Design pressure — DP — is the single number on a window schedule that tells you how much wind and water-driven force an opening can take before it fails. On the coast, it is the difference between a window and a liability.
A DP rating combines three tests: resistance to air infiltration, to water penetration, and to structural load. The number is expressed in pounds per square foot. A DP 65 unit has been tested to withstand a structural load equivalent to roughly 65 PSF — which corresponds to sustained winds in the region of 140 mph.
Why the coast is different
Inland, a builder-grade window at DP 30–40 is often enough. On a barrier island or oceanfront lot, the combination of sustained wind, wind-driven rain, and flying debris raises the bar dramatically. Building codes in hurricane zones require both a high design pressure and impact resistance — the ability to survive a Large Missile strike followed by thousands of cycles of positive and negative pressure.
Design pressure is not impact resistance
They are tested separately and a window needs both. Our systems are rated to DP 65+ and carry Large Missile Impact Level D approval, having come through some of the worst recent hurricanes intact. Multi-point, opposing-direction locking keeps the sash compressed against its seals under load, which is what holds water out when the wind reverses.
What to put on the schedule
- A design pressure that meets or exceeds your local code for the building’s exposure category.
- Impact rating appropriate to the wind-borne debris region — Large Missile for most oceanfront work.
- Oceanfront, salt-air-rated hardware, so the locking that earns the rating still works in ten years.
Specify all three together. A high DP number means little if the hardware corrodes or the glazing isn’t impact-rated.