The sill at the northwest corner of my garage has rotted in about a 3 ft. long area. I can see the edge of the concrete slab (garage floor) and the ground outside through the opening where the sill is gone. My carpenter wants to jack it up and replace the sill in that spot. Question is, will jacking it up in this one spot compromise the rest of the structure? The sill in the southwest corner (diagonal from the other one) is also in bad shape. The garage at some point has had an additional brace made of wooden beams placed diagonally from corner to corner, shaped in an "x" and where they meet in the middle, braced at the peak of the ceiling with a thinner board. The bracing obstructs my ability to install a garage door opener which is now low priority until the sill/foundation issue is resolved. Any thoughts on the jacking up and replacing sill and taking down the "x" bracing? Additionally, I get water draining from the alley way which is higher than the garage. What is a good drainage system to divert this?

, We sawed 24" off of the dangling walls to make them even with the front part.
