I’m re-roofing an old farmhouse, and the roof is sagging in the middle. It’s about 40 feet long and the roof is an eight pitch and the rafters are 16‘6“ from wall to peak in our space up to 3 feet apart and our 4 x 4‘s.
As far as I can tell from the rafter charts a 2 x 12 is recommended for 16 feet, but I can’t find any tables for 3 foot spacing
So my question is if I would sister a 2 x 8 on both sides of each beam, with that be strong enough?
Replies
Is the 16'-6" a horizontal distance or a slope distance along the rafter?
Why not add a rafter in between each existing rafter instead of sistering 2 to each side? This would put you around 18" o.c. and you could then use standard span tables for the rafter sizing.
Many times roof sag has as much to do with lack of ridge support rather than overspanned rafters (although sometimes it is both). I would take a close look whether there is a ridge beam, ridge board, or neither, and at what is being used as a rafter tie for these rafters. Are there ceiling joists parallel to the rafters acting as a tie? If not, you may need to add some rafter ties in the bottom third of the rafter span while you are doing this work.
16‘6“ is the span from the ridge to the wall along the beams they are 4 x 4‘s spaced 2 to 3 feet apart and they have a tie about head height from the front to the back rafter.
I hear your idea, but I don’t want nails to pull loose so that’s why I wanna sister along the existing ones since I have to move the roof several inches to get it back into place.
There is no ridge beam, they are notched to fit with each other at the top and held together with a wooden dowel.
The sag is definitely due to undersized and overspaced
I have seen several roofs framed like you describe.
I would still lean towards adding a midspan member along with a sistered member. Depending on your snow load 2x8 rafters may still work (1 sistered and 1 between existing rafters). The advantage to adding one in between is that the roof decking is not having to span as far. If you ever decide to replace decking with plywood you would thank yourself. If you are worried about pulling nails, put in the sistered member first, and then follow with the others.
See link below for span tables for Southern Pine rafters. Remember that "span" for rafters is generally the horizontal distance (horizontal projection) of the rafter. So for a 16'-6" slope distance @ 8:12 you should be looking at basically a 13'-9" "span" in the tables. If you are in an area that uses DF, you can find similar information in the codes.
https://www.southernpine.com/span-tables/joists-rafters/
Thanks, the 2x8 sister and one in between is the best option so far.
Measure eave to eave at each and and then halfway in between.
If all 3 dimensions are close, then your sag is most likely due to undersized rafters that have "creeped" over time.
If you pull a string over the ridge from one end to the other and find the the ridge sags in the middle AND if the eave-to-eave dimensions vary, then you've got the typical swayback condition resulting from inadequate tie from eave-to-eave. Common for many older barns, sheds & farmhouses.
That (3 foot) spacing between rafters only made sense with roof decking that was stronger and thicker than you likely want to replace. How thick are the roofing deck boards?
My free advice would be to remove the whole thing and replace with structure per tables and code. Trusses would be good if you can get the top of the walls level.
Trusses would be ideal, I could get them a whole additional floor out of it, but I don’t think they’re looking to spend that much money!
Try looking at this code Section, for roof framing requirements, it should assist you:
https://up.codes/viewer/pennsylvania/irc-2018/chapter/8/roof-ceiling-construction#R802.4