Each commercial rooftop unit (RTU) needs to be mounted to a curb. A curb supports the RTU and provides openings for the supply and return ductwork (which can be massive compared to what we see in residential and light commercial HVAC). Occasionally, we’ll need to do an RTU changeout and put in a bigger or smaller unit than what was there before. If the size is different, we can either install a new flat curb on the roof (commonly called a knockdown or OEM curb) or put a new curb adapter on top of the existing curb. If we’re going to install a new adapter, we’ll need to know the size of the original curb and where the ducts run in relation to it.

Getting these measurements can be tricky, especially when there’s an existing adapter of a different size, so it pays to know what the original curb size is, where the duct openings are, and how big those openings are in case we ever need to add or replace an adapter. 

Find and Measure the Original Curb

If you already have an adapter, you’ll want to look for the original curb. Curbs are structurally connected to the building envelope. Adapters are usually made of metal on top of an existing curb (not directly connected to the structure). Below, you can see that the metal adapter extends further than the actual curb, which is the raised white part that’s still part of the building structure. 

You’ll want to measure the exterior of the original curb. That will usually be pretty straightforward; measure the length and width with your tape measure. (Try to get these measurements as high as possible on the existing roof curb. Sometimes the roofing material that encases the original curb extends past the curb closer to the roof and can give you a false measurement.)  Make sure you write those numbers down somewhere, as we’ll want to use them to draw and label a diagram later. In the case we’ll be walking through in this tech tip, our curb exterior is 70” x 77 ¼” (or 77.25”).

Return and Supply Air Measurements 

Your return and supply ducts are fed through and will come out of the top side of the curb. We’ll need to measure those openings, too. Accessibility will vary by manufacturer, but we had to take off the entire panel to see the ductwork for this Trane RTU.

We want to measure the length and width of the interior duct openings and write those numbers down. When you look inside at the ductwork to do that, make sure you’re measuring from the duct’s interior wall, not the adapter. You’ll also need to watch out for lips or other extended pieces that might appear to add or subtract from the total measurement. In the picture below, we have a lip that adds an inch to our return duct. The return duct opening’s total measurements are 23” x 68”.

The supply duct will most likely be different, so you’ll want to repeat those steps, staying mindful of where the duct interior begins (and these may extend into hard-to-see areas), not the curb adapter. Below, you can see that there’s about 12” of difference between the curb adapter and the supply duct wall we’re trying to measure from. 

Our supply duct opening is a bit longer and thinner than our return, coming out to 20” x 74” in total.

Measuring the Distance from Duct to Curb

Now that we know what our duct dimensions are, we need to determine their distance from the original curb, NOT the adapter. You’ll need a marker on hand to draw some references for this part.

First, you’ll want to mark where the curb ends onto the adapter with a permanent marker. Below, you can see how the bottom curb aligns with the black mark on the adapter. We’ll use this mark as a reference point.

To measure the distance between the long side of the ductwork and the curb, take your measuring tape or another long, thin object and see where that line would extend into the RTU interior. Then, we’ll measure the distance between that reference point and the duct wall. In our case, that’s 2” between the curb’s edge and the return duct’s edge. 

After that, we’ll want to measure the distance between the duct’s short side and the curb. The ductwork usually extends a few inches behind the adapter, so you’ll want to take your measuring tape and mark out the duct wall’s location on the adapter.

Then you measure the distance between that line and the curb’s edge to get that distance. That distance was 6” for the return duct.

Repeat for the supply side. We got 1” between the short side and the curb’s edge and 14” between the long side and the curb’s edge. 

Drawing a Diagram

Let’s go over all the measurements we collected:

• Curb exterior: 70” x 77 ¼”
• Return duct opening: 23” x 68”
• Supply duct opening: 20” x 74”
• Distance between R/A duct long side and curb: 2”
• Distance between R/A duct short side and curb: 6”
• Distance between S/A duct long side and curb: 1”
• Distance between S/A duct short side and curb: 14”

When we draw everything out, it looks like this:

It’s clearly not to scale, but the important thing is that we’ve documented the measurements of the curb, supply duct, return duct, and the distance between the duct and curb edges. Now we know the size of the original curb in case we need a new adapter during the next changeout. 

It also helps to draw a basic compass rose beside the drawing. You have apps that can tell you which way you’re facing, but also remember that the sun rises in the east and sets in the west. 

The estimation can be tricky sometimes when you have to guess where the ductwork ends and mark it on the adapter. You might end up being off by an inch or two, but that’s usually not the end of the world in a changeout. In any case, measuring curbs, ducts, and the distances between their edges is a good skill to know for RTU changeouts.