zero-slope drain line refers to grey-water drain piping that is installed on a level surface. Such lines require no slope to drain, but carefully calculated relations between length from fixture to drainage manifold, pipe diameter and number of elbow fittings. Based on testing and demonstrated performance, zero-slope drains have been certified for installation as part of specific infill products in some jurisdictions. by Prof. Dr. Stephen Kendall, Building Futures Institute, Ball State University