Jacking wood rafters

Hello,
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.
Thanks, Wayne
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Anyway to put a strongback or beam at midspan of existing rafters? Even like an overhead beam ontop of or above ceiling joist supported at either end. Then support rafters on that? It sounds like doubling is a good way to go. I would try to raise rafters in a group if possible. john
"Wayne Whitney" wrote in message
Hello,
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.
Thanks, Wayne
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     Thanks for your response.
The problem with supporting the rafters at midspan is there's no foundation underneath that area. So I'd have to transfer the point loads to doubled or tripled 2x6 ceiling joists or 2x10 floor joists to carry the load to the perimeter foundation and the central girder. And so it seems better just to triple the rafters directly.

I had been thinking of using a screw jack to raise the rafters one at a time. I could get several screw jacks to lift a temporary beam under all 7 rafters.
Or I could try raising each rafter with a 2x4 running diagonally from rafter midspan to the house centerline, cut to the proper length so the rafter will be straight once the 2x4 is in place. Then I can install the 2x4s with a sledgehammer, doing all seven roughly in parallel. Anybody done that before, will it work?
Thanks, Wayne
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Wayne-
rafter midspan to the house centerline, cut to the proper length so the rafter will be straight once the 2x4 is in place. Then I can install the 2x4s with a sledgehammer, doing all seven roughly in parallel. Anybody done that before, will it work? <<<<<<
Sounds like you're approaching a site built truss arrangement.
I have 4ea jack posts & screws that you can borrow.
My house (1930) has 2x4 "rafters" but they're braced at midspan back to the house centerline creating a site built truss. Do oyu have space in the attic to add the members and do the work?
cheers Bob
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Hi Bob, it's been a while. How've you been?
I'm not going to go the truss route for a couple reasons. First the chords would be obstructions for using the attic, although that is not a large concern as I don't use it much. More importantly, the rafters are not in plane with the ceiling joists! The ceiling joists are consistently 16" o.c., but the rafters vary from 30" - 37" o.c. and average around 34" o.c.
So really the issue is just the best way to straighten the existing rafters so I can fit the sisters in place. I'm first going to try pounding some temporary chords into place; if that doesn't work I'll try screw jacks.
Cheers, Wayne
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Update: the temporary chord method worked well, although I ended up needing two per 16' 2x4 rafter to get them straight. With the rafters 32" o.c., the roof deck was flexible enough that I could straighten the rafters one by one.
Cheers, Wayne
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On Saturday, March 17, 2012 4:15:26 PM UTC-5, Wayne Whitney wrote:

Wayne, Having just put a new metal roof on my house (1928), I would keep a close eye out during the next rainstorm to make sure you don't have any leaks generated by moving all that asphalt on the roof around. Between the time I closed in the existing shingles with the R-panel, I had a new leak develop just from us mucking with the existing framing.
Good luck, and it feels really good to fix your own stuff, doesn't it? Phil
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