I have a 1910 home in which the roof is grossly underframed, and I would like to strengthen it so I can install solar panels. The original 2x4 rafters at 32" o.c. span 12' 3" in plan from plate to ridge and support the original 1x skip sheathing, 1/2" plywood, tar paper, and one layer of architectural asphalt shingles. I checked a couple of the rafters with a string line, and they are deflected about 1 3/8" at midspan.
My plan is to strengthen the roof framing by adding rafters to modern framing standards. AWC's on-line span calculator indicates that DF-North #1 2x6s rafters at 16" o.c. will handle the span with 10 psf dead load and 20 psf live load. Since I don't see any way to attach the new rafters to the sheathing without disturbing the asphalt shingles, I am planning to install doubled 2x6 rafters alongside each of the original 2x4 rafters and sistering them together.
Is there a better way to go about strengthening the roof framing?
To install the new rafters, I will have to remove the deflection of the existing rafters so the new ones will fit and properly carry the roof load. I can jack against the ceiling joists over a bearing wall below. I am tempted to install the new rafters with the crown down to reduce the jacking required. The roof is hipped, so the ridge is only 16' long and there are only 7 common rafters on each side of the roof. I'm going to leave the rest of the rafters as is.
What are my chances of getting the existing rafters to move 1 1/2" or so without damaging the plywood sheathing or ashphalt shingles?
Can I get away with jacking the rafters one at a time, or should I arrange to jack all seven rafters on a side at once?
In case it is important, the rafters don't bear directly on the exterior wall plate; instead, there is a flat 1x plate installed on top of the ceiling joists (2x6s at 16" o.c.), and the rafters bear on that. I'm planning to install the new rafters on the same 1x plate, add squash blocks beneath them, and figure out a strap detail to tie the new rafters to the wall plate.