Corner Mullion Leg Calculator - "L" Corner Mullion

Regarding defining a precise inside mullion width at corners of curtain walls: Craig Thomas with Leo A. Daly shared this concept with me. It goes something like this: (Craig writes)

I did some experimenting and found I could make the “L Corner Mullion” work to my satisfaction (your expectations may be different) for angled curtain walls. (Craig wants to define the inside edge exactly) This is what I found.

If you know the angle between the two curtain walls (A), the desired width of the mullion along the face (in my case 2 ½”) and ½ the frame depth (y), you can calculate how long to make each leg on the mullion (X”)(see image below – from Type Properties of “L Corner Mullion”). Here’s the formula:

X” = 2 ½” + (y * tan(90-(A/2)))


At first it may look like a lot of work, but with the calculator on your PC you can easily do the math. Then save the mullion based on the angle. In my example, I had a 7” deep mullion at 150°, with 2 ½” exposed in the corner.

I named it “L Corner Mullion – 150 degrees”.

Steve says, "You can enter this formula in the Leg 1 and Leg 2 field directly too! and make sure you use inches!!"


This is the result in the project:



Thanks for sharing Craig!!

I thought I'd "pile on" and add something to this concept. I built an annotation symbol that consists of parameters to enter the data required for the formula and does the calculation for you. Place the symbol next to your corner. Enter values, get the result. Then you can edit the mullion, create a new type and enter the values.
If you'd like to play with this you can download the file here!