What is the minimum diameter required for sub-soil drains made of plastic pipe?

The minimum diameter required for sub-soil drains made of plastic pipe is 4 inches. This is based on standard plumbing and drainage codes, which specify that a minimum size helps ensure adequate flow capacity and prevents clogging. Sub-soil drains, also known as French drains, are designed to manage groundwater and reduce surface water accumulation, so having a larger diameter aids in effectively handling the volume of water that can flow through the system. A diameter of 4 inches is generally sufficient for most residential applications, allowing the system to function well without being prone to blockages.

For context, smaller diameters might not provide the necessary capacity for drainage, while larger diameters, although they could theoretically work, are typically unnecessary for standard applications and may lead to increased costs and installation complexity.