Sidewalks shall be constructed and laid either with cement, bricks or dressed flat stones, and shall be secured with curbstones along the outside thereof, in a manner as hereinafter prescribed, and according to the regulations of the City, sidewalks shall have a pitch of 3/8 of an inch to the foot, from the front line of the house or lot to the curb, and the cement bricks or flatstone used for laying or paving any sidewalk shall not be less than two inches in thickness. The flat stone shall be square blocks with an even surface on the top and closely joined to the curbs and to each other. The curbstones required to support such sidewalks shall be cut granite, sand, blue or limestones, and shall not be less than 20 inches in width, with an even bottom. Each stone shall be not less than three feet six inches in length, and the thickness thereof shall be not less than four inches. In streets of a width of 20 feet and less, curbstones 16 inches in width and three inches in thickness may be used. Sidewalks shall be laid in good sand of not less than eight inches in depth and shall be excavated and filled with cinder to a depth sufficient to protect the same from frost. In all cases where the street pavement is constructed and composed of the following improved street pavements: granite block, asphalt block, sheet asphalt and vitrified brick, the curbing used in and along all of such properties abutting on such improved pavements shall consist of granite curbing of not less than five inches, 20 inches in depth and four feet in length. The curbing shall be properly faced and jointed, set in a firm bed of not less than six inches of sand, gravel or such material as the Director of Public Works shall approve and backed up to the top of the stone by not less than four inches of the same material, except that the use of concrete is permitted in lieu of granite for the construction of curbs set to the lines of circular arcs for the rounding of street intersections as provided by § 508-513A.