All flues shall extend at least three feet above the roof and always above the comb of the roof and shall be coped with well-burnt terra cotta, stone, cast iron or cement. In all buildings erected after February 20 1917 the stone or brickwork of all flues and the chimney shafts of all furnaces, boilers, bakers’ ovens, large cooking ranges and laundry stoves and all flues used for similar purposes shall be at least eight inches in thickness, with the exception of smoke flues which are lined with fire-clay lining or cast iron. These may be four inches in thickness, but this shall not apply to metal stacks or boiler houses where properly constructed and arranged at a safe distance from wood or other inflammable material. All buildings erected after said date shall have smoke flues constructed, either in walls of eight inches thickness or with smoke flues lined with cast-iron or fire-clay lining, the walls of which may be four inches in thickness, the lining to commence at the bottom of the flue or at the throat of the fireplace and to be carried up continuously the entire height of the flue. All joints shall be closely fitted, and the lining shall be built in as the flue is carried up. All chimneys which shall be dangerous in any manner whatever shall be repaired and made safe or taken down.