Gassho-zukuri is a Japanese architectural style with a distinct steep roof.
The group of buildings in the villages of Shirakawago and Gokayama are especially well known and are registered as a World Heritage site (Cultural Heritage) by the United Nations Educational, Scientific and Cultural Organization (UNESCO).
The main feature is a steep, thatched kirizuma yane (gable roof). It is said that the term of Gassho-zukuri came from the fact that the shape of the roof resembles that of praying hands.
The koyagumi (roof truss or framework) of the Gassho-zukuri in Shirakawago and Gokayama has become famous, but this structure was previously widely used in Japanese private houses. Gassho-zukuri is advantageous because a steep roof is necessary to prevent rain from seeping into the house with a thatched roof. In addition, it is also well-suited for supporting the weight of snow that accumulates in heavy snowfall areas.
Its structure is considerably different from the koyagumi (wagoya structure) such as shoin-zukuri or sukiya-zukuri, used for houses of people in higher classes. More specifically, the ridgepoles and roof purlines are supported vertically from below in the wagoya structure. However, in gassho-zukuri, materials from both sides lean against each other in the shape of the Chinese character 人 cross at the ridgepoles. This is generally called the sasu structure. It is a truss structure and ideal considering the characteristics of wooden material because it reduces bending moments and concentrates tensile strength of the beams.
Gassho-zukuri houses have a spacious area under the roof without koyazuka (vertically-placed wood materials). Around the middle of the Edo period, farmers started to place shelves for raising silkworms in this space under their roof when the sericultural industry grew. It is structurally difficult to make roofs that are not steep with Gassho-zukuri style. However, it is thought that the roof was made higher and steeper to increase the space available for shelves to raise silkworms.
Thatched roofs are re-thatched once every 30 to 40 years. When snow slides off the roof, it sometimes brings pieces of thatch with it. Therefore, the roof must be repaired once or twice a year. The work to renew or repair a thatched roof is conducted in cooperation with people in the community. This system is called 'yui' (bonding).