What is the exception to the "one pipe diameter" slope rule for vent openings?

The exception to the "one pipe diameter" slope rule for vent openings pertains specifically to water closets. This rule generally states that the vent must be installed with a slope of at least 1/4 inch per foot to facilitate proper drainage and air movement through the plumbing system. However, water closets have unique requirements due to the potential for waste accumulation and the higher likelihood of blockages in their connection to the drainage system.

In the case of water closets, a different approach is often utilized. These fixtures typically require a larger venting system to accommodate the volume of waste they handle. As a result, the slope and size of the venting may deviate from the standard rule to ensure adequate performance. By allowing this exception, codes aim to maintain system efficacy and reduce the risks of malfunctions.

Other fixtures, such as urinals, bathtubs, and showers, do not have the same high-pressure waste disposal demands, allowing them to conform to the one pipe diameter rule without exceptions. This is why water closets are identified as the exception in this scenario.