It is not practicable to attempt to exclude rats from poultry houses, but such buildings can easily be made proof against serious trouble by the elimination of all places where the rodents can obtain safe harborage. Most rat infestation around poultry plants is due to the presence of numerous shelters and suitable breeding places. Three things are particularly to be avoided: Wooden floors on or within a few inches of the ground; double walls; and nest boxes, feed hoppers, and other fixtures placed so as to provide shelter for rats under or behind them. From a rat-proofing standpoint the floors as well as the foundation should be made of concrete. (Fig. 13.) If this is not considered practicable, wooden floors should be elevated so as to insure a clear space of 2 or more feet between the floor and the ground. Warmth can be provided, if necessary, by two thicknesses of flooring with tar paper between. Hollow walls almost invariably furnish harborage for rats. The inner stud covering therefore, should be torn out, but if warmth is a factor to be considered, siding should be put over sheathing on the outside of the studs with building paper between.[2] [2] The construction of poultry houses and fixtures is described in Farmers' Bulletin 1554, Poultry Houses and Fixtures. [Illustration: B4390M Figure 11.--Rat proof pigpens and feeding troughs are easy to keep clean and sanitary, and rats have little opportunity to steal the feed] [Illustration: Figure 12.--A concrete curtain wall or apron under a feeding floor prevents raveling of earth and consequent breaking of the slab, as well as the harboring of rats] Portable laying and brooder houses frequently become heavily infested because they are usually built with wooden floors removed from the ground only by the height of the runners on which they are constructed and are seldom moved as frequently as originally intended. Feed, sifting through the floors, attracts rats, which after finding desirable shelter soon establish themselves in burrows beneath the houses and multiply rapidly. Portable houses, therefore, should be raised off the ground 2 or more feet. Nests should be raised 2 or more feet above the floor, and feed and grit hoppers at least 1 foot. Drinking vessels for water and skim milk should supported on a platform 1 to 1½ feet above the floor, so as to eliminate the possibility of rat shelters and keep the liquids in a more sanitary condition. Other equipment should be given the same consideration.