ALTERATION. Any construction or renovation to an existing structure other than a repair or addition . Alterations are classified as Level 1, Level 2, and Level 3.
REPAIR. The restoration to good or sound condition of any part of an existing building for the purpose of its maintenance.
I am assuming you are not in a high seismic zone so you only need to follow 606.2 if it is just a re-roof. If the re-roof is in conjunction with other alterations then more sections come into play
606.2 Addition or replacement of roofing or replacement of equipment.
Where addition or replacement of roofing or replacement of equipment results in additional dead loads, structural components supporting such reproofing or equipment shall comply with the gravity load requirements of the International Building Code .
Exceptions:
1. Structural elements where the additional dead load from the roofing or equipment is not increased by more than 5 percent.
2. Buildings constructed in accordance with the International Residential Code or the conventional light-frame construction methods of the International Building Code and where the additional dead load from the roofing or equipment is not increased by more than 5 percent.
3. Addition of a second layer of roof covering weighing 3 pounds per square foot (0.1437 kN/m2) or less over an existing, single layer of roof covering.
BTW FYI
The IEBC provides three main options for a designer in dealing with rehabilitation of existing buildings. These are laid out in Section 101.5 of this code:
OPTION 1: Work for alteration, repair, change of occupancy, addition or relocation of all existing buildings shall be done in accordance with the Prescriptive Compliance Method given in Chapter 3. It should be noted that this same method is provided in Chapter 34 of the International Building Code .
OPTION 2: Work for alteration, repair, change of occupancy, addition or relocation of all existing buildings shall be done in accordance with the Work Area Compliance Method given in Chapters 4 through 12.
OPTION 3: Work for alteration, repair, change of occupancy, addition or relocation of all existing buildings shall be done in accordance with the Performance Compliance Method given in Chapter 13. It should be noted that this option is also provided in Chapter 34 of the International Building Code .