What is the difference between Zone 4 and Zone 5 in wind load calculations?

Zone 4 is the 'wall field' - the middle sections of walls away from corners. Zone 5 is 'wall corners' - areas within a distance 'a' from building corners. Zone 5 experiences 20-40% higher wind pressures than Zone 4 because wind accelerates and creates stronger suction as it wraps around corners.

Why are Zone 5 corner pressures higher than Zone 4?

When wind hits a building and flows around corners, it speeds up (like water flowing through a narrow channel). This acceleration creates stronger suction forces. Additionally, vortices form at corners, creating localized high-pressure areas. This is why corner windows and doors need higher pressure ratings.

How wide is Zone 5 on a building wall?

Zone 5 width equals the smaller of: 10% of the least horizontal dimension OR 40% of building height, but not less than 4% of least dimension or 3 feet. For a typical home, Zone 5 is usually 3-6 feet from each corner.

Zone 4 vs Zone 5 Wind Pressure Comparison

Why Corners Get Hit Harder

Imagine holding your hand out a car window. When your palm faces the wind, you feel the push. Now angle your hand - the wind flows around it faster. The same thing happens at building corners, but much more dramatically.

When wind hits a building and wraps around the corners, it speeds up. This is called the Venturi effect. Faster wind creates stronger suction, which is why Zone 5 (corners) can experience 40% more pressure than Zone 4 (the middle of the wall).

Factor	Zone 4 (Field)	Zone 5 (Corner)
Location	Middle of wall	Within 4-6 ft of corner
Negative Pressure	-48.3 PSF	-67.4 PSF
Window Cost Impact	Standard rating	Premium rating needed
Why Higher?	Uniform flow	Wind acceleration + vortices

This is why building codes require engineers to calculate different pressures for different zones. A window in the middle of your wall might be fine at 50 PSF rating, but that same window in a corner would fail - it needs at least 70 PSF rating!

Zone 4 vs Zone 5

Why Corners Get Hit Harder

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