Masonry composed of unsquared stones is called rubble. This class of masonry covers a wide range of construction, from the com monest kind of dry-stone work to a class of work composed of large stones laid in mortar. It comprises two classes: (1) uncoursed rub ble, in which irregular-shaped stones are laid without any attempt at regular courses, and (2) coursed rubble, in which the blocks of unsquared stones are levelled off at specified heights to an approx imately horizontal surface. Coursed rubble is often built in random courses; that is to say, each course rests on a plane bed, but is not necessarily of the same depth or at the same level throughout, so that the beds occasionally rise or fall by steps. Sometimes it is required that the stone shall be roughly shaped with the hammer.
In building rubble masonry of any of the classes above men tioned the stone should be prepared by knocking off all the weak angles of the block. It should be cleansed from dust, etc., and moistened before being placed on its bed. Each stone should be
firmly imbedded in the mortar. Care should be taken not only that each stone shall rest on its natural bed, but that the sides parallel to that natural bed shall be the largest, so that the stone may lie flat, and not be set on edge or on end. However small and irregular the stones, care should be taken to break joints. Side joints should not form an angle with the Led joint sharper than 60°. The hollows or interstices between the larger stones must be filled with smaller stones and carefully bedded in mortar.
One-fourth part at least of the face of the wall should consist of bond stones extending into the wall a length of at least 3 to 5 times their depth, as in ashlar.
Amount of Mortar. If rubble masonry is composed of small and irregular stones, about 3 of the mass will consist of mortar; if the stones are larger and more regular 6 to will be mortar. Laid in 1 to 2 mortar, ordinary rubble requires from to 1 barrel of cement per cubic yard of masonry.