What is the minimum height for a clothes washer standpipe above the trap?

The minimum height for a clothes washer standpipe above the trap is specified as 18 inches. This height is necessary to prevent siphoning, where water can be pulled from the trap due to a negative pressure created by the washing machine pump.

By setting the standpipe at this height, it ensures that there is sufficient space for the water to discharge into the standpipe while maintaining the necessary air gap to prevent backflow into the washing machine. This height also allows for proper drainage and aligns with plumbing codes which aim to maintain sanitary conditions within the plumbing system.

Maintaining proper heights for standpipes is essential for the effective functioning of the drainage system, as it helps avoid issues like clogs, odors, and the potential for wastewater to enter living spaces.