The ends of joists or beams entering masonry walls shall be cut to a three-inch bevel and where they enter from opposite sides they must be staggered so as to have eight inches of solid masonry between. No timber shall be placed within two inches of the outside face of any chimney or smoke flue. All chimney and smoke flues shall be lined throughout with terra-cotta flue lining or else shall have all walls at least eight inches in thickness. In no case shall flues be supported upon woodwork nor corbelled out from the wall more than four inches. The roofs of all school buildings more than one story high shall be covered with fire-retarding materials.