Udatsu (a fire-preventative structural element used in residential houses) （卯建） is a building term and is also written as 宇立.
The pronunciation of udachi was used in the Heian period, but was changed to udatsu in the Moromachi period and later. Originally, it indicated a small pillar placed on an utsubari (beam). Later, a small fire-preventive wall was placed between gabled roofs of adjacent houses so that the wall protruded between the first floor roof and the second floor roof, and this wall became to be called udatsu. Originally, udatsu were built as a wall to prevent a fire spreading to adjacent houses when a row of machiya (a traditional form of townhouse found mainly in Kyoto) houses were built with little space between adjacent houses. However, more importance came to be placed on its decorative nature in later years. Mainly in the Kansai region, merchants competed with each other to attach splendid udatsu to their houses as an indication of their financial power. It is said that udatsu came to be built in and around the middle of the Edo period.
Installing udatsu was very expensive, therefore, many of the houses with udatsu were those of relatively wealthy people. The idiomatic expression of Udatsu-ga-agaranai originated from such a situation. Udatsu-ga-agaranai refers to a condition where no good result are being obtained, in work for example, with no future prospects. However, there are many theories about the origin of the expression.
Today, there exists a few streets where udatsu can still be seen. Parts of Waki-machi, Mima City, Tokushima Prefecture and Mino City, Gifu Prefecture are trying to preserve udatsu as their regional symbol.
When the station building of Aoyagi Station of the East Japan Railway Company (JR East) in Nagano Prefecture was renovated in 2002,it featured a design based on udatsu. In Tokushima Prefecture, there is Michi no Eki (highway service area) Ai-land Udatsu.