My garage ceiling joists are not sitting on top of the walls, they are sitting on a rail that has been relieved into the studs about 18” below the top plate. The living space of the house has normal construction, but the garage has this design, I’m assuming for added attic height, at the expense of an unnecessarily tall garage (since the garage is on the ground, and the living area is on a crawl space).

Tension or compression, keeping the tops of the walls in place is the job at hand. Putting a 2x10 on one side of the stud and taking the 2x6 on the other side out 18 times seems like it would a more practical solution than trying to sister 18 new 2x10s to 18 existing (and now redundant) 2x6s. Sistering a few bad 2x6s and reworking the door opening to maintain the existing build? Sure. But if I don’t have a reason to maintain the existing build (no tile bathroom above these joists), why bother with the time and expense of trying to? 2x6s are just not well suited to spanning a two car garage. I can’t claim to guess why they were used back when the house was built, but I can replace them with more suitable lumber.

I’m going to buy the lumber at the local lumber yard that sells the lumber I need. That will not be Home Depot or Lowe’s.

I’m going to transport them on my 24’ car hauler behind my cargo van.

I’m not going to attach them to the rafters at all, since there is no place they will be in contact with the rafters.

As with most 2 car garages, there is no support anywhere except the ends. That’s why I’m replacing the 6 inch lumber with 10 inch lumber.