Cruck frame building with Jack Sobon.

   A cruck or crook frame is a curved timber, one of a pair, which supports the roof of a building, used particularly in England. This type of timber framing consists of long, generally naturally curved, timber members that lean inwards and form the ridge of the roof. These posts are then generally secured by a horizontal beam which then forms an "A" shape. Several of these "crucks" are constructed on the ground and then lifted into position. They are then joined together by either solid walls or cross beams which aid in preventing racking (the action of each individual frame going out of square with the rest of the frame, and thus risking collapse).

   Crucks were chiefly in use in the medieval period for structures such as large tithe barns. However, these bent timbers were comparatively rare, as they were also in high demand for the ship building industry. (source)