Shoulder-to-shoulder dimensions provide a direct reading of the actual length of pipe required. Therefore, they are more accurate and save time because additional calculations are not required. However, this method is useful only with DWV fittings. Pressure fittings do not have shoulders. Center-to-center measurements require you to know the laying length of the fitting. The laying length is the amount of a run of pipe, measured along the centerline, that is taken up by the fitting, Figure 19-2. Therefore, if the center-to-center dimension is known, the length of pipe needed can be determined by subtracting the laying length of the fitting at each end of the pipe. Laying lengths for common types of fittings are given along with the fitting allowances later in this chapter. Various combinations of these measuring techniques are used. For example, you can measure face-to-centerline, face-to-shoulder, or shoulder- to-centerline. The key is to remember that any measurement taken to the face of a fitting requires adding the appropriate fitting allowance. Any measurement taken to the centerline of a pipe requires subtracting the laying length for the fitting to be installed. Plumbing codes include many detailed requirements for DWV piping. These must be applied correctly for the piping system to pass inspection. Quality of components, sizing of pipe, selection of fittings, location of cleanouts, location of vents, joining techniques, and supporting pipe are among the requirements that must be understood and applied. Face-to-face: Method of measuring pipe requiring that the plumber know the fitting allowance for the type and size of fitting being installed. Shoulder-to-shoulder: Method of measuring pipe in which dimensions provide a direct reading of the actual length of pipe required. Center-to-center: Method of determining pipe size in which measurements require you to know the laying length of the fitting. Laying length: The amount of a run of pipe, measured along the centerline, that is taken up by the fitting. Measure from face to face and add fitting allowance for each fitting. Measure from center to center and subtract laying length for each fitting. Face-to-Face Measure from shoulder to shoulder distance (DWV only). Shoulder-to-Shoulder Center-to-Center Goodheart-Willcox Publisher Figure 19-1. Three ways to determine the length of pipe needed between two fittings. Laying Length Laying Length Center-to-Center Laying Length Laying Length Goodheart-Willcox Publisher Figure 19-2. Laying length is the amount of a run of pipe, measured along the centerline, that is replaced by a fitting. 298 Section 3 Plumbing System Design and Installation Copyright Goodheart-Willcox Co., Inc.