Slightly off topic, but I was in rural Portugal recently, and my impression was that about one in three buildings was in some state of ruin, from otherwise delightful little cottages with holes in their roofs and empty windows, to remains of ancient walls just clinging together by the sheer will of their graffiti. (This isn't to pick on Portugal either - I've seen it in several different parts of Europe.)
For the less damaged ones, I don't really understand why it's not economical to throw a new roof on them to make them habitable. In the US, when a house rots it rots, because it's going to be made out of wood and gypsum and insulation that can't come back from exposure to the elements, but the buildings I was seeing were all stone. Is there some special failure mode that's harder to see in them, like black mold in the cracks or structural instability that makes them unrepairable, or is just lack of demand?
A good example is this building: https://goo.gl/maps/7uzuvfrUQHD2 This is right next to the national palace in one of the wealthiest regions of the country, and instead of fixing it up and putting it on Air BnB as "The Cottage by the Palace" you can see from google that it's abandoned and the roof has slowly been caving in for the last ten years.