I spoke to man that said he had 3 A/C units stolen from his rental houses. Said he is now putting all his A/C units on the roof.
He said one of the houses was even rented at the time.
I am in no way implying that doing so will not violate any codes, or that it will comply with the manufactures installation instructions. I am only stating that it is sometimes done, and a possible reason as to why the buyer would request it to be installed on the roof.
