Cockroaches live in the soil under the houses, around water and sewage pipes, in cesspools, etc. They migrate into the house searching for food or shelter. They eat clothing, books, and furniture, as well as food, voraciously. They do not only crawl, they fly as well. A mixture of mashed potatoes and boric acid is lethal to cockroaches but this is a dangerous poison and must be kept far from the reach of children and pets.
Cockroaches especially like to build their homes in the subterranean areas of bathrooms.
Although you may not be able to liquidate all of them that are breeding on the level below your bath, you can keep them from entering the house. All cracks in or between the tile must be filled. Before sundown every day, the stopper should be placed on each overflow drain. Cockroaches do not like moth balls. They avoid any area that carries the odor of moth balls.