DETROIT:
Sorry if I started something that led off topic. Let me try to help.
Hopefully, I can make a statement without violating board policy (against claiming expertise) but ... some structures can be rehabilitated and some structures cannot. People exist who are called upon to occasionally make that call and the decision can be driven by two criteria.
1. sometimes, water infiltration beyond the envelope of a structure over time has rendered it structurally unsafe or so badly decayed that rehabilitation is a meaningless exercise ... replacing every rotted board in a wooden house may result in a structure in which only the brass house number is original.
2. sometimes rehabilitation is physically possible, but is actually more expensive than simple demolition and rebuilding a new structure.
Wooden houses and flat roofed commercial structures are particularly vulnerable to damage beyond the point of rehabilitation. Sadly, much of the urban housing stock in the 'rust belt' appears to be in this condition. Including Detroit.
While I do not have the exact figures handy, I believe the population of Detroit has fallen from a peak of somewhere around 1.7 million to a projected stable population of 400,000. That is a lot of empty structures just because no one is living there.
I mentioned Detroit partially in jest and partially because it is a modern area that actually is surviving an 'apocalypse'. Whatever the reason, the end result of rapid depopulation will have certain impacts on an area. Detroit is having to deal with that reality and offers people pre-toughened for your Nuclear Winter or Zombie Hoard.
###
I have a friend who is a retired career Marine (Master Sergeant, I think) who recently visited some friends still living in Detroit (and not the better area). He told about walking 10 blocks to a Music Store to order a CD and requiring his Marine training and knife to get home with his wallet and life. He said he would not go out at night ... his knees are just getting too old for that crap.
Those are the people that I want to rebuild MY civilization, thank you very much. Ooh Raahh!