Ban's work encompasses several schools of architecture. First he is a Japanese architect, and uses many themes and methods found in traditional Japanese architecture (such as shōji) and the idea of a "universal floor" to allow continuity between all rooms in a house. In his buildings, this translates to a floor without change in elevation. By choosing to study under Hejduk, Ban opted to do something different. Hejduk's rationalist views on architecture provided a way of revisiting Western modernism and gaining a richer appreciation than the reductive vision of it as a rationalized version of the traditionalist—yet ultra-modern—Japanese space. With his Western education and influences, Ban has become one of the forerunning Japanese architects who embrace the expression of Western and Eastern building forms and methods. Perhaps most influential from Hejduk was the study of the structure of architectural systems. Ban is most famous now for his innovative work with paper and cardboard tubing as a building material. He was the first architect in Japan to construct a building primarily out of paper with his paper house, and required special approval for his building to pass Japan's building code. Ban is attracted to using paper because it is low cost, recyclable, low-tech and replaceable. The last aspect of Ban's influences is his humanitarianism and his attraction to ecological architecture. Ban's work with paper and other materials is heavily based on its sustainability and because it produces very little waste. As a result of this, Ban's DIY refugee shelters (used in Japan after the Kobe earthquake, in Turkey, Rwanda and around the world) are very popular and effective for low-cost disaster relief-housing.