Why is this roof covered in pigeons?
Rock Pigeons (or Rock Doves) live almost exclusively in human structures. There must be a bridge or old mill building or something nearby. 

Then, pigeons, like all other animals, operate under the guidance of their instincts. They're instinctively attracted to perches that offer a good vantage point to watch for predators, socialize, and move out to feeding areas. This home happens to be in a location that suits those biological imperatives. 

I doubt that there's anything particular about the construction of the home that's attractive to the birds. They're mostly happy with its location, or rather the location of the perching surface.