Why Seismic Roof Curb Specifications Matter
It’s the responsibility of the project’s design team to ensure every component meets the safety, function, and budget requirements of a building. Rooftop curbs—often overlooked—are part of that responsibility.
Your curb’s seismic requirements may appear:
- In the mechanical schedule
- In written specifications
- In accompanying structural documents
Submitting an incorrect curb can result in costly rework, delays, or rejection during inspection.
Step 1: Gather the Required Design Information
Before determining the correct curb, you’ll need several critical pieces of information. These come from the building’s design documents and the governing building codes.
1. The Applicable Design Code
Seismic design must adhere to standards such as:
- ASCE/SEI 7 (2005 or 2010 + 2013 Errata)
- IBC 2006 / 2009 / 2012
- NEHRP 2003 / 2009
- ASSHTO 2009
- 2013 ASCE 41
- AASHTO Guide Specifications (2003/2009)
Your project’s specifications dictate which code applies. Selecting the correct curb begins with knowing precisely which standard governs the installation.
2. Site Soil Classification
Soil Class affects seismic behavior. It is designated as:
- Class A – Hard Rock
- Class B – Rock
- Class C – Very Dense Soil / Soft Rock
- Class D – Stiff Soil (Default)
- *Class E – Soft Clay
- *Class F – Special Soil Conditions (not supported in USGS tool)
Consult the structural or geotechnical report to confirm the correct classification.
3. Risk Category (Seismic Design Category)
Determined by building use and occupancy:
- Category I – Low Hazard
- Category II – Standard
- Category III – High Hazard
- Category IV – Essential Facilities
These categories influence seismic loading and required curb design strength.
4. Site Location
This defines seismic acceleration forces for the project.
You can enter:
- Latitude & Longitude
- Physical address
- Map selection
Into the U.S. Geological Survey (USGS) Seismic Design Maps Tool.
This generates the necessary seismic parameters for your project.
Step 2: Understand the Key Design Values
Once you enter the required information, you’ll receive the seismic design result—including the critical “Ss” value.
What Is “Ss”?
“Ss” = Spectral Response Acceleration Parameter at short periods
- Represents the maximum credible ground motion
- Measured with 5% damping
- Defined in ASCE 7-10 Section 11.4.1
The value typically ranges from 0 to 3.73, with higher values found in California.
How to Use “Ss”
Compare your project’s “Ss” value to the value listed on the MicroMetl structurally stamped submittal for the seismic curb.
- If MicroMetl’s curb rating meets or exceeds the required “Ss,” you’re compliant.
- If not—STOP. Contact the manufacturer immediately.
Other Important Factors
Ip – Importance Factor
Found on the seismic curb submittal.
Reflects the life-safety importance of the equipment, per:
- ASCE 7 Section 13.1.3
Attachment Requirements
Your curb is only as strong as its installation.
Each seismic curb submittal includes:
- Anchor specifications
- Fastener pattern
- Structural attachment details
Failure to follow the attachment detail can result in structural failure—even if the curb itself is properly rated.
Bonus: Wind Load Calculations Included
MicroMetl seismic-rated curbs also include wind design criteria, making them compliant for hurricane-prone regions.
Example:
- Houston, TX requires 145 MPH / 3-second gust
- MicroMetl’s structural curb is rated up to:
155 MPH / 3-second gust
(Risk Category IV / Exposure C / building up to 60 ft high)
For more wind design information, the Applied Technology Council (ATC) offers a wind speed lookup tool.
When Do You Need Hurricane-Level Curbs?
For coastal locations such as La Marque, TX, wind calculations may exceed:
- 157 MPH
In that case, a specially designed hurricane-rated curb is required.
Again—proper anchoring is essential.
Final Thoughts
Choosing a seismic roof curb is not just an equipment decision—it’s a safety-critical engineering requirement. To ensure compliance:
- Verify the correct design code
- Confirm site soil classification
- Identify the Risk Category
- Use the USGS tool for location-based seismic values
- Compare your “Ss” requirement to your curb’s stamped rating
- Follow all installation and anchoring specifications
With accurate information and proper installation, you can be confident your curb will withstand the designed seismic or wind event.
