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Wind Force Fundamentals

Suction vs Positive Pressure

Wind does not just push on buildings - it also pulls. Understanding this difference explains why roofs blow off and walls get sucked outward.

WIND
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Positive Pressure Pushes TOWARD surface
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Suction (Negative) Pulls AWAY from surface
PUSH
Front Wall
+0.8 Cp
PULL
Back Wall
-0.5 Cp

Wind Creates Both Forces

When wind hits a building, it does not just push on the front wall and stop. As the wind flows around the building, it creates different pressures on different surfaces.

Windward wall (front): Wind hits directly and creates positive pressure - it pushes inward.

Leeward wall (back): Wind flows past and creates suction - it pulls outward.

Side walls: Also experience suction as wind accelerates around corners.

Roof: Almost always suction, pulling upward like a giant vacuum.

Why Roofs Blow OFF, Not IN

Have you ever wondered why hurricane damage pictures always show roofs missing? The roof did not blow downward - it was sucked upward!

When wind flows over a roof, it speeds up (like water through a narrow channel). Faster moving air has lower pressure. This creates suction that lifts the roof.

The effect is like holding a piece of paper in front of your mouth and blowing across the top - the paper rises because of the low pressure above it.

In Miami-Dade at 180 mph, roof suction can exceed 80 psf at corners - like having an 80-pound weight pulling up on every square foot!

Suction pulls roof UPWARD
Corner (Zone 3)
Edge (Zone 2)
Field (Zone 1)

Corners Get Hit Hardest

Wind does not affect all parts of a roof equally. Corners and edges experience much stronger suction than the middle of the roof.

Why? Wind accelerates as it wraps around corners, creating vortices (spinning wind patterns) that dramatically increase local suction.

That is why building codes define different pressure zones:

Zone 3 (Corners): Highest suction, up to 3x the field value
Zone 2 (Edges): Moderate suction, about 2x field
Zone 1 (Field): Lowest suction, but still significant

This explains why hurricane damage often starts at roof corners - they see the worst forces.

Frequently Asked Questions

What is the difference between suction and positive pressure? +
Positive pressure pushes toward a surface (like wind hitting your face). Suction (negative pressure) pulls away from a surface (like wind on the back side of a building trying to pull the wall outward). Both forces must be designed for in wind loads.
Why is wind suction often more dangerous than positive pressure? +
Suction can be stronger than positive pressure, especially at roof corners and edges. Buildings are typically designed to resist gravity pushing down, but suction pulls upward and outward. Connections that seem strong against pushing forces may fail when pulled apart.
Which parts of a building experience the highest suction? +
Roof corners and edges experience the highest suction, often 2-3 times stronger than the main field of the roof. This is why hurricane damage often starts at corners - the suction literally lifts roofing materials and tears connections apart.
How do pressure signs work in wind load calculations? +
Positive signs indicate pressure toward the surface (inward for walls). Negative signs indicate suction away from the surface (outward for walls, upward for roofs). A wall with positive external pressure and negative internal pressure experiences net inward force.

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Our calculator provides both positive and negative pressures for all building surfaces and zones. Accurate wind loads for Miami-Dade projects.

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