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What is a French Drain?

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What is a French drain? Depending on your location, you may also hear this humble gravel- or rock-filled trench referred to as:

  • Agricultural drain
  • Blind drain
  • Drain tile
  • French ditch
  • Land drain
  • Perimeter drain
  • Rock drain
  • Rubble drain
  • Sub-soil drain
  • Sub-surface drain
  • Weeping tile

Each of those terms refers to the same style of drain. If you’re still left wondering “What is a French drain?”, All will be revealed below.

What Is A French Drain?

In its simplest terms, a French drain is a usually-shallow trench, with a perforated hollow pipe (or pipes) set at its lowest point, that’s been refilled with gravel or rock.

The purpose of a French drain is seldom, if ever, to route wastewater away from its source to a point of disposal. Downpipes, sewers and other enclosed conduits take care of that function. Rather, French drains redirect surface water and groundwater, keeping it away from areas where it could do damage.

Water that’s passing over the surface, or moving through the earth, is interrupted by the trench. Instead of progressing toward the structure or installation protected by the French drain, that water then seeps downward, through the gravel or rock, until it enters the perforated hollow pipe(s). That ducting then routes the collected water away from the site where it might do damage, terminating it in a safe location.

As with most things, the simplest solution is usually the best; French drains have no moving parts to wear out, no automated assemblies to maintain, and cause no obstructions to the surface traffic going on above them.


French drains are most commonly used to exclude ground and surface water from damage-prone building foundations, or actively penetrating sub-surface voids (basements, parking structures and below-grade accommodations of all sorts). The same logic is applied to French drains used to protect retaining walls from excessive ground water pressure.

A very similar system is used to distribute run-off water generated by, or sent to, septic drain fields. In this application, the French drain may be at the system outlet, channeling away the purified water, or may be an integral part of the drain field itself, routing water around the arrangement and allowing it to gradually leach away.

Modern Evolution

In answer to the construction industry’s cry for ever simpler installations, that cost less in terms of time and inconvenience, the original French drain principle has been incorporated in pre-engineered systems that eliminate any need for infill.

With these designs, perforated pipes are sold already surrounded with a lighter-weight gravel/rock substitute wrapped around them. An outer skin of filter fabric also serves to hold the entire assembly together until it’s sited within the trench.


It should be noted that the words “virtually faultless” were used for a reason. What is a French drain fail-point? They can – and do – clog up, becoming progressively less effective with age and regular use. For all your Toronto-area drain and sewer cleaning and replacement services, including video camera damage and blockage location, one call to ExpressRooter Plumbing does it all.

What is a French drain? It’s our area of expertise. You’ll find direct local numbers here for our East, Central and West GTA offices, along with our Burlington, Hamilton, Mississauga, and Oakville locations; or you can email us at

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