Horizontal diaphragm sheathing shear values 1

I'd agree with your assessment and be telling the homeowner that unfortunately that's not possible without substantial structural intervention. Not to mention that's a substantial looking mechanical duct going down the middle as well.

I don't think the ceiling provides a diaphragm. Not only are there not any chords to the ceiling "diaphragm", but the diaphragm material is neither strong nor ductile. What the ceiling framing provides is the bottom chord of a truss. By removing the ceiling, you destroy the truss.

You need to install a ridge beam just below the existing ridge board and ensure the existing ridge board bears on the new ridge beam and is positively connected to it (e.g., Simpson H2As, twist straps, etc.). You also need to ensure the ridge beam is stiff enough. Now that the roof rafters are merely joists spanning between the top plates of the exterior walls to the ridge beam, they also go into compression. If the ridge beam is not very stiff, the rafters will push out the exterior walls putting the top chords in tension. If you visualize a very soft ridge beam, you will see what I mean. This kind of stuff is more challenging (and entertaining) than most engineers realize.

Have fun!

Alos, need to reinforce the gable end with balloon framing as it is now unbraced for out-of-plane loads.

SE2608 said:

Now that the roof rafters are merely joists spanning between the top plates of the exterior walls to the ridge beam, they also go into compression. If the ridge beam is not very stiff, the rafters will push out the exterior walls putting the top chords in tension. If you visualize a very soft ridge beam, you will see what I mean.

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I don't know anyone who considers this as long as the ridge meets code deflection

I also don't think many people consider the gable end when doing these retrofits. Most times it doesn't matter because between the sheathing overlap and double top plate, unless the ridge is really high, problems aren't common.

jerseyshore said:

unless the ridge is really high, problems aren't common.

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Sheathing overlap? That is usually the spot where it doesn't overlap! How much can that be doing anyway? Maybe for wind suction, it would help.

Depends on the width of the room and/or if there is a door is the wall. Slam the door and the wall can shake considerably.

Interior or exterior sheathing I mean. Not saying it does a lot, but the combination of all the things nailed on those end walls keeps them upright IRL, even without any overlaps. Like I said, tall walls are certainly more susceptible to problems, but there are probably a million houses like OP's that people just cut the CJ's out with no thought to the gable ends.

Ha - XR and I have had it out over this topic before. I'm on the coast a bit south you, jerseyshore, so those problems are a bit more common here. Granted, they're mostly in houses within a half mile or so of the beach, but I've seen complete failures where the the interior drywall ruptured and the wall was leaning in a few inches midway between perpendicular walls after a good storm with a powerful sea "breeze".

In most cases (especially away from the beach and high wind events - probably away from earthquakes, too) the amalgamation of members and fasteners is enough to hold it together, but as XR noted as the wall gets taller/longer you can get some weird vibrations and shaking if that fastening isn't right.

I have honestly have only seen one failure and that was after hurricane Fran. Wall pushed in 7". Have had numerous complaints about shaking, however.
Most contractors don't realize the code now requires them to balloon frame in these situations - not just in high wind areas.
Around here, the plan reviewers are now getting really picky about the exterior wall framing that is adjacent to stairwells. Similar issue. We usually end up framing these with 2x6's and letting the band joist capped with the 2x6's span horizontally.

I just got back from a house they want to vault the ceiling on. I told them to just run new 2x4 studs sistered to the existing on the gable ends. That's usually easier for a house contractor than making a detailed connection.

And funny you mentioned the stairwells, I went to a house 2 weeks ago that had their stair wall bulging inward from a lack of lateral support as the stairway went down. This was in the middle of the house as well, not even at the exterior.

jerseyshore said:

I told them to just run new 2x4 studs sistered to the existing on the gable ends.

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That is my go to or 2x6 if needed due to height or large openings