“Dear PoPville,
I live in a late 1930s brick wardman style home with a flat rubber roof. After it rains I notice this small debris washes off the main roof onto the roof over our back room and eventually down the gutter. I don’t think it’s mortar because motar is usually finer as it erodes, this stuff is a little coarser than kosher salt. Given the number of similar houses in the area, I’m wondering if anyone knows what it is?”