in 1399 Richard II commissioned a hammer-beam roof to arch across the entire span of Westminster Hall. It is the largest medieval timber roof in Northern Europe with a span of 69 feet. The roof spans this great space, supported only by the buttressed walls and by certain inherent elements of strength developed by the interlocking on geometrical principles of the timbers of Sussex oak. The hammerbeam roof, invented in England in the late 1300's, made it possible to span wide spaces without rows of columns or stone vaulting. The trick was to make use of a series of short horizontal and vertical oak beams, rising in steps from the tops of the walls to the centre of the roof space. These distributed the weight of the roof to the sides. Not only serving a structural purpose, hammerbeam roofs are also very beautiful. In the tradition of great hall architecture it was Hugh Herland, the King's Master Carpenter, who swung this wonderful piece of open timber carpentry across the hall. His creation is one of the acknowledged structural and artistic masterpieces of the Middle Ages. To cover the gigantic 69ft (21.1m) width of the hall, he constructed a so-called hammer-beam roof, which effectively spanned the space in two structural stages. From the ground the whole roof seems to be supported on the backs of the huge, hovering figures of angels that are carved on each hammerbeam, holding shields that bear alternately the arms of Edward the Confessor and the ill-fated Richard.