Copyright Goodheart-Willcox Co., Inc. 326 Section 3 Plumbing System Design and Installation The location of the 2 1/2 × 2 × 1 1/2 × 1 1/2 dou- ble wye fitting that joins the two tub drains before they enter the double sanitary tee is determined by the lo- cation of the vertical drain to the lavatories. Therefore, it is best to work back from the lavatory stub-outs to locate the 2 × 1 1/2 × 1 1/4 × 1 1/4 double wye. This work is done using procedures like those described for installing the laundry tub stub-out. Once this work is completed, a length of 2″ pipe can be inserted into the bottom hub of the double wye and the measurements can be made below the floor frame to locate the elbow that joins the 2″ stack to the double wye below the floor joists, Figure 21-22. To determine how far the 90° ell must be below the floor joists (B), calculate the fall for the 36″ distance between the ell and the side inlet of the double sanitary tee. (Fall = 1/4″ per foot of run × 3′ = 3/4″.) Since the total fall from the bathtub P-trap was 2 3/8″, the fall from the 90° ell will be 1 5/8″ (2 3/8″ – 3/4″). This means that the distance from the bottom of the floor joists to the centerline of the 90° ell (B) will be 2 5/8″ (1 5/8″ + 1″). Position the 90° ell with the centerline of the hub 2 5/8″ below the floor joists and mark the 2″ pipe at the inlet to the 90° ell. Remove the 2″ pipe and add the appropri- ate fitting allowance before cutting the pipe. Test tee w/ cleanout Mark here and add fitting allowance to determine length of pipe connecting test tee and double sanitary tee. 1uni2033 23⁄8″ fall 33⁄8uni2033 Floor joist Studs Sole plate Subfloor 3uni2033 pipe fitted in test tee @ base of stack Alternate: If the P-trap will extend too far below the floor joist, the end of the run can be turned up between the floor joists. Goodheart-Willcox Publisher Figure 21-20. Location of the double sanitary tee depends upon the fall required for the bathtub. L = Laying length L Level to check plumb Shims equal in thickness to finished floor 3″ pipe Floor joist Stud Sole plate Closet flange Double sanitary tee 3uni2033 pipe B A A Goodheart-Willcox Publisher Figure 21-21. Joining the closet flange to the double sanitary tee requires two short lengths of pipe and a sanitary ell.