See how hurricane winds attack rooftop HVAC equipment. This simulation shows the uplift, horizontal forces, and overturning moments that your curb anchors must resist.
How wind forces travel from equipment to foundation.
1
Wind Hits Unit
Horizontal pressure on equipment surfaces creates drag force.
2
Roof Suction
Negative pressure under curb tries to lift the entire assembly.
3
Curb Resists
Curb transfers loads to anchors through bending and shear.
4
Anchors Hold
Anchors resist uplift and shear, transferring to roof structure.
Anchor Design Requirements
What it takes to keep your RTU attached during a hurricane.
Expansion Anchors
Wedge-type expansion anchors for solid concrete decks. Minimum 4" embedment with 3" edge distance. Verify concrete strength meets anchor requirements.
Adhesive Anchors
Epoxy or hybrid adhesive anchors for cracked concrete or close spacing. Requires hole cleaning and proper cure time before loading.
Welded Embeds
Pre-installed embed plates with welded anchor rods. Best for new construction. Requires coordination during concrete pour.
Mechanical Curb FAQs
Common questions about RTU wind load design.
What wind loads do rooftop mechanical curbs experience?
Mechanical curbs experience both horizontal wind forces on the equipment and vertical uplift forces from roof suction beneath the curb. In Miami-Dade HVHZ at 180 mph, a typical 4x8 foot RTU curb might see 3,000-5,000 lbs of uplift and 1,500-2,500 lbs of horizontal force, creating significant overturning moments.
How do you anchor a mechanical curb to resist wind loads?
Mechanical curbs are typically anchored with expansion anchors, adhesive anchors, or welded embed plates. The anchors must resist both uplift and horizontal forces. In Miami-Dade, anchor spacing is typically 12-24 inches depending on loads. Each anchor must be sized for the tributary load including safety factors.
Does a rooftop unit need wind load calculations for permits?
Yes. Miami-Dade County requires wind load calculations for rooftop mechanical equipment installations. The calculations must show the design wind pressure, uplift force, horizontal force, overturning moment, and proof that the curb anchorage can resist these forces. This applies to new installations and replacements.
What is the effective wind area for a mechanical curb?
The effective wind area for a mechanical curb is the projected area of the equipment for horizontal loads and the curb footprint for vertical uplift. For a 4x8 foot RTU that is 4 feet tall, the horizontal effective area is 32 sq ft (8x4) and the uplift area is 32 sq ft (4x8). Corner and edge zone factors apply based on location.
Why do mechanical units blow off roofs during hurricanes?
RTUs blow off during hurricanes due to: undersized anchors, corroded fasteners, curbs not properly sealed to the roof, equipment heavier than curb design load, and improper roof zone consideration. Once the curb lifts, the roof membrane tears and water intrusion causes major damage.
Calculate Your RTU Wind Loads
Get the exact uplift, horizontal force, and anchor requirements for your rooftop equipment installation in Miami-Dade.
WindLoad.co