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I need to enclose the shown duct work in a soffit (shown in blue). The ceiling joists run parallel to the soffit and (shown in red). They are 24” spaced and the spot I need to attach the soffit to, doesn’t have a joist right above to secure to. The studs (shown in yellow), are 16” spaced. There is a small, non-structural wall on the left, with HVAC duct running up it, and at the right end, there is a double LVL beam (shown in green).

Soffit only needs to be finished at its face and a little bit at the bottom with a corner bead. Below the soffit there will be kitchen cabinets.

What is the best way to frame this soffit? Ordinarily, I would build a ladder and attach it to the joists above (if they were perpendicular).

I’m thinking, instead building the same ladder with top 2x4 “standing up”, securing it to the wall and LVL at each end, and then running some 2x4s toward joists for the bottom part. I haven’t come up with a good way to do the same on the right side of that LVL beam with the can lights to its right. enter image description here

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  • Do you have access to the area above the ceiling such that you could add supports across the joist bays? Nov 12 '20 at 1:19
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    @jimmyfix-it I do, hate to remove so much extra drywall though. I could cut the drywall upto where the soffit ends and perhaps I can “tuck in” some 2x4s. Edit: I do but only from this floor. There is a room above with finished floors.
    – David
    Nov 12 '20 at 1:54
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    Were you able to fasten the soffit frame to the ceiling joists?
    – SteveSh
    Dec 13 '20 at 11:57
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    @SteveSh, yes. I updated the photos to show detail. I dis cut back drywall to ensure the soffit frame had a full contact with the braces.
    – David
    Dec 13 '20 at 16:42
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    @TylerH No, there will be a flashing that goes above the cabinet, that will account for any unevenness the soffit will have. Once it is finished, there will be no gap at the top or the sides.
    – David
    Dec 13 '20 at 21:26
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This is how I ended up building the soffit. I decided to write this up as an answer, since there is not a lot of information, including on YouTube, dealing with building kitchen or basement soffits or bulkheads, where the joists are parallel to the planned soffit. Hopefully the readers can benefit.

Step 1. I cut back the existing ceiling drywall to the point where it wouldn’t interfere with the new soffit frame. I made sure that I didn’t cut beyond that, as you want your soffit drywall to butt under, and against the ceiling drywall, and not against the frame above. This way the soffit vertical drywall piece will support ceiling drywall from any possible sagging and will make this joint much less susceptible to cracks.

Step 2. I took the measurements and built the soffit frame on a level floor. First I built the front of the frame using a 2x4 as a top piece and a (ripped from 2x4) 2x2 as a bottom piece. I used the lumber as straight as I could find - a challenge in today’s breakneck lumber production, especially for boards 10’ and up. The ladder "steps" were made with 2x4 pieces at 24” at the center (you could go 16” but unnecessary for narrow soffit). I used long clamps like this, which made it MUCH easier to keep the ladder together while assembling than it otherwise would. I used 3" these decking screws to assemble the whole ladder.

Step 3. Next, I cut some 2x4s at a length that would fit between two joists and "toenail" them with 3” decking screws to the joists in a perpendicular fashion.

Step 4. Lifted the assembled front of the soffit, and attached it with the same decking screws to the 2x4 braces I toenailed to joists earlier, as well as the side walls.

Step 5. I attached the second piece of 2x2 from that ripped 2x4 with 2x4 ladder "steps" to make up the bottom of the soffit.

Step 6. Hang drywall as usual using drywall screws, tape the joints, install a corner bead (a lot of people use vinyl beads on soffits, I just prefer metal ones), mud, and finish.

I left the soffit on the back open (no drywall), as new kitchen cabinets, as well as the trim (still to be installed), would fully cover the opening. Had I had no cabinets, I would additionally screw the bottom of the soffit to the studs on the back wall and run the drywall all the way to the wall. In this instance it was unnecessary, and besides, I had a range hood vent hanging behind it.

Here are some photos of the build. I didn't take detailed photos of the whole process but it does show some detail.

enter image description here

enter image description here

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enter image description here

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I like to use OSB for soffit walls. I'll run a 2x2 rail on the ceiling, hang an OSB strip of the appropriate height, and attach another 2x2. Add a 2x2 at the wall and drywall it all. This way you're not relying on end-grain fastener pullout or shear strength in drywall. Everything is in shear except the screws up into the framing, but they're cross-grain and will not let go.

 ______________________________________________
|                                   [ ]|||
|                                      |||
|                           OSB ------>|||
|                                      |||<-- drywall 
|                                      |||
|[ ]<-- 2x2                         [ ]|||
|=======================================
|          ^--- drywall

There's no need to cut back ceiling drywall for a soffit unless you have to add backing, but even then you just cut access openings. Use screws long enough to penetrate the framing above at least 1-1/4" and it'll hang solid.

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This is a great video for this: https://youtu.be/lAM_196_B7I?t=400

Measure what you need and go to the home depot or whatever and have them cut some OSB. they have those big saws that will cut the pieces nice and straight, and I think the first like 3 or 4 cuts are free.

then just a matter of hanging it.

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    The first few cuts are usually free. And if you ask for more (as long is it isn't a huge amount) they will often just do them for free because they don't want to bother with paperwork. And if they do charge, the price is ridiculously low. The problem is that a lot of the time, the machines are "out of order". Nov 12 '20 at 18:06
  • Link-only answers aren't considered valuable on Stack Exchange. Please provide the relevant information here, so if the link does your answer doesn't go with it.
    – isherwood
    Dec 14 '20 at 15:45

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