What is the maximum allowable length of a 2" horizontal waste arm from the combination wye and 1/8 bend fitting to the fixture connection before a revent is required?

The maximum allowable length of a 2" horizontal waste arm from the combination wye and 1/8 bend fitting to the fixture connection is indeed 5 feet before a revent is required. This regulation is in place to ensure proper drainage and venting of the waste system.

In plumbing, it is essential to maintain adequate venting to prevent siphoning of the trap seal and to allow for the smooth flow of wastewater. The horizontal waste arm must have a limited length to ensure that the pressure differences created by the flow of wastewater do not create a vacuum that would draw water out of the trap.

A length of 5 feet is specified because it ensures that there is enough vertical drop for gravity to assist in waste flow while minimizing the potential for any clogs or backup. If the horizontal waste arm exceeds this length without adequate venting, there is a risk of creating negative pressure conditions that could compromise the drainage system's effectiveness.

Therefore, the maximum length of 5 feet is critical for maintaining the integrity and functionality of the plumbing system, directly relating to the importance of proper venting and drainage practices.