The earth-pit privy is the simplest to build and the one most widely used. It is not generally recommended in localities where underground rock has crevices. For a sanitary type of privy with reinforced concrete[9] floor, riser, and supporting sills see figure 14. Because privy units are commonly used as urinals, the use of impervious materials for risers and floors facilitates cleanliness. In the colder climates, wood treated with a preservative is durable and reduces the problem of moisture condensation. Therefore, wood could be used if approved by the State department of health. [9] For information on making concrete see Farmers' Bulletin 1772, Use of Concrete on the Farm. When it is considered impracticable to build the slab and riser of concrete, these parts may be of wood, as shown in figure 16. The building itself may be as shown in either illustration. A wood structure is easy to move to a new location. A pit with a minimum capacity of 50 cubic feet[10] will usually serve five people over a period of 5 to 10 years. The privy should be moved when the pit is filled to within 18 or 20 inches of the top and a strong disinfectant spread in the old pit before covering it with earth. [10] Recommended by the Committee on Promotion of Rural Sanitation, Public Health Engineering Section of the American Public Health Association, 1932. [Illustration: Figure 14.--Sanitary type of privy. Detailed plans and a bill of materials for this design can be had from the United States Public Health Service, Washington 25, D. C.] It is important to have the earth-pit privy more than 50 feet from the well even where the water table is not near the surface. The ground water should flow from the well toward the privy, and it is important that this direction of flow be determined in advance. Wood is most commonly employed for the main part of the building. The ground outside should be sloped as shown, to shed water away from the building, and the roof should extend beyond the walls to shed water away from the pit. CARE AND MAINTENANCE All privies require periodic attention. Seats and covers should be washed weekly with soap and water or with disinfectants, such as cresol, pine oil, and hypochlorite or chloride of lime. These have deodorant properties and are available at most grocery or drug stores. Druggists generally carry a more refined product and consequently the price is higher. During the fly season fly and mosquito eggs will be destroyed by pouring half a pint of crude oil, crankcase oil, fuel oil, kerosene, or borax solution (1 pound powdered borax dissolved in about 10 gallons of water) over the contents of the pit about once a week. [Illustration: Figure 15.--Pit privy of all-wood construction. The sills and riser of this type should either be treated or made of cypress, redwood, cedar, locust, fir, or other decay-resistant wood.]