For any/all zones which have the UNDESTR flag set to true - decay does not cause any damage. The appropriate amount of resources are consumed from the TC for upkeep but once these resources are gone (or if the structure had no TC at all) the building does not actually decay. Everything remains at 100% health.
I am using these zones to create several small pockets of raid safety on the island for specific buildings but I still need the players to have to deal with upkeep. Fortunately only one player has noticed but I'm sure I won't be able to keep this hidden for very long. Any help would be appreciated.