[Illustration: Thickness of rubble-stone wall]
The kind of mortar which should be used for the rubble wall depends
upon its location and desired appearance. All foundation-walls,
and all walls which are subject to dampness, should be built with
Portland-cement mortar. Lime mortar may be used in walls above grade,
although cement mortar, or cement-lime mortar is superior. As the
strength of a rubble wall depends more upon the mortar than the bond,
it is well to use the best. However, care should be taken that the
wall is well bonded. A wall which consists of two faces, not bonded
together, should not be built. A bond stone which carries through from
one face to the other should be set into the wall every 2 feet in
height, and every 3 feet in length. This bond stone should be flat and
about 12 inches in width and 8 inches thick. The usual thickness of
walls for dwellings not over three stories in height is 16 inches, and
the foundation-walls are made 8 inches thicker than the wall above or 2
feet.
The footings under a stone wall should be of concrete, not less than 12
inches thick, and should rest upon solid ground at a depth equal to,
or greater than, the frost-line below the surface, unless solid rock
occurs above this point. The width of the footings should be such that
it projects outward on both sides of the wall at least 4½ inches.