Because that seam is the hardest detail on the building: two roof systems with different movement, different materials, and different ages meet in one line, and ponding water on the flat section sits against it. The local symptom leader's own copy names Peterstown and Elizabethport flat-roof rowhouses leaking at exactly this shingle-to-flat seam. The repair is a seam rebuild in the flat system's chemistry with the pitched flashing woven correctly, plus drain clearing so water stops sitting at the joint. Caulk over the seam is the repair that has already failed on most of these roofs once.
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