What i mean is, at some point, 1' from the exterior wall plate or 5', i assume your repair cant support the roof. Too much stress. And you have to replace the whole rafter.
I have several 2x6 roof raters and several 2x4 ceiling joists that have rotted at different lengths from the exteroier wall plate. Some are after the wall plate, in the soffit. Others are a few inches from the wall plate, inside, meaning they are not supported by the wall plate. Other are about 1' from the wall plate. All are 12' long. Can they he repaired? Or should they be replaced?
The roof deck is off to repair the leak, i am planning on fixing the roof this year and the exterior wall next year. I have not opened up the wall yet, but a few feet of the 40' wall is rotted. Not sure how bad. The top plate is rotted and a screw driver went through the wall from exterior to interior in a few spots, so i might have to replace that section, from footing to roof.
So, repair or replace the rafters and joists?

 

You really have to look at your situation to determine if the entire member should be replaced. Often if it's just one end that's rotten you can sister on to the side of the existing one. It would have to be pretty bad to make me replace the entire rafter but if you have the roof sheeting off replacing the rafter might be pretty easy. The ceiling joists though I'd try to keep as removing them would probably mess-up the sheetrock of the rooms below.

 

First, the drywall inside is gone due to the leak. rafters and joists are fully exposed from the interior and exterior. so, access to them or damaging something to get to them, is not a concern.
my only question is, do i have to replace the whole rafter or joist, or can i sister a new section.
i assumed there was a point where you had to replace the rafter.
just to make sure i understand what you are saying, 12' long rafter. 2x6. last 2' rotted. the end that sits on top of the exterior wall and extends for the soffit. are you saying i can cut off , say, 2' of the rafter, rafter is now 8' long, sister a 4' long 2x6, and thats it? Nothing else?
I just cant belive it is that easy.
i was assuming there was a limit to how long a sister could be. i assumed the joint for the sister would be weaker the closer to the middle of the rafter that you got. i assumed a sister could not be longer then a foot or two.
dont want to sound like i am arguing with you, just want to make sure i understand before i go the easy way and sister.

 

oh wait, i just realized.
since the roof deck and the interior sheet rock are gone and the rafter is totally exposed, replacing the rafter would be easier then sistering, wouldnt it?
at least on the rafters that are missing 2' and I have to cut another 2' to get to solid wood.
the rafters that are only missing the soffit part, past the exterior, wall I can sister.

 

Yea, if you've got everything else removed and the rafters or joists are exposed then replacing them is usually easier. I'd go that route. Sistering is generally done when you're trying to save the member because it's attached to other stuff and replacing it would open a can of worms. In your case it sounds easier just to replace the whole thing and have a clean & proper repair.