The short version
Design pressure shows up as two numbers on a window or door rating: a positive and a negative. Positive pressure pushes the assembly inward, toward the building. Negative pressure sucks it outward, away from the building face.
They are not the same load, they don’t act on the same surfaces, and they aren’t interchangeable. On an HVHZ submittal the negative number is the one that usually catches people — because a product rated +50/−50 is not the same product as one rated +50/−35, and if your submittal only shows the positive, it is incomplete before the reviewer gets to it.
Why pressure comes in two signs
Wind flowing around a building does not just push. As it wraps the corners and spills over the roof and off the leeward face, it accelerates and creates suction — exactly the way air moving over an airplane wing lowers pressure above it. That suction acts on the outside of your fenestration and pulls the assembly outward.
So every opening carries two demands at once:
- A positive value when wind pushes directly into that face.
- A negative value when that same face is in suction.
The rating on a product approval lists both, written as something like +50/−50 psf. The first number is the inward capacity, the second the outward. A submittal that treats them as one number is missing half the structural condition the opening has to survive.
Where the negative number actually comes from
The design pressures for your project come off the structural drawings, not the product data sheet. The engineer computes them for the building — at each opening, or by zone — under the applicable standard.
In Miami-Dade and Broward, that means the High Velocity Hurricane Zone provisions of the Florida Building Code, which reference ASCE 7 wind loads. Fenestration is treated as components and cladding under ASCE 7, and components and cladding can see higher localized pressures than the main wind-force-resisting system — because a small element like a window takes the full brunt of a local gust, with no structure to share it.
The negative side of that is frequently the driver. On interior zones and leeward faces, the suction can be larger, relative to its positive counterpart, than many estimators expect. If your window schedule only notes what looks like the bigger positive pressure, the negative column in the engineer’s table is what will trip you up.
What happens when only one sign is listed
The reviewing authority sees an opening with a combined +/− requirement on the drawings and a submittal that substantiates one side. The result is a deficiency, and a deficiency becomes a resubmittal.
For a window submittal under HVHZ, the cost of that resubmittal is rarely the window itself. It is the week of your schedule that goes into the revision, the reinstated review clock, and the line item in your allowance that now has to absorb the second pass. On a project with dozens of openings, one missing sign can gate the whole package.
Reading the rating the right way
When you check a product against a pressure from the drawings:
- Pull both the positive and negative pressures for that opening from the structural schedule.
- Find the product’s +/− rating on its approval sheet, not the brochure.
- Confirm the product’s positive rating clears the positive requirement and its negative rating clears the negative requirement — independently.
- Watch the units. Both sides must be in pounds per square foot, and if the engineer mixed in a kilometer-speed or a different load case, reconcile it before the submittal, not after.
A product that clears +50 on its positive side but only −35 on its negative side does not satisfy a +50/−50 requirement, regardless of how strong it looks in the positive direction.
Need the full design-pressure picture? How design pressure drives a window submittal walks the numbers from the drawings; reading a Miami-Dade NOA shows where the ±DP rating lives. Confirm a required pressure on a live opening with our free wind-load calculator.
The takeaway
On an HVHZ submittal, design pressure is a two-number condition, and the negative number is the one that most often decides the outcome. Pull it off the structural drawings, verify the product’s negative rating independently, and never round the two signs into a single number. Getting this right is cheap; getting it wrong costs you the resubmittal week every time.