What is a French drain?

A French drain (also called a rubble drain or land drain) is a trench filled with gravel or crushed stone, usually containing a perforated pipe, that collects groundwater and surface water and carries it away to a soakaway, ditch, or watercourse. It works by providing a permeable path of least resistance for water to flow into, intercepting waterlogged ground or diverting water away from a building before it can cause damp problems.

When would you use a French drain?

French drains are used to: intercept surface water or groundwater running toward a building and divert it away before it can cause rising damp or penetrating damp; drain a waterlogged garden or lawn; relieve hydrostatic water pressure against a basement or retaining wall; and provide cut-off drainage at the top of a slope to stop water running downhill onto an area to be protected. They are a common first-line solution for persistent damp problems affecting ground floors and basements.

How deep should a French drain be?

The required depth depends on the purpose. A perimeter drain protecting a building from groundwater typically needs to be at least as deep as the base of the wall construction it is protecting - usually 600mm to 1m. A garden land drain intercepting surface water only needs to be 300-450mm deep. The drain must have a continuous fall of at least 1:200 to the discharge point to ensure water flows away and does not pond in the trench.

What pipe is used in a French drain?

The perforated pipe in a French drain is typically 100mm diameter perforated PVC land drain pipe or flexidrain coil. It is laid at the bottom of the trench on a bed of clean angular gravel (20mm or 10mm single-size) and surrounded by more gravel to within 150mm of the surface. The pipe and gravel are wrapped in geotextile membrane to prevent fine soil particles migrating in and blocking the system. The trench is then topped with topsoil or paving.

Drainage & Utilities noun

French Drain

/ frɛntʃ dreɪn /

Also known as: rubble drain, land drain, perimeter drain, cut-off drain

Definition

A French drain is a trench filled with clean angular gravel or crushed stone - usually containing a perforated pipe at the base - that intercepts groundwater and surface water and redirects it to a soakaway, ditch, or watercourse. It works by creating a permeable pathway of low resistance that attracts water away from adjacent structures or waterlogged ground. French drains are one of the most effective and economical solutions for damp problems caused by groundwater or poor site drainage.

In practice

The trench is excavated to the required depth and fall (minimum 1:200), lined with geotextile membrane, and filled with 20mm clean angular gravel to about two-thirds full. A 100mm perforated PVC land drain pipe is laid at the bottom, and more gravel is placed on top. The geotextile is folded over to enclose the gravel, and the trench is backfilled with soil or reinstated with paving.

For a perimeter drain protecting a building's foundations from groundwater ingress, the drain is positioned at the toe of the external wall, just below DPC level, and the water is led away to a soakaway or combined drain. The geotextile membrane prevents fine soil particles from migrating into the gravel and blocking the system over time - this is the most common failure mode of older French drains that were installed without membrane protection.

Building Regulations

Relevant PartPart H - Drainage and waste disposal

Minimum fall1:200 (0.5%) continuous to discharge point

Discharge pointSoakaway, watercourse, or surface water sewer (with consent)

Approved Document H requires that subsoil and surface water drainage is provided where it is needed to protect the building and its foundations from damage. French drains discharging to a watercourse or surface water sewer require consent from the sewerage undertaker or the Environment Agency. Where they discharge to a soakaway, the soakaway must be designed to BRE Digest 365. French drains may not discharge to a foul sewer.

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