I think the issue is that there are so few things that CAN result from flammability early on (basically the one burn event and not much else), so it cannot possibly be progressive. That's why I'm suggesting splitting the event into multiple events that all look the same to the user but have increasing probabilities of occurring (makes higher flammability progressively more likely to trigger a burn down, rather than it being basically an on/off thing as it is now). This approach has the benefit of not requiring any new code, or buildings. However, I also like your idea of an auto-build small anti-flammability building that comes with a water source. Obviously they are not mutually exclusive.