Masonry units should be laid in full beds of 1:3 cement mortar and the walls and floor plastered with at least a 1/2-inch coat of 1:2 mortar. Cells of concrete blocks and tile must be filled with concrete. Masonry walls are generally 8 inches thick, and care must be taken to follow _inside_ dimensions given for concrete tanks. Directions for laying structural tile, brick, and concrete blocks can be obtained from dealers or trade associations. Commercial tanks are suitable if they embody the essential features given in this bulletin. Capacities should be as recommended in figure 4 for concrete tanks. Proper installation and periodic servicing also are essential. Tanks badly damaged in handling should not be used. Rapid corrosion of steel tanks will result if the asphalt coating is impaired. Minor defects in precast masonry tanks may often be overcome by plastering the interior with cement mortar. BUILDING A CONCRETE TANK[5] [5] For information on making and placing concrete, see Farmers' Bulletin 1772, Use of Concrete on the Farm. A convenient method of assuring correct location of the tank is to build a frame as shown in figure 6. Care is necessary to aline it with the center line of the inlet and outlet and to level it so that the distance from the bottom of the 2 by 4's on the form to the lower edge of the inlet hole in the form will permit it to be set at the grade of the house sewer. This frame is used to support the form for the tank. To avoid caving the edges, drive the stakes supporting the frame before beginning the excavation. The lumber in the frame can be used later to make part of the tank baffles. [Illustration: Figure 6.--Method of outlining a septic-tank excavation on the ground surface.] Figure 7 shows how an inside form can be built and hung in place. The inlet and outlet tees should be carefully set and tied in place before the concrete is poured. A single length of pipe should be joined to the tee, so that the two can be set in the form as one unit. In most cases the earth walls of the excavations will serve as the outside forms unless the soil is sandy or gravelly and the excavation is deeper than 5 feet. If outside forms are used, space must also be provided for them. Forms should be constructed before the excavation is made and the tank built as soon as practical, to avoid warping of forms and caving of earth walls. [Illustration: Figure 7.--Inside form hung in place for single-chamber septic tank, also a form for casting concrete-slab cover in sections.] County agricultural agents, local health departments, building-material dealers, and other agencies often have forms that may be borrowed or rented. THE EFFLUENT SEWER