BEAM, in Architecture, the largest piece of wood in a building, which lies cross the walls, and serves to support the principal rafters of the roof, and into which the feet of these rafters are framed. No building has less than two of these beams, viz. one at each end; and into these the girders of the garret roof are also framed. The proportions of beams in or near London, are fixed by statute, as follows: a beam 15 feet long, must be 7 inches on one side its square, and 5 on the other; if it be 16 feet long, one side must be 8 inches, the other 6, and so proportionally to their lengths. In the country, where wood is more plenty, they usually make their beams stronger.