I've got to sister five 17' long ceiling joists, due to removing a bearing wall. I've done the engineering, and given the existing roof construction, I've got two options that work:
1) Use double 2x6 select struct at 16" o.c.
2) Use 2x8 select struct at 16" o.c. with a 3.5" long by 1.75" deep top edge end notch.
The selection is deflection controlled, as 2x6 select struct at 16" o.c. meets bending and shear.
Which option is better? The material cost for the 2x6s is about $50 more. I'm mostly working alone, so the 2x6s will be somewhat easier to wrangle into place. Otherwise, it is notching the 2x8s versus the extra nailing to double the 2x6s.