Porous roof tiles. Over time, almost any type of tiled roof (terracotta, concrete, etc.) will end up this way. While it will take a long time for this to happen, it's important to be aware of the problems that can come with porous tiles. Whether it's the fact that tiles become more susceptible to accumulating dirt and other grit or the very real dangers that exist as they weaken, here are some of the most key things to remember with porous roof tiles.