Wet and dry rot are conditions related to moisture being absorbed and released. Certain materials are more prone to dry/wet rot than other. Rubber, for example, will dry rot over time when left in the elements. The pores allow it to absorb and release moisture, and that process removes/breaks down some of the fibrous materials that make up the rubber over time. There are many materials subject to this process other than rubber, but I am not going to provide a comprehensive list. More or less, anything that can absorb moisture will rot over time if left in the elements.
Thanks, lets say you have the joists to a first floor and then around 2ft below a ceiling to a kitchen. The flat above has leaked a lot and there has been no where for the water to go except out of a light fitting in the floating ceiling. Could any moisture in there that has not been aired mean a case of rotting joists/timbers?? I am trying to figure out what to do before taking down chunks of our ceiling to have a look.