In building construction, a horizontal member spanning an opening and carrying a load.
The load may be a wall above the opening (see post-and-beam system ) or it may be a floor or roof. Beams may be of wood, steel or other metals, reinforced or prestressed concrete, plastic, or even brick with steel reinforcement. For weight reduction, metal beams are I-shaped, having a thin vertical web and thicker horizontal flanges where greater stress occurs. A joist is any of a series of small parallel beams supporting a floor or roof. See also girder , spandrel .