This situation is more common than you might think — and it doesn’t require removing the curb or disturbing the roof structure.
Why Measuring the Existing Curb Matters
Removing a curb from the roof is rarely practical. Doing so can:
- Break roof integrity
- Compromise waterproofing
- Add unnecessary labor and cost
Instead, the safest and most efficient solution is to measure the existing curb and order the adapter based on those dimensions.
The Challenge: The RTU Is Still in Place
The biggest obstacle is that the unit itself is covering the curb. This blocks your view of the dimensions you need — specifically the inside and outside measurements that determine which adapter is required.
Fortunately, there is a proven method to gather accurate measurements without removing the RTU.
How to Measure an Existing Curb with the RTU Installed
The process involves:
- Identifying access points beneath the RTU skirt
- Taking outside curb measurements from exposed areas
- Measuring supply and return openings from inside the unit (when accessible)
- Checking duct locations and alignment
- Recording height, pitch, and any accessories attached to the curb
These steps help you capture the exact curb footprint, ensuring your curb adapter will fit the existing curb without roof modifications.
Bookmark Our Guide
We’ve created a detailed instructional video that walks you through the complete measurement process — step-by-step — while the HVAC unit remains in place. This is an essential reference for installers, service techs, and project managers.
If you need assistance verifying your measurements or ordering the correct curb adapter, please contact MicroMetl’s Customer Service Department. Our team can help you identify the correct fit and ensure your replacement project goes smoothly.
What Is an HVAC Curb Adapter?
For anyone new to the HVAC industry, the term curb adapter can be confusing. Here’s a simple, clear explanation of what curb adapters are and why they matter.
When a Building Is New
During construction, before a rooftop unit (RTU) is installed, the roof is cut and a roof curb is put into place.
This curb:
- Supports the weight of the RTU
- Provides openings for supply and return air
- Serves as the structural base for duct connections
- Is sealed and waterproofed to protect the roof
Once installed, the building’s supply and return ducts are brought up to the curb and sealed tightly to prevent air leakage.
The Challenge: Replacing an RTU
After years of service, an RTU will eventually need to be replaced. Here’s where contractors often run into problems:
- The new RTU is usually different in size, shape, or configuration
- Supply and return openings rarely align with the existing ductwork
- Removing the original curb requires cutting into the roof
- Re-roofing after removing a curb is time-consuming, costly, and risks leaks
That’s why most pros avoid tearing out the existing curb unless absolutely necessary.
The Solution: Curb Adapters
A curb adapter is a custom-designed, engineered transition that allows a new rooftop unit to be installed on an existing curb.
The adapter:
- Seats and supports the new RTU
- Aligns the new unit’s supply/return openings with the existing ductwork
- Preserves the integrity of the roof
- Saves installation time and cost
Think of it as a precision “bridge” between the new unit and the old curb.
Why Alignment Matters
Manufacturers design their rooftop units with specific supply and return configurations. Even small differences between brands—such as Carrier vs. Trane—can dramatically impact how the airflow lines up.
For example:
- Unit A may have supply and return together on one side
- Unit B may split them across the cabinet
- Opening sizes, positions, and spacing may differ by inches
A well-designed curb adapter accounts for these differences while minimizing static pressure and ensuring efficient airflow.
The Contractor’s Advantage
Switching to a different manufacturer or upgrading to a more efficient model often changes unit dimensions. Rather than reworking the roof and duct system, you can:
- Install a curb adapter
- Keep downtime to a minimum
- Avoid expensive roof modifications
- Maintain proper airflow and system performance
Choosing the correct adapter is crucial to protecting efficiency, airflow, and structural integrity.
Need Support?
Curb adapters can be complex, and every roof situation is unique. If you have questions about sizing, layout, airflow, or design strategy, MicroMetl’s team is here to help.
Feel free to reach out to our Technical Services or Customer Support team for guidance.
