Illinois Plumbing Code Practice

What is the minimum grade required per foot for horizontal drainage piping of 3 inches or less in diameter?

1. 1/4 inch

2. 1/2 inch

3. 3/4 inch

4. 1 inch

The correct answer is: 1/4 inch

The minimum grade required for horizontal drainage piping of 3 inches or less in diameter is indeed 1/4 inch per foot. This standard is established to ensure proper drainage flow and prevent clogging in smaller diameter pipes. A slope of 1/4 inch per foot allows wastewater to flow efficiently towards the drainage outlet, thereby minimizing the risk of stagnation and potential backflow. While larger grades may seem beneficial for faster drainage, they can actually cause issues by creating excessive velocity, leading to material erosion or inefficiency in carrying solids. A 1/4 inch slope strikes a balance, promoting adequate drainage while reducing the likelihood of problems associated with higher slopes. The other options present steeper slopes, which are not necessary and could create complications in the plumbing system.