I have been looking at Vienna on Google maps, satellite view and street view---something I like to do before visiting a new place. Just now I noticed something curious on Satellite view: the colors of the rooftops. I was looking specifically at the Rathaus and the Volksgarten nearby, and when "zoomed out" I noticed a distinct difference in the rooftops east and west of Landesgerichtstraße). To the west of this line, the many red roofs give a reddish cast to the whole area, while on the east side there are no red roofs, rather green and grey ones.
I assume this reflects a difference in preferred building materials at the time of construction, or perhaps different suitability of materials for large public buildings (Rathaus and Parlament) versus smaller ones. I am curious specifically about the red roofs---what is the material? I first thought it was red tile (like Venice, for instance), but on close inspection (as close as I can get with Satellite view) many of the surfaces appear to be completely smooth.
Does anyone know what material this is and why the abrupt difference? (The reddish roofs dominate again east of the government buildings and public squares, past St. Stephansdom, but not with such a sharp dividing line as Landesgerichtstraße).