I need to put a soffit around 18"(W) x 8"(H) x 23'(L) rectangular HVAC ductwork in my basement and I am very near to the minimal code requirements for ceiling height for obstructions before adding a soffit.
If possible, the ideal solution would result in a soffit that is up to code (International Residential Code), structurally sound, and takes up less than 1 inch of additional space below the ducts. The one inch requirement is a finished height that includes 1/2" drywall or another material approved for fireblocking that would result in a nice finish. The soffit framing can extend horizontally as far as needed without creating any issues.
If my request of one inch or less in lost height is not possible, then I am still interested methods that minimize height loss and lead to a code-approved and structurally sound soffit.
Best Answer
I usually use wood for everything. This is how I would put it together with 2x4s. Personally, I think 2x4s are overkill for this, but that's just me. I suggest using screws as your fasteners, and with screws your "bottom plate" is probably sound. Screws will be fine in tension. If you are using nails, I'd stick to the sketch so that your nails are loaded in shear.
The main code issue you are going to be dealing with here is fireblocking. You have this open plenum that connects all the joist cavities which allows fire to spread quickly across the joists. You'll want to stuff insulation in areas that will prevent airflow from one joist (and wall) cavity to another.