(1) Thickness.- Empirically constructed natural stone walls shall comply with the following requirements for thickness-

a) the thickness of ashlar stone walls shall be not less than that specified under the same conditions for solid brick walls in sections 44 to 56;

b) the thickness of squared rubble walls shall be not less than that specified under the same conditions for solid brick walls in sections 44 to 56, and in no case less than 230 millimetres for bearing-walls or less than 150 millimetres for non-bearing walls.

c) The thickness of random rubble walls shall be 150 millimetres greater than that specified under the same conditions for solid rock walls in sections 44 to 56, and in no case less than 330 millimetres for bearing-walls or less than 230 millimetres for non-bearing walls.

(2) Height and length of empirically constructed natural stone walls.- The height and length of any empirically constructed wall of natural stone shall be as prescribed by sections 44 to 56 for a solid brick wall of the same thickness:

Provided that, in the case of a random rubble wall, the height and length shall be as prescribed for a solid brick wall having a thickness 150 millimetres less than the thickness of the random rubble wall.

(3) Natural stone retaining walls.- Every empirically constructed natural stone retaining wall shall comply with the following requirements-

a) it shall comply with provisions of subsections (2) to (5) of section 56 for brick retaining walls:

Provided that a batter may be used on the outside face of the wall if it does not exceed 1 in 10;

b) its thickness at the top shall be not less that 230 millimetres, and, at any point below the top, its thickness shall be not less than 40 per centum of the height of wall above that point;

c) it shall be constructed on a concrete foundation at least 250 millimetres in thickness, projecting at least 150 millimetres beyond the edge of the base of the wall at either side, and constructed of concrete complying with the requirements of section 14;

d) its height shall not exceed 2 metres;

e) the mortar used in such wall shall be class A mortar.