French Drain is a gravel-filled trench — normally containing a perforated drainage pipe wrapped in geotextile — that intercepts and redirects subsurface water. French drains are widely used in the UK to dewater wet ground, protect foundations and embankments, drain sports pitches, and manage garden run-off.
How a French drain works
Water in the surrounding ground flows through the surrounding geotextile filter, into the high-void gravel fill, and along the perforated pipe at the base of the trench. The pipe carries the water to a discharge point — a surface water sewer, soakaway, watercourse or rainwater garden — at a controlled gradient.
Components and typical design
- Trench excavation — typically 300–500 mm wide and 600–1,000 mm deep.
- Geotextile filter wrap — non-woven needle-punched fabric wrapping the gravel fill on all sides.
- Gravel fill — clean, hard, angular 20–40 mm gravel filling the trench.
- Perforated pipe — DN 80–160 mm corrugated or twin-wall plastic pipe at the base of the trench, laid to a fall of 1:200 or steeper.
UK applications and sizing
French drains in UK contexts serve five common applications: foundation drainage around new and refurbished buildings; highway and railway cutting drainage; sports pitch drainage; agricultural land drainage; and SuDS infiltration trenches where ground permeability is sufficient.
For SuDS applications, French drain design follows the principles of BRE Digest 365 (where infiltration is intended) and CIRIA C753 (where the drain discharges to sewer or watercourse).
Related ViaCon solutions
ViaCon supplies the geotextiles, geocomposites and drainage pipe components used in French drain construction. See our stormwater solutions and geotechnical solutions. Related glossary entries: soakaway, geotextile, permeability and surface water drainage.
Frequently asked questions about french drain
What is a French drain?
A French drain is a gravel-filled trench containing a perforated drainage pipe wrapped in geotextile filter fabric. It intercepts and redirects subsurface water — used for foundation drainage, highway cuttings, sports pitches, agricultural land and as a SuDS infiltration trench. UK design follows CIRIA C753 and BRE Digest 365 principles.
How deep should a French drain be?
Typical depths are 600–1,000 mm depending on application. Foundation drains should sit at least 200 mm below the lowest floor level. Highway and field drains are typically 600–800 mm.
Does a French drain need a perforated pipe?
Modern practice is to include a perforated pipe at the base of the trench. The pipe dramatically increases hydraulic capacity, makes maintenance possible (CCTV inspection, jetting) and provides predictable flow rates.
Do French drains need geotextile?
Yes, in nearly all UK applications. The geotextile filter wrap prevents fines from migrating into the gravel and clogging it. Without geotextile, a French drain typically loses 70–90% of its hydraulic capacity within 5–10 years.