Kaphis,“beam,” is supposed by some to be a cross-beam for binding together the walls of a building. Jerome says it is “the beam which is placed in the middle of any building to hold the walls together, and is generally called ἱμάντωσις by the Greeks.” Henderson, however, objects to this rendering. He says: “That it was not the wood itself is evident from the following: from, or out of the wood.” He prefers the interpretation given by the Mishna, and followed by some Jewish writers. According to these kaphis signifies a half brick. Rashi, the celebrated commentator and Talmudist, explains it to be “half a brick, which is usually laid between two layers of wood.”
There are numerous evidences to show that ancient architects used wood to unite and bind walls, and it may have been some such custom to which the prophet refers in the text.