What are roof wind uplift Zones 1, 2, and 3?

Per ASCE 7-22 Components and Cladding, the roof is split into pressure zones. Zone 1 is the interior field, Zone 2 wraps every edge in a band of width a, and Zone 3 sits in the corners where two edges meet. Corner Zone 3 carries the worst suction, because conical vortices spin off the corners and concentrate low pressure there.

Why are roof corners and edges the worst for wind uplift?

As wind separates over a roof edge it rolls into tight conical vortices anchored at the corners, creating concentrated suction. That is why the verified flat-roof Zone 3 corner coefficient (GCp of -3.2 at 10 sqft) is nearly double the Zone 1 interior value of -1.7, with the Zone 2 edge band (-2.3) in between. Larger effective wind areas average the peaks down.

How do I calculate roof wind uplift pressure per ASCE 7?

Use p = qh (GCp - GCpi). The velocity pressure is qh = 0.00256 Kz Kzt Kd Ke V squared, with Kd = 0.85 for buildings and enclosed internal pressure GCpi = plus or minus 0.18 (partially enclosed is plus or minus 0.55). Pick GCp by zone and effective wind area; the most negative GCp paired with positive internal pressure governs uplift.

How is the roof edge and corner zone width a determined?

Dimension a sets the edge and corner zone widths. It is the smaller of 10 percent of the least horizontal building dimension or 0.4 times the mean roof height h, but not less than 4 percent of that least dimension or 3 feet. The corner Zone 3 squares and the Zone 2 edge band are both built from this value of a.

What resists roof wind uplift in a building?

Net uplift is resisted by a continuous load path, link by link: covering to sheathing (fasteners, clips, sealant), sheathing to framing (nailing), framing to wall (hurricane straps and clips at the top plate), and wall to foundation (hold-downs and anchor bolts). Fasteners are spaced tighter in Zones 2 and 3, and the chain is only as strong as its weakest connection.

ASCE 7-22 · COMPONENTS & CLADDING · ROOF

Roof wind uplift is a corner-first failure

Wind never lifts a roof evenly. Vortices spin off the corners, and the suction there runs far past the calm interior field. Design the zones, not the average.

3C&C ROOF ZONES (1 · 2 · 3)

−3.2FLAT-ROOF CORNER GCp @10 SQFT

±0.18ENCLOSED GCpi

0.85Kd · DIRECTIONALITY

Calculate roof uplift C&C vs MWFRS

FIG 30.3-2 · ROOF PLAN

The roof, mapped by pressure zone

Zone 1 is the interior field. Zone 2 wraps every edge a width a. Zone 3 sits in the corners where two edges meet — the worst suction on the roof.

Zone 3 corners carry the highest suction; Zone 1' / edge-and-corner widths are set by dimension a — the smaller of 10% of the least horizontal dimension or 0.4h, but not less than 4% of that dimension or 3 ft.

VERIFIED GCp · h ≤ 60 FT · ENCLOSED

Uplift by zone, straight from ASCE 7-22

External pressure coefficients (GCp) ranged by effective wind area, smallest area to largest, log-interpolated between. Suction is negative.

Zone	Flat roof Fig 30.3-2A	Gable 7–20° Fig 30.3-2B
Zone 1 — interior	−1.7 @10 sqft → −1.0 @500	−2.0 @10 sqft → −0.5 @500
Zone 2 — edge	−2.3 @10 → −1.4 @500	−2.7 @10 → −1.0 @500
Zone 3 — corner	−3.2 @10 → −1.4 @500	−3.6 @10 → −1.8 @500
Positive (toward surface)	+0.3 @10 → +0.2 @100	—

Design pressure: p = q_h (GCp − GCp_i), with enclosed GCp_i = ±0.18 and q_h = 0.00256 K_z K_zt K_d K_e V². Use K_d = 0.85 for buildings. The most negative GCp paired with positive internal pressure governs uplift.

THE MECHANISM

Why corners and edges lift first

As wind separates over a roof edge it rolls into conical corner vortices — tight spirals of fast, low-pressure air anchored at the corners. That concentrated suction is why the verified Zone 3 corner coefficient (flat roof −3.2 at 10 sqft) is nearly double the Zone 1 interior value (−1.7). The edge band (Zone 2, −2.3) sits between. Larger effective wind areas average the peaks down — which is why every coefficient eases toward the 500 sqft end of the range.

CONTINUOUS LOAD PATH

How the roof holds down

Net uplift is resisted link by link — covering, sheathing, framing, wall, foundation. The chain is only as strong as its weakest connection.

Covering to sheathing

Shingle sealant, panel clips, or membrane fasteners resist covering uplift — spaced tighter in Zones 2 and 3.

FASTENERS

Sheathing to framing

Nailing carries diaphragm shear and uplift into rafters and trusses; the edge zone gets the closer schedule.

NAILING

Framing to wall

Hurricane straps and clips wrap rafter or truss to the top plate — sized for net uplift over each member's tributary area.

STRAPS

Wall to foundation

Hold-downs and anchor bolts close the load path to ground, completing continuous resistance from ridge to footing.

HOLD-DOWNS

PUT THE ZONES TO WORK

Get zone-by-zone roof pressures

WindLoadCalc.com returns C&C pressures for Zones 1, 2, and 3 per ASCE 7-22 — ready for fastener schedules and connection design.

Try the wind load calculator Full C&C calculation guide

KEEP READING