After I greeted the homeowner, she took me to the side of her garage at the front of her house and showed me the staining—evidence that water had indeed been dripping from her siding (1). This was going to be interesting.
Not seeing anything unusual in the siding or in the roof, fascia, gutter, or soffit above the leaking wall, I went to take a look from inside the garage. Fortunately, there was no wall or ceiling covering, and the roof had been framed using raised-heel trusses. The exterior wall sheathing ran up the heel but stopped below the 2x4 truss tail. This space allowed me to see inside the soffit.
Granules that had sloughed off the new shingles lay on the inside surface of the soffit, a clear sign that wherever this water was coming from, it had first run down the shingles.
Looking both ways down the length of the soffit, I could see which direction the water was coming from. Working my way toward the source, it seemed that the water originated where the garage soffit intersected with the roof plane of the house, about 10 feet from the site of the staining. Now it was time to get out of the garage and back onto the roof.
Still, these factors alone didn't create the problem. The soffit material had been run tight to the shingles and past the angled edge of the fascia.
This combination created a scoop that allowed water to flow under the fascia and on top of the soffit. From there the water made its way down the soffit and finally behind the siding. I thought, "Look on the bright side: At least the water-resistive barrier behind the siding did its job well!"
At this point, my work was complete, except for the apologies. I called the project contractor and told him what I'd found. In light of missing the problem the first time, and knowing that it could be fixed with a single piece of step flashing