How to Build A Soffit Around Ductwork
Building a Soffit:
I build most of my soffits with 2x material and plywood. It’s faster, stronger and more reliable. It’s also less prone to error.
When creating a soffit I try to keep it simple. I use a combination of parallel tape measuring points, string lines and use the 3-4-5 triangle method to mark the location of my soffit. The goal is to create an equal ceiling space and this is critical if you’re using an acoustical ceiling grid.
Tip: DO NOT trust that a duct or pipe is lined up parallel or perpendicular to a wall. The last thing you want is a crooked soffit and a series of differently cut acoustical ceiling tiles running along it. It’s amateurish and I’ll make you sit in the truck for the rest of the day if you get caught doing it!
Once I have located my soffit I determine how I am going to build it.
Build A Simple and Straightforward Soffit:
I use a caulk string line to mark a continuous line along my tape measure measurements.
Then I install the longest, straightest 2×4 I have along the length of the line and fasten it into each floor joist. I keep this 2×4 or “smaller” 2x nailer on the inside [not finished side] of the soffit.
Then I determine the depth of coverage that I need in order to cover the obstruction and then cut a length of 5/8 or ¾ inch plywood to that depth. To create a straight, rigid soffit, try to maximize plywood along it’s length and minimize joists.
Tip: Use a table saw to keep your cuts perfectly straight.
Straight, Level and Square:
You should be concerned about leveling the soffit and this is the time to think about it. Check your nailing along the floor joists with a level. You can install your plywood to 
lower [out of level] section of the joist nailer.
When securing the plywood, push it almost tight to the joist and install only one nail. [Too tight may create squeaks] On the other end of this plywood you can then use a level to move the plywood up and down [pivot on nail] to achieve a level edge.
Tip: Make sure to use a string ling to check the soffit for straightness.
Once level, secure the plywood to the horizontal nailer with a nailer every 12 to 16 inches with 8 penny nails.
At the bottom of the plywood 
edge I screw on another straight 2×4 or larger to the inside edge. This is also another opportunity for you to adjust for level. Move the bottom edge 2×4 up or down a bit as needed. The 2x material at this edge becomes a nailer for a bottom panel or in the case of a skirt wall – for wall board corner bead.
Depending on the soffit I may need to do this same procedure on the other side of a duct and then add a bottom piece of plywood, or I may need to return the bottom of the soffit to an adjacent wall.
