If you want to decorate aged buildings and floors which are modified into living spaces, the popular style to do it is industrial style. The style is also applicable for conventional houses and apartments. Creating an illusion of rough substances and surfaces which represent industrial history is what you need. Just let some surfaces or walls in raw condition, that is the easiest way to emerge the roughness look. Additionally, the industrial environment could be created by some rough metal materials.
Feel free to check out the gallery below to find more ideas.