Figure 19.--Defects in foundations, such as the opening to the right of the step, are often the cause of rat infestation in old buildings] All openings between floors and in partitions made for the passage of pipes and wires and any defects in the wall should be closed with metal flashing. All dead spaces throughout the building, such as boxed-in plumbing, spaces behind or beneath built-in cabinets, counters, shelving, bins, show windows, and many similar places, should be removed, opened up, or effectively and permanently proofed against rats. In the Southern States, where the roof rat occurs, similar care must be taken to make the upper floors and roofs of buildings rat proof, as this rat is an expert climber and frequently enters buildings by way of the roof. Doors at the top of stairs and elevators should fit snugly, and all ventilators, exhaust fans, unused chimney flues, and other openings should be screened. Broken skylights and openings under eaves and places where electric wires enter the building should be repaired or closed. [Illustration: Figure 20.--Methods of rat proofing stud spaces in old buildings: A, Construction at outer wall. Open stud spaces are filled with weak concrete, which is placed by removing the skirting above. If the work is done a little at a time, the wooden forms can be removed when the concrete has set, and used again. B, Another method employing sheet metal secured to sill, joist, and flooring. C, Post and girder in basement supporting partition with open stud spaces. Sheet metal nailed to joists and floor and fitted about the stud prevents access to upper floors] [Illustration: Figure 21.--A, Typical construction of frame building on wooden girders and posts with Joists more than 2 feet above ground; B, sheet metal placed as shown serves to prevent the rats from climbing to a point where they can gnaw through the wood] Buildings that have neither basements nor continuous masonry foundations present more difficult rat-proofing problems. The most effective procedure is to construct a concrete foundation wall between the existing supports and, after the wall has hardened, remove the supports, if of wood, and replace them with concrete to make the wall continuous. Where the cost prohibits following this plan and where the supporting sill and joists are at least 2 feet above the ground level, satisfactory rat proofing may be attained by stopping the spaces between the studs with weak concrete or other material resistant to rats for a distance of 8 inches above the floor level, or with galvanized-metal flashing nailed to the joists, plate, and floor. (Fig. 21.) The space beneath the building must be free from all rubbish and other material that would afford shelter for rats. A continuous masonry foundation, with screened openings to provide ventilation, presents a more pleasing appearance.