Copyright Goodheart-Willcox Co., Inc. Chapter 21 DWV Pipe and Fitting Installation 325 run to each of the P-trap inlets for the bathtubs. Given that the pipe going to each of the bathtubs is relatively long, the fall necessary for these pipes will be the deter- mining factor in locating the double sanitary tee. Drawing lines on the subfloor to represent these pipe runs is an accurate means of determining the total run to each bathtub P-trap, Figure 21-19. Once the total run is known (9 1/2′), the total fall is calculated by multiplying by 1/4″ per foot of run (1/4″ × 9 1/2′ = 2 3/8″). See Figure 21-20. The 1″ vertical distance from the centerline of the pipe to the floor joist must be added to the fall to determine the vertical distance of the side inlet of the double sanitary tee below the floor joists (2 3/8″ + 1″ = 3 3/8″). Hold the double sanitary tee in position so the center of the side inlet is 3 3/8″ below the bottom of the floor joist and mark the pipe at the bottom of the fitting. Add the appropriate fitting allowance and cut the pipe to join the test tee with the double sanitary tee with side inlet. In our example, two short lengths of 3″ pipe and a 90° sanitary ell are used to connect the closet flange to the double sanitary tee. The horizontal pipe from the dou- ble sanitary tee must have a fall of 1/4″ per foot of run. The length of the horizontal pipe can be determined by temporarily inserting a 3″ pipe in the closet flange and measuring A from the centerline of the pipe to the 3″ inlet at the double sanitary tee, Figure 21-21. The lay- ing length (3″) of the 90° ell must be subtracted from measurement A and the fitting allowance (1 1/2″) for the double sanitary tee must be added to determine the length B of the pipe (16″ – 3″ + 1 1/2″ = 14 1/2″). Once the horizontal pipe is cut, temporarily assem- ble the pipe in the double sanitary tee and slip the 90° ell into place. Check the fall of the horizontal pipe and measure the distance between the 90° ell and the closet flange. Adding the fitting allowances to this measure- ment gives the length of the vertical pipe. The closet flange will not be permanently secured until after the finish floor is installed. Strainer Molded shower base Floor level To sewer 2uni2033 waste pipe Strainer Lead Molded shower base Floor level To sewer 2uni2033 waste pipe Oakum Compression gasket A B Goodheart-Willcox Publisher Figure 21-17. The connection between the shower drain and DWV piping must be sealed. A—Today, a compression gasket is typically used. B—Some older installation may be joined with lead and oakum. Check accuracy Blocking equal in thickness to the finish floor Closet flange Closet bend Double sanitary tee goes here Opening for stack Goodheart-Willcox Publisher Figure 21-18. The closet flange must be temporarily supported on blocks equal to the thickness of the finished floor before measurements are made for the pipe connecting it to the stack. A 18uni2033 B 24″ C 72uni2033 Total run = A + B + C = 18uni2033 + 24uni2033 + 72″ = 114uni2033 = 9½uni2032 Total fall = Total run(uni2032) × ¼uni2033 per ft. = 9½uni2032 × ¼uni2033 per ft. = 9⁄4 = 1⁄8 = 23⁄8uni2033 Goodheart-Willcox Publisher Figure 21-19. The total run of the drain pipe to the bathtub can be determined by a simple layout on the subfloor.