ASCE 7-22 · C&C · FENESTRATION
Window & Door Design Pressure
Design Pressure (DP) is how you specify a window or door for wind. The unit's DP rating must meet or exceed both the positive and the negative C&C pressures calculated for its location. Not sure where that DP number comes from? See how wind load pressure is calculated under ASCE 7-22.
THE TWO DIRECTIONS DP MUST COVER
One unit, two pressures
Wind pushes inward (positive) on the windward face and pulls outward (negative suction) on side and corner zones. A DP rating is symmetric: DP-50 means the unit is rated for +50 psf inward and −50 psf outward.
POSITIVE vs NEGATIVE DESIGN PRESSURE
Both must be satisfied
Positive (inward)
Windward pressure pushing the unit into the building. Driven by external coefficient +1.0 (Zone 4 @10 sf) minus the enclosed internal pressure −0.18.
GCp +1.0 @10 SFNegative (suction)
Outward pull that usually governs DP selection. Strongest at corners (Zone 5, GCp −1.4 @10 sf) versus the wall field (Zone 4, −1.1).
ZONE 5 −1.4 @10 SFHOW TO PICK A DP RATING
Calculate → cover → select
Net C&C pressure is p = qh[(GCp) − (GCpi)], with qh = 0.00256·Kz·Kzt·Kd·Ke·V² and Kd = 0.85. The worked numbers below use an enclosed building (GCpi ±0.18) at a corner window, with a site velocity pressure qh = 40 psf.
1 · Calculate
Positive: 40×(1.0−(−0.18)) = +47.2 psf. Corner suction: 40×(−1.4−0.18) = −63.2 psf.
+47.2 / −63.2 PSF2 · Cover both
The DP demand is the larger magnitude of the two. Here suction −63.2 psf governs over +47.2 psf.
DEMAND 63.2 PSF3 · Add margin
Round up to a rated DP step so the unit meets or exceeds demand — never specify a DP below the governing pressure.
ROUND UP4 · Select DP
63.2 psf → specify DP-65. It is rated +65 / −65 psf, covering both the +47.2 and −63.2 demands.
DP-65 ≥ DEMANDEFFECTIVE AREA & ZONE
Bigger units & corners change the number
GCp is a function of effective wind area. A larger unit (bigger effective area) sees a lower GCp magnitude than the −1.4 / −1.1 / +1.0 values tabulated at 10 sq ft. Zone 5 (corners) always demands more suction capacity than Zone 4 (field). Always check the unit's actual area and its zone before fixing the DP.
HVHZ · MIAMI-DADE & BROWARD
Impact rating is also required
In the High-Velocity Hurricane Zone, a sufficient DP rating is not enough on its own — fenestration must also carry impact approval under the Florida Testing Application Standards.
SPECIFY WITH CONFIDENCE
Get the C&C numbers your DP must beat
Calculate zone-by-zone positive and negative C&C pressures so you can specify the right DP rating for every opening.