, roof systems often need:

• closer fastener spacing or extra fasteners in perimeter and corner zones;
• additional adhesive or attachment for low-slope membranes and insulation;
• stronger edge metal, coping and gutters;
• higher-rated tile or panel clips near edges.

Other national standards, such as Eurocode 1 in Europe and AS/NZS 1170.2 in Australia and New Zealand, follow the same principle with their own zone layouts and terminology.

Field evidence

Damage surveys after hurricanes and tornadoes consistently show roof coverings and decks failing first at corners, eaves, rakes, hips and ridges, confirming the zone approach.