Why Seismic Roof Curbs Exist
Seismic requirements stem directly from building codes and structural engineering standards. Design engineers determine the specific seismic criteria for each building, guided by:
- International Building Code (IBC)
- California Building Code (CBC)
- American Society of Civil Engineers (ASCE) Standards
Equipment mounted on a roof—especially heavier HVAC units—is vulnerable to shifting, tipping, or detaching during strong motion events. A seismic-rated curb solves this by providing a reinforced, engineered foundation designed to resist these forces.
What Qualifies as “Seismic Certification”?
A true seismic-rated curb is more than just heavy-duty metal:
It must meet two core requirements after a Design Earthquake (DE):
- Maintain Structural Integrity
- Remain Functional—meaning the HVAC unit still operates as intended
To achieve this, manufacturers provide:
- A professionally engineered (PE) stamped submittal
- Documentation verifying compliance with applicable ASCE, IBC, and CBC standards
- Required anchoring and attachment instructions
Important:
Ordering a curb with a PE stamp is not enough on its own. Installation must follow the stamped details exactly—attachment points, fastener types, spacing, and anchoring all matter. Deviating from the engineered plan invalidates the certification.
Where Are Seismic Curbs Required?
While California is the most seismic-active region and has the strictest regulations, it’s not the only area that uses seismic-rated curbs. Other regions require or recommend them due to:
- Proximity to fault lines
- Soil characteristics
- Local code adoption of ASCE standards
- Government or military building classifications
- Essential facilities (hospitals, emergency operations centers, etc.)
Even in areas where earthquakes are less common, older buildings or structures not designed for seismic loads may need additional protection.
Types of Seismic Roof Curbs
There are two main categories:
Reinforced Seismic Curbs
Heavy-gauge steel, reinforced internal bracing, and engineered welds.
Vibration Isolation Curbs
Designed to both absorb vibration and meet seismic calculations—but not all isolation curbs are seismic-certified.
Only curbs accompanied by a PE-stamped submittal count as certified.
Additional Benefits: Wind Resistance
Seismic-rated curbs do more than protect against earthquakes. MicroMetl’s structurally certified curbs also meet:
- Wind requirements for up to 155 MPH, 3-second gusts
- Exposure C conditions as defined by IBC standards
This makes them suitable for inland hurricane-prone regions where high winds are a concern. For coastal or extreme wind zones (e.g., Miami-Dade), additional specialized ratings may be required.
Understanding Wind Exposure Categories (Simplified)
- Exposure B: Urban/suburban areas with lots of buildings
- Exposure C: Open terrain—fields, grasslands, shorelines
- Exposure D: Directly facing large bodies of water
Most seismic-rated curbs are engineered for Exposure C, which covers many commercial installations.
The Bottom Line
A seismic roof curb ensures:
- Code compliance
- Enhanced safety
- Protection of your investment
- Continued operation of critical HVAC equipment
If your project requires seismic certification—or if you suspect it might—MicroMetl can supply compliant, professionally engineered curbs with documentation for all 50 states.
