" We have heard two different approaches from two different structural engineers."

There's your problem. Contact a GC that's done this previously locally.

Gotta start over rebuilding the whole house. It's cheaper to move.

I agree this is a costly endeavor for very little ROI . It adds no sq. footage and requires an actual rebuild of the house honestly .I have a house with 8’ ceilings and I have never missed my 9’ ones in my old house

I would trust a structural engineer if he told me that it was ok. He's the one that's going to be putting his reputation and liability behind it.

Just thinking through it off the top of my head, I wouldn't use a 4x12, I would build 12" frame ladders for a number of reasons.

As I see it, the biggest issue is the hinge point between the old and new pieces of wall. If you cut the old exterior ply down a foot and overlap the new plywood down that same 12", that would help towards creating a more rigid connection.

BLDG Workshop:

There is no problem with trusting a structural engineer's math when it comes to actual building, the problem is they usually don't do any cost/benefit analysis as Sophie did in two sentences above.

I agree on most of the other comments - its a bunch of money that will not add any real value to the house. Rafters put outward pressure on the tops of exterior walls, and the ceiling joists resist that. To get that 4x12 in there would mean cutting the connections where the joists & rafters meet the existing top plate. You'd then have to jack up the roof/attic floor as an assembly to make space for the 4x12. That's a big job on a house. I actually did this on a 20'x20' garage, and even with bracing everything got so wracked and out of square that it took longer to square it back up than to actually raise the roof.

If you think of wall studs as vertical joists, you'll realize that removing the old top plates, and then sistering in 9' studs would be a better choice for the walls. But you might as well start over before doing that. If you are in an earthquake zone, adding the 4x12 will create a weak link in the structure. Maybe Simpson makes some straps for that.

Not. Worth. It.

Thank you for all the replies. We are expanding the back & side of the house so we have to replace approximately 80% of the roof due to the additions anyway. ROI is always in the back of our minds, although this will be our house for another 20 years. Without getting too much into this our main question is whether there are any limitations stopping us from just building up the plate height without tearing down exterior walls.

I would trust the engineer that said it could be done, because it can be done.

The biggest issue (as identified by both Great Oak and myself above) is the lateral instability that is created at the point where the existing wall connects to the new piece of wall you're adding.

The plywood overlap detail I suggested could be a possibility, as could some type of Simpson strap. That would be the biggest point of concern that I would want the structural engineer to satisfy.

Best of luck!

Also consider insulation values as the solid 4x will prove to be much less energy efficient.

^ Right, which is the main reason why frame ladders is likely a better option. Another reason is making it easier to pass services through.