You are correct that the roof is unobstructed, however, the roof material in the problem statement makes you use the solid line. I don't have the book in front of me, but if you read section 7.4, it states what is slippery and what is not. "Slippery surfaces shall include metal, slate, glass, and bituminous, and plastic membranes with smooth surface. Membranes with embedded aggregate or mineral granule surface shall not be considered smooth. Asphalt shingles, wood shingles, and shakes shall not be considered slippery."
Let me know if I am off somewhere or am not remembering the problem statement correctly.
