necessary to close all openings either with caps, plugs, or test plugs so that the system can be tested. These procedures are discussed near the end of this chapter. Polyvinyl chloride (PVC) was discovered accidentally in the 19th century. It appeared as a white solid inside flakes of vinyl chloride that had been exposed to light. It was not until 1926, when Waldo Semon of the B.F. Goodrich Company developed a method of plasticizing PVC into a more flexible and workable form, that it became a commercially valuable material. Installing DWV Plastic Pipe and Fittings DWV plastic piping is generally measured by either the face-to-face or the shoulder-to-shoulder method. The fitting allowances (engagement) for DWV fittings are given in Figure 19-24. These allowances are the same for either ABS or PVC materials. For example, if the face-to-face mea- surement for a 2″ PVC pipe is 4′-6″ and the pipe must engage a fitting at both ends, then the pipe must be cut 4′-7½″ long. This is an allowance of ¾″ for each fitting, or a total of 1½″. See Figure 19-25. joists and mark the 2″ pipe at the inlet to the 90° ell. Remove the 2″ pipe and add the appropriate fitting allowance before cutting the pipe. The double wye can be located anywhere between the 90° ell and the double sanitary tee. For this example, the midpoint was chosen. To determine the length of the 2½″ and 2″ lengths of pipe, subtract the overall length of the fitting from the 3′ distance between the double sanitary tee and the double sanitary wye and divide by two. Add the appropriate fitting allowances to deter- mine the correct length of each pipe. The pipes and fittings that extend from the double wye to the bathtubs can then be installed. The key is to locate the 1½″ 45° ell. Begin by temporarily inserting a length of 1½″ pipe in the double wye and supporting it below the floor joist. Identify which part of the structure the 1½″ pipe to the bathtub P-trap will be parallel to. Typically, this pipe will be parallel to a foundation wall, and it may be parallel to a floor joist. Measure the distance (A) from the parallel surface to the point where the 1½″ pipe connects with the P-trap, Figure 19-23. Once this measurement is transferred to the 1½″ pipe (B), the laying length of the fitting is subtracted and the pipe can be cut to fit. Measure the length of pipe from the ell to the P-trap. Add the appropriate fitting allowance and cut the pipe. Procedures for installing the remainder of the pipe and fittings for the bath stack are the same as those described previously for the kitchen/laundry stack. Once the DWV piping is installed, it is A B Mark here To lavatories Foundation wall End of pipe to tub P-trap Goodheart-Willcox Publisher Figure 19-23. The length of the pipe connecting the double sanitary wye to the 45° ell of the bathtub can be determined by transferring dimension A to location B. Chapter 19 DWV Pipe and Fitting Installation 313 Copyright Goodheart-Willcox Co., Inc.