What is the recommended Manning's “n” value for PVC pipe?

The appropriate conservative "n" value for minimum slope design of PVC sewer pipe is 0.009, as correctly justified by actual test data. The justification of this value is briefly described below.

For many years it has been popular and convenient to use a single "n" value for all pipe materials. However, no data technically support the single "n" value approach, and the most cost-effective sewer designs are precluded by such an over-simplified design criterion. The recognition of "n" value variations among the commonly used sewer pipe materials is long overdue.

No published technical study has ever reported an "n" value as high as 0.013 for a PVC sewer pipeline either in-service or in the laboratory. No published technical study has ever reported PVC sewer pipe as having the same hydraulic characteristics as clay, concrete or asbestos cement under any conditions. The major engineering textbooks dealing with sewer design have yet to address plastic pipe hydraulics. The ASCE and WPCF Manual for Design and Construction of Sanitary and Storm Sewers lists a range of "n" values for "plastic" pipe, i.e., 0.011-0.015, but the authors have not supplied any evidence supporting their recommendations. There is absolutely no scientific basis or technical justification for requiring the use of a 0.013 "n" value when designing PVC sewer pipelines.