The raw materials used in Hanok, such as soil, timber, and rock, are all natural and recyclable and do not cause pollution. Hanok's have their own tiled roofs (Giwa), wooden beams and stone-block construction. Cheoma is the edge of Hanok's curvy roofs. The lengths of the Cheoma can be adjusted to control the amount of sunlight that enters the house. Hanji (Korean traditional paper) is lubricated with bean oil making it waterproof and polished. Windows and doors made with Hanji are beautiful and breathable.