(a) segmental brick arches, having a rise of not less than one inch per foot of span, and a thickness of four inches and eight inches respectively, for spans less or greater than five feet;

(b) or hard-burned, semi-porous or porous terra cotta hollow tile arches, with shells not less than three-eighths inch thick and webs not less than five-eighths inch thick, and laid in Portland cement mortar, and having an effective depth of not less than one and one-half inches per foot of span in the case of flat arches, and a rise of not less than one inch per foot of span in the case of segmental arches. Segmental arches shall be not less than six inches in thickness; flat arches shall be not less than eight inches in thickness;

(c) or, reinforced stone concrete, consisting of one part Portland cement and not more than six parts of a properly proportioned aggregate consisting of sand and stone passing a one-inch ring, and not less than four inches thick in the case of floors, and not less than three and one-half inches thick in the case of roofs designed in accordance with the current regulations suggested in the report of the joint committee on concrete and reinforced concrete of the American Society of Civil Engineers, American Society for Testing Materials, American Railway Engineers' Association and the Association of American Portland Cement Manufacturers;

(d) or, any form of construction not less than four inches thick in the case of floors and not less than three and one-half inches thick in the case of roofs which shall have passed successfully a standard four-hour fire, load and water test.