What fitting should be used instead of a single or double sanitary T fitting in a nominally horizontal soil-or-waste pipe?

The correct choice is a single or double wye with a 45-degree angle fitting because it is specifically designed to allow for a smoother flow of wastewater in a horizontal soil or waste pipe. The wye fitting facilitates the connection of multiple branches into the main line, ensuring that flow dynamics are optimized, minimizing turbulence and reducing the potential for blockages. The 45-degree angle helps to direct the flow more efficiently than a standard T fitting would, allowing for better drainage and less likelihood of build-up at the junction points.

Using a fully vertical or horizontal T fitting can create issues such as stagnation or inadequate flow, which can cause clogs or backups in the system. The wye's design addresses these concerns, promoting a natural flow that is essential in waste management systems. Additionally, this fitting is compliant with plumbing codes that dictate the proper methods for joining pipes in horizontal installations.