impervious to rats as to prevent the ingress or egress of rats: _Provided_, That all such wall spaces shall be closed with cement, mortar, or other material impervious to rats, which closure shall extend the full thickness of the wall and shall extend upward at least twelve inches above the floor level. The cellar of every building hereafter erected within the building limits shall be made rat proof by the use of masonry or metal. All openings in foundations, cellars and basements In such buildings, except for doors and hatchways, and except also for such windows wholly above ground as may be exempted by the -------- in his discretion, shall be completely covered with screens of metal having meshes of not more than one-half of an inch in least dimension and constructed of rods or wire of not less than twelve gauge. All buildings, outhouses, and other superstructures of class B separated from any other building on three sides by at least ten feet and lacking any basement or cellar may be rat proofed in the following manner, to wit: Said building, outhouse, or other superstructure shall be set upon pillars or underpinning of concrete, stone, or brick laid in cement mortar, or may be set upon underpinning of substantial timber, such pillars or underpinning to be not less than eighteen inches high, the height to be measured from the ground level to the top of said pillars or underpinning; and the intervening space between said building and the ground level to be open on three sides and to be free from all rubbish and other rat harboring material, or may be made rat proof by constructing at the margin of the ground area of said building a wall of concrete or brick or stone laid in cement; such wall to extend into and below the surface of the ground at least two feet and to meet the floor of the building above closely and without any intervening space, to be at least four inches thick and extend entirely around said building _Provided_, That said walls may be built with openings therein for ventilation only: _And provided further_, That such openings for ventilation may be all of such size as the owner may elect and shall be securely screened with metallic gratings having openings between the bars of said gratings of not more than one-half inch or with wire mesh of not less than twelve gauge, having openings between the wires of said mesh of not more than one-half inch and the whole so constructed and closed as to prevent the entrance of rats beneath such building.