True they were old and some were a little worse for wear, rotted at the ends or showing signs of termites, but most were quite usable.
This is an awesome diagram with much of what you need to know about roofs in general. Terms you'll need for our shantyboat roof are common rafter, ridgeboard, birdsmouth, eave, gable end and gable end stud, ladder, collar tie, and rafter tie (also called a ceiling joist).
I sorted the good rafters from the marginal. The usable but marginal ones I treated with CopperGreen Clear and cut off the bad parts.
Since our shantyboat is smaller than the original chicken coop, I cut the rafters down to size, decreasing the overall length as well as the length of the eaves. I carefully cut the angle where they met in the center and re-cut the angle of the birdsmouth.
I laid them out on the floor of the barn, and used a temporary plywood collar tie (leaving a slot at the top for the ridgeboard) to keep everything from going wonky while I struggled to secure them into place.
With some ridiculously awkward effort, I got the two end rafters up supporting the ridgeboard.
Then one by one, I installed each of the rafter pairs.
Finally, I added a permanent collar tie to each of the inside rafter. I suppose soon I will have to add a ladder to support the gable overhang and a fly rafter.
At the end of the day, for the first time, I was able to see the shape and size and height of this crazy boat.