Retrofitting rooftop HVAC units is a common part of commercial building upgrades. Whether you're replacing an aging RTU or upgrading to a more efficient model, one challenge often gets overlooked until it causes delays: the rooftop curb.

Enter the adapter curb—a smart, efficient solution that bridges the gap between your old unit and your new one, without needing major roof modifications. In this blog, we’ll walk you through when and why to use an adapter curb, how it works, and how it can streamline your retrofit projects.

What Is an Adapter Curb?

An adapter curb is a specially designed rooftop frame that allows a new HVAC unit to be installed on an existing roof opening or curb. It “adapts” the footprint and duct connections of your new equipment to fit the layout of the previous unit.

Instead of tearing out the old curb, patching the roof, and custom-building new ductwork from scratch, an adapter curb provides a fast, efficient, and cost-effective alternative.

When to Use an Adapter Curb

You’ll want to use an adapter curb when the new unit’s dimensions, airflow configuration, or duct placements don’t match the existing setup on the roof. Common retrofit scenarios that call for an adapter curb include:

1. Mismatched Unit Footprints

This is the most common reason for needing an adapter curb. RTU sizes and layouts vary between brands—and even between models from the same manufacturer. If your new unit has a different footprint than the old one, it likely won’t align with the existing ductwork, curbing, or roof opening.

Rather than cutting a new hole in the roof (and inviting leaks or structural issues), an adapter curb can align the new unit precisely over the existing opening, while routing the airflow correctly to your existing duct system.

2. Changing HVAC Brands or Models

Switching brands? Even switching between product generations? Chances are, your duct layout and curb dimensions won’t line up.

Adapter curbs are custom-built to match your old unit’s curb dimensions and the new unit’s specs. This makes it easy to swap brands while maintaining proper airflow and performance—without compromising the roof.

3. Avoiding Roof Penetration and Costly Modifications

Every time you cut into a commercial roof, you're not just adding cost—you’re adding risk. Improper sealing, poor flashing, or structural damage can lead to leaks, energy loss, and expensive repairs down the road.

By using an adapter curb, you can reuse the existing roof curb and penetrations, significantly reducing labor time and minimizing disruption to the building or business operations.

4. Keeping Retrofit Projects on Schedule

Time is money—especially when it comes to commercial HVAC replacements. Adapter curbs allow for quicker, cleaner installations with fewer surprises on-site. No delays while fabricating new ductwork, no structural changes to account for, and fewer headaches for everyone involved.

This is especially valuable during peak heating or cooling seasons, where every hour counts in restoring climate control.

What to Know Before Ordering an Adapter Curb

To ensure a successful fit and performance, you’ll need to gather a few key details before placing an order:

• Make and model of both the existing and new rooftop units

• Duct configuration (side or bottom discharge, supply/return locations)

• Curb dimensions (existing curb width, length, and height)

• Roof pitch (if applicable)

• Any special requirements (seismic rating, extra insulation, filter racks, etc.)

At Complete Curb Products, we help you through the entire process to make sure your adapter curb is built to spec—no guesswork required.

Why Choose a Custom Adapter Curb from Complete Curb Products?

Not all adapter curbs are created equal. At Complete Curb Products, we’ve spent over 25 years designing custom solutions that:

• Match OEM specs for top brands and models

• Ensure airtight fit and proper airflow

• Simplify installation and reduce field modifications

• Ship nationwide with fast turnaround times

• Support both standard and complex retrofit needs

We understand that every retrofit project is different, and we take pride in getting it right the first time—because your rooftop unit deserves a secure, high-performing foundation.

Final Thoughts: The Retrofit Secret Weapon

Replacing a rooftop unit doesn’t have to mean redesigning your whole rooftop. With the right adapter curb, you can save time, reduce labor costs, and avoid unnecessary roofing work—without sacrificing performance.

If you're planning a retrofit, don’t wait until the install day to realize your new unit doesn’t fit. Let us help you plan ahead with a custom adapter curb solution that’s built for your exact specs.

Ready to get started?
Use our Curb Finder Tool or contact our team today to make your next retrofit job faster, cleaner, and smarter—from the curb up.