The effluent sewer should be constructed in similar manner and of the same materials as the house sewer and on a slope of 1/8 inch to 1 foot. This line, however, may be laid of terra-cotta pipe, as cast-iron is not considered necessary except in unusual cases. This line should always terminate in a distribution box from which the tile lines of the disposal field lead away. For steep slopes the arrangement shown in figure 9 (p. 15) is practical. Joints must be of root-tight construction if the sewer is in the vicinity of trees or shrubs. The length of the sewer depends upon the distance from the tank to a safe site for the disposal field. THE DISPOSAL FIELD Correct installation of the disposal field is of great importance for proper functioning of the septic tank. Therefore, the width, depth, and spacing of the tile trenches must be carefully selected. Line of 4-inch, open- jointed, agricultural drain tile laid in shallow trenches are ordinarily used. Perforated fiber drain pipes also may be used and are obtainable in 4-foot lengths. A distribution box with an inlet for the effluent sewer and an outlet for each individual run of disposal tile is the best means of dividing the flow. The outlet serving a large or double disposal field may be alternately opened and closed by means of a sewage switch that permits half the disposal field to work and rest alternately several weeks. A switch is especially helpful in tight soils but should not be provided unless proper maintenance is assured, so that a portion of the disposal field will not be left to handle the entire load of the system for an indefinite period. There are many variations of boxes, but figure 8 shows a practical type. [Illustration: Figure 8.--Typical distribution box.] Shallow Tile Lines The disposal tile should not be more than 18 to 24 inches below the surface, and where the ground-water level rises to the bottom of the trench special underdrains, described on page 16, are necessary. Special provisions must also be made where tight soils are encountered. These methods are described in the section entitled "Disposal methods in tight or wet soils." The table in figure 9, together with the information given in table 1, below, may be used for estimating the number of tiles needed in any particular soil type. If there is any doubt about this requirement, a percolation test should be made in the disposal field, as follows: