How far apart should land drains be?

Drain spacing depends on soil type: heavy clay 5m, clay 7m, loam 10m, sandy loam 15m. In a herringbone pattern, lateral drains run at 45-60 degrees to the main collector pipe. Closer spacing is needed for heavier soils with lower permeability. Always test soil permeability before designing a drainage system.

What is a herringbone drainage pattern?

A herringbone pattern has a main collector pipe running down the centre (or along one edge) with lateral drains running diagonally at 45-60 degrees from both sides, like a fish skeleton. This is the most efficient layout for draining large areas. The collector pipe should run in the direction of the natural fall to the outfall.

How deep should land drains be?

Land drains are typically installed at 600-900mm depth. Minimum depth is 450mm to avoid damage from cultivation or foot traffic. The collector pipe should be deeper than the laterals to ensure water flows by gravity. A minimum gradient of 1:200 is needed (5mm fall per metre). Check the outfall level first — this determines maximum drain depth.

What gravel do I need for land drains?

Use clean, washed pea gravel (10-20mm) to surround the perforated drainage pipe. Fill the trench to approximately half depth with gravel (around the pipe and above it). Cover the gravel with geotextile fabric to prevent soil washing into the gravel and clogging the system. Backfill above with topsoil.

What size collector pipe do I need?

The collector pipe is typically 80-100mm diameter perforated twinwall pipe for domestic/garden use. For larger areas or commercial land drainage, use 100-150mm. The collector must be larger than the laterals (usually 60-80mm). Use solid (non-perforated) pipe for the final run to the outfall to prevent water re-entering the pipe.

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Land Drainage Calculator — Pipe Layout, Gravel & Trenching

Calculate land drain pipe quantities, herringbone layout spacing, gravel fill and excavation for field drainage.

Field Length

Field Width

Soil Type

Drain spacing: 7m

Layout Pattern

Pipe Price

Perforated twinwall land drain

Gravel Price

How We Calculate This

This calculator determines the pipe layout, gravel and excavation quantities for a herringbone or parallel land drainage system.

Layout calculation

Lateral Runs = ceil(Field Width / Drain Spacing)
Total Pipe = (Lateral Runs x Field Length) + Collector Length
Drain spacing determined by soil type: 5m (clay) to 15m (sandy).

Trench fill

Gravel fill surrounds the perforated pipe in the lower half of the trench. Geotextile membrane wraps the base and sides to prevent soil ingress.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.