What slope should underground ducts have according to code?

Underground ducts are required to have a slope to ensure proper drainage and prevent the accumulation of water, which can cause issues like corrosion and reduced airflow. A slope of 1/4 inch per foot is specified as it effectively allows any condensation or water that may form inside the duct to drain away efficiently. This particular slope is a standard widely recognized in building codes because it balances the need for drainage with practical considerations for installation and maintenance.

While other options do exist, they either do not provide enough slope (like the option with no slope or 1/8 inch per foot), which might not adequately facilitate drainage, or they present too steep a slope (such as 1/2 inch per foot), possibly leading to other complications such as creating stresses in the duct material or making installation more challenging. Therefore, the 1/4 inch per foot slope is generally seen as the optimal choice to maintain the integrity of underground duct systems.