How to Measure for a Curb Adapter

by | Jan 15, 2026 | Curb Adaptors, Featured, Roof Curbs

When replacing a rooftop unit (RTU), accurate measurements are essential for ordering the correct curb adapter. But what happens when the existing unit’s nameplate is worn, unreadable, or missing entirely? And how do you measure the curb when the RTU is still sitting on top of it?

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:

  1. Identifying access points beneath the RTU skirt
  2. Taking outside curb measurements from exposed areas
  3. Measuring supply and return openings from inside the unit (when accessible)
  4. Checking duct locations and alignment
  5. 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