An addition was added on to my home 25 years ago. I discovered the sill plate and band joist are rotted. Possibly the floor joist have rotted also. The expanse is 30 feet for the exterior load bearing wall. The wall would need to be supported from the exterior as there is no access from inside.
1. I plan to add braces to lift and support the wall off the foundation. How far apart should I position the supports?
2. I then plan to cut out and replace the sill plate. I'm not sure how the sill plate was secured to the cinder block foundation but will find out when I remove the sill plate. How are sill plates secured to cinder blocks?
3. I will remove the band joist in sections and scab 2x10 pieces to any rotted floor joists before closing the space with the new band joist.
4. Once all of the rotted wood has been replaced, I'll lower the wall back down onto the sill plate by removing the braces. How do I secure the wall to the sill plate?

Never having tackled a project of this size, I would appreciate any recommendations or comments. Thanks.

How do you plan to attach the braces and where? What will you lift the house with? Normally a job like this calls for building a bridge beam across the floor joists and then jacking up the house with screw or hydraulic jacks. For 30 ft. you would need 5-6 jacks and a large bridge beam. Are you lagging to the wall? Don't think you should try to lift off the foundation on the walls. Sill plates are bolted with J bolts embedded in concrete filled cavities of the block or held with Simpson ties that are bent over the sill plate. From your description of what you will be doing, you wouldn't attach the wall to the sill plate, the wall sits on the floor sheathing which is on top of the floor joists and band joist. Normally the wall plate is through nailed to the band joist but in your case, it would have to be toenailed from the outside. This is not an easy task, even for experienced carpenters. The whole house is going to be sitting on your braces and that's a lot of dead weight. If you try to lift small sections, you will rack the house frame. Replacing 30" might not be too bad but 30' is a VERY big task, particularly with no underneath access. Please give this a lot of thought before attempting this repair yourself!