Design pressure ratings, explained

Positive and negative pressure are different loads

Wind striking the windward face of a building pushes inward — positive pressure. As that air accelerates around corners and over the roof, it creates suction on the leeward and side faces, pulling outward — negative pressure. A window has to resist both, and they are not equal.

Negative pressure is frequently the governing case, particularly near building corners and at height, where suction peaks. This is why a DP rating is properly expressed as a pair, such as +50/-55, and why an opening on the corner of a house may need a higher-rated unit than an identical opening in the middle of the same wall.

It also explains something that surprises people after a storm: windows can be pulled out of their openings rather than pushed in. The failure was suction, not debris.

Why a higher number is not automatically better

It is tempting to treat DP like horsepower and assume more is better value. It is not, for two reasons.

First, DP is a property of the whole assembly at a specific size. The same window design tested at a smaller size yields a higher DP than at a larger one, because a bigger pane of glass and a longer unsupported frame span deflect more under the same pressure. A DP65 rating quoted without the size it was achieved at tells you almost nothing. This is the single most common way DP numbers are used misleadingly in a sales conversation.

Second, reaching a higher DP at a given size means more material — thicker frames, heavier glass, sometimes a mullion through the opening. You pay for that in money, in weight on the hardware, and in sightlines. Specifying well above what the calculation requires buys you a narrower view and a larger invoice, not more safety.

How the required figure is arrived at

The Florida Building Code sets the method. The inputs are the ultimate design wind speed for your location, which varies across South Florida and is highest on the coast; the building's exposure category, which reflects how much surrounding terrain and development shelters it; the mean roof height; the shape of the building; and the size and position of each individual opening, since corners and higher elevations see greater suction.

Those inputs produce a required design pressure per opening. Your contractor then specifies units whose tested DP at the actual ordered size meets or exceeds it. On a permit submission the building department checks exactly this: that the approved product documentation covers the size being installed at the pressure the calculation demands.

This is why a quote that specifies one window type for an entire house should prompt a question. A three-storey home with a coastal exposure will legitimately need different ratings on different elevations.

How DP relates to impact approval

Design pressure and impact rating are separate tests and a product needs both. Impact testing fires a missile at the assembly — the large missile test used in the High-Velocity Hurricane Zone uses a nine-pound piece of lumber at approximately 34 miles per hour — and then cycles the assembly through thousands of pressure reversals to simulate a storm's duration.

Design pressure testing is the static load test. A window can hold a high DP and have no impact rating whatsoever, which is exactly what an ordinary non-impact window in a windy location is. In Miami-Dade and Broward the assembly must carry a Miami-Dade Notice of Acceptance covering both; elsewhere in South Florida, Florida Product Approval serves the same function.

What to ask when you are quoted a DP

Three questions settle it. What is the DP at the size you are actually ordering, not at the manufacturer's test size? What does the code calculation require for this specific opening on this specific elevation? And is the approval documentation on file at the size being installed?

Any contractor pulling the permit has to answer all three for the building department regardless. There is no reason they cannot answer them for you first.