Attic remodel / sistering rafters – GreenBuildingAdvisor

After early framing help from a capenter, I’m taking on an attic renovation. The carpenter installed knee walls at 35 inches. Because the existing 2×6″ collar ties were only halfway up the 2×6″ rafters, we removed them and installed new 2×4″ collar ties at 10 ft from the floor and 2 ft from the peak. 

The current rafters are pretty bowed, so the carpenter suggested that I sister 2×8″ x 10′ planks to them, running them from the knee walls up past the collar ties on the opposite side from where we placed the collar ties. Doing this will give us a flat surface for the drywall. It will also add needed depth to the rafter cavities when we add closed cell spray foam insulation. 

I’d be grateful for any reactions and for advice on the following:

1. Does this sound like a good approach? I just lugged the 2×8″ planks up three floors so fingers crossed.

2. I plan to run string lines at the bottom, middle and top of the existing rafters to get a level surface for the new rafters. Can I start the string lines at the first and last rafters, or do I need to find a place along the end walls to secure the lines? In other words, where is the best place to run my string lines?

3. What size (and brand) of fasteners should I use to sister the new rafters to the old? 

4. I will likely be doing all of this on my own and assume I will need to use clamps to hold the new rafters in place while I attach them. Any suggestions on the approach here? I’ve noticed that most of the hand clamps I see are not deep enough because the new 2x8s will extend at least 2 or 3 inches past the old rafters. 

Thanks in advance for any advice you all can provide here! John