Design for structural repairs

The subject is a 1100 sq ft house with subterranean termite damage. County inspectors asked us to have a structural engineer letter proposing repair procedures for termite damaged ceiling joists and roof rafters. House is 26' D x 43' W, their are no ceiling joists nor termite damage on the left 10' W of house. There is no termite damage on the right 15' W of house. The termite damage is limited to the center 18' W of house. The 13 or so ceiling joists are spaced 16" on center and are either attached with joist hangers to either a 10' W or 3' W load bearing beam or overlap one another by 2' on top of a 5' W load bearing wall. The front of house ceiling joists are 15' L and the rear of house ceiling joists are 11' L. The shingled 4/12 pitch roof is supported by appx. 13' L rafters. The sheathing is mostly plank with some plywood from recent repairs. 1. We are replacing most all rear of house 11' ceiling joists. 2. We are replacing 3 or so front of house 15' ceiling joists. The rest of them have the bad ends cut off appx. 2'-4' short of the load bearing beams or wall. 3. We have several rafters in the rear of the house that have termite damage. They have anywhere from 1'-8' of termite damage. Considerations 1. We have removed the ceiling and have access to the roof framing from the underside. We'd like to make the repairs (if possible) w/o having to remove the roofing (shingles, underlayment, sheathing, etc.). 2. For the front ceiling joists, if is feasible to replace the cut away termite-damaged lumber with new 2x6 lumber and then sister a 2x6 over the repaired area. This would eliminate the need to completely replace the joist. 3. For the rafter repairs, is it possible to cut away termite-damaged lumber with new 2x6 lumber and then sister a 2x6 over the repaired area. We understand that the replaced lumber will not be attached to the sheathing from the outside. So could we affix the replaced lumber to the sheathing from the underside using adhesive directly on the rafter/sheathing connection or using blocks? Also, for the rafter repairs, is there a standard that states if the damage is more than x% of the length or some other metric then it must be replaced? Repair Specifications 1. ceiling joist repairs/replacements 2. roof rafter repairs/replacements Timing Ready for immediate hire. We need a fast turnaround on this job. Please let me know if you need any additional information.