Friedrich Caspar David's Abtei im Eichwald.
Peter Pauls Rubens, Daniel in the Lion's Den.
Francis Bacon, Self Potrait.
Shots from the movie Stalker by Andrei Tarskovsky.
I don't want to tell you how to do your job or anything like that, but these are sugestions of ideas you might want to think when you work about "decay".
Ruins as the conflict of man vs nature. Death(decay) as nature over men. If a city is mean of decay, does it mean is not for men or is over mens? Ruin as reason vs the unreasonable. Ruin as order (impose by men) over chaos. What I'm saying is that, decay is a concept, but ontologically has different features which can be taken from nature, irrational, chaos, etc. And with exploring the concept you can also explore the problem.
There was also a architect (you might now it better than my) who said that ruins are the detachment of building to men, making them pure (I tried to find the quote but I was to lazy and busy to keep searching).