Master roof wind uplift calculations for zones 1, 2, and 3. Learn fastener design, hurricane straps, and roof-to-wall connections to prevent roof failures in high-wind events.
Calculate Roof UpliftRoof uplift = Negative (suction) wind pressure that tries to lift the roof off the building. Corners (Zone 3) experience the highest uplift, often 2-3x higher than interior areas (Zone 1).
Key concept: Roofs fail when uplift force exceeds the weight of roofing plus connection capacity. In hurricanes, roofs often fail at corners first due to concentrated vortex suction.
Critical design: Roof-to-wall connections (hurricane straps) and fastener patterns must be designed for C&C pressures, not MWFRS. This is where most roof failures originate.
ASCE 7 divides roofs into three zones based on pressure intensity. Understanding these zones is critical for proper fastener spacing, roofing selection, and connection design.
Zone 3 corner width = Zone 2 edge width = dimension 'a' (typically 10% of least horizontal dimension or 0.4h, not less than 4% of least dimension or 3 ft)
Zone 1 covers the central portion of the roof away from edges and corners. While Zone 1 pressures are the lowest, they still exceed MWFRS pressures and must not be ignored. This zone typically comprises the majority of roof area.
Zone 2 extends along all roof edges for a width of dimension 'a'. As wind flows over the building, it accelerates at roof edges causing higher suction. Ridge lines on gable roofs also experience Zone 2 pressures.
Zone 3 occupies corners where two Zone 2 areas meet. Corner vortices create intense localized suction that can be 2-3 times higher than Zone 1. This is where most roof failures initiate during hurricanes and severe storms.
Post-storm damage surveys consistently show roof failures initiating at corners (Zone 3). Once a corner lifts, the failure propagates across the roof as wind gets under the membrane or sheathing. Proper Zone 3 design is the first line of defense.
Roof systems must resist significant suction (negative) pressures during high winds. Unlike walls that experience both positive and negative pressures, roofs primarily experience suction. Key factors:
Net roof uplift = C&C suction pressure + internal pressure - dead load
Zone 3 C&C pressure: -85 psf (suction)
Internal pressure (enclosed): +18 psf (adds to uplift)
Combined uplift: -85 + (-18) = -103 psf
Dead load (roofing + structure): +15 psf
Net uplift: -103 + 15 = -88 psf
Connections must resist 88 psf net uplift in Zone 3.
The roof-to-wall connection is the critical link in the continuous load path. These connections must transfer roof uplift forces to wall framing and ultimately to the foundation.
| Connector Type | Typical Capacity | Application |
|---|---|---|
| Toe Nails | 100-200 lbs | Low-wind areas only; not permitted in high-wind |
| Hurricane Clips | 200-500 lbs | Moderate wind areas; single-sided attachment |
| Hurricane Straps | 500-1000+ lbs | High-wind areas; wraps over truss/rafter |
| Engineered Connectors | 1500-3000+ lbs | Extreme wind; custom designed for loads |
Connection uplift capacity = Net uplift pressure × Tributary area
For a rafter spaced at 24" on center with 15 ft span:
Connections at roof corners (Zone 3) require the highest capacity. Don't use field area (Zone 1) pressures for corner connections. Zone 3 may require capacities 2-3 times higher than Zone 1.
Roof covering fasteners (for shingles, metal panels, membrane attachment) must resist C&C uplift pressures. Proper fastener selection and spacing prevents roof cover failure.
| Zone | Typical Edge Spacing | Typical Field Spacing |
|---|---|---|
| Zone 1 (Field) | 6" o.c. | 12" o.c. |
| Zone 2 (Edges) | 4-6" o.c. | 6" o.c. |
| Zone 3 (Corners) | 4" o.c. | 4-6" o.c. |
Most roofing manufacturers provide high-wind installation instructions that specify:
The roof is one component in the continuous load path that must transfer wind forces from roof to foundation. Every connection must be designed for the loads passing through it:
The continuous load path is only as strong as its weakest link. Missing or undersized hurricane straps can cause roof failure even if all other connections are adequate. Every connection in the chain must resist its share of loads.
WindLoadCalc.com calculates C&C roof pressures for all three zones per ASCE 7-22 and ASCE 7-16. Get zone-by-zone pressures for accurate fastener and connection design.
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