2.T18.004-21 is a type of construction element used in the Roztocze region of Poland. It is a double or single T-shaped truss, made of either wood or steel, used for structural support in roofs, walls, ceilings and other structural components. It consists of two parts, the top tension chord, which is a steel or wood beam pre-formed in an 'I' or 'T' section and the bottom compression chord, which is usually made of wood, although steel can also be used. It is designed to be strong, lightweight and easy to install. Its use dates back to the 19th century and it is still in use today in some parts of the country.