Every Roof Leak is Different
In this example, the leak is coming from around the cricket on the chimney. To fix the leak, the shingles and siding need to be removed. Then, new flashing that extends out past the corner boards needs to be installed along with weather shield, a water-proofing membrane. A piece of plywood also needs to be replaced as it is rotten. Once all of this is done, new shingles can be installed in the area the old ones were removed from.
The condition of the rest of this roof is okay, therefore we recommended to this client that they go with a repair versus re-roofing their home. If the roof looked old or if we saw multiple repairs up there, we would give the opposite opinion as it may not be cost effective long term to continue to repair a roof that should be replaced.