[ygra]

OpenStreetMap has the building outlines (sometimes even parts of the buildings) as well as tagging that specifies

    • wall and roof color
    • wall and roof material
    • roof shape
    • height (or levels, which
      typically is then multiplied
      with 3 m to get a rough height)
    • starting height (or level), for
      building parts above ground, e.g.
      bridges between buildings.

This can then get rendered as upwards extruded polygons with a cap in the shape of the roof. There are a few more complicated ways of specifying complex roofs with ridge lines, etc. but few renderers support them and usage is fairly limited.