We do have some different policies, most of them only covers "sudden unexpected events", meaning that damages that has accumulated over time due to leakages, fungus, animals or other pests won't be covered.

I don't think it's reasonable to expect the insurance company to cover the replacement of a nearly 70 years old sewage pipe with expected lifetime 40 years. At the other hand, I'm pretty sure damages caused by a sewage overflow should be covered.