that joins the 2″ stack to the double wye below the floor joists, Figure 19-22. 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 = ¼″ per foot of run × 3′ = ¾″). Since the total fall from the bathtub P-trap was 2⅜″, the fall from the 90° ell will be 1⅝″ (2⅜″ – ¾″). This means that the distance from the bottom of the floor joists to the centerline of the 90° ell (B) will be 2⅝″ (1⅝″ + 1″). Position the 90° ell with the centerline of the hub 2⅝″ below the floor The location of the 2½ × 2 × 1¼ × 1¼ double wye fitting that joins the two tub drains before they enter the double sanitary tee is determined by the location of the vertical drain to the lavato- ries. Therefore, it is best to work back from the lavatory stub-outs to locate the 2 × 1½ × 1¼ × 1¼ double wye. This work is accomplished 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 2 × 1½ × 1¼ × 1¼ double sanitary tee Level to check plumb Length of 2″ pipe 36″ 23⁄8″ fall 33⁄8″ 21⁄2 × 2 × 11⁄2 × 11⁄2 double wye Wall between lavatories and tubs Vent thru roof 1½″ vent can also be installed Mark here and add fitting allowance to determine length of pipe. 1″ A B 3′ Goodheart-Willcox Publisher Figure 19-22. Installing the 90° ell and double sanitary wye requires calculating the fall for the horizontal portion of this run of pipe. 312 Section 3 Plumbing System Design and Installation Copyright Goodheart-Willcox Co., Inc.