Designing a French Drain Grid for Clay Soil Yards

A small front yard (25×40 ft) in Vancouver with heavy clay soil stays wet long after rain. The homeowner wants to know the effective spacing for French drains.

Grid Layout for Clay

In low-permeability clay, a single drain line only affects a narrow zone. A grid or herringbone pattern of parallel drains spaced 4–6 feet on center, all connected to a single trunk line, dramatically increases the capture zone.

Components

• Perforated pipe (4-inch) wrapped in filter sock or surrounded by gravel and fabric.
• Trunk line (solid pipe) to storm drain or daylight outlet.
• Cleanouts at junctions for maintenance.

Product Resource

NDS Pro offers modern, high-flow drain components (EZ Flow, EZ Drain) that simplify installation in tight spaces.

Key Point

The closer the spacing, the faster the water table drops. For a 25-ft deep yard, 4–5 parallel lines tied to a side-yard trunk line will effectively dry the entire area.