Take the pressure off the wall.
A french drain is a gravel-filled trench, usually with a perforated pipe in the base, that intercepts water moving through the soil and redirects it somewhere harmless.
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The classic application is a line rather than an area: along the uphill side of a house, behind a retaining wall, or across a slope above something you want to keep dry. If you have waterlogged ground across a whole yard, sub-soil drainage is the broader version of the same idea.
Water building up behind a retaining wall is the single most common reason those walls fail. Saturated soil is heavier than dry soil, and it exerts hydrostatic pressure the wall may not have been designed to hold.
Signs to take seriously:
A french drain behind the wall relieves that pressure. It’s substantially cheaper than rebuilding a wall, and considerably cheaper than dealing with one that has failed. Where a wall has already moved, a structural engineer should be involved. That’s beyond plumbing, and you should be told so rather than sold a drain.
Describe what’s happening and we’ll tell you straight, including when the simple fix is the whole job.
The failure mode for a french drain is silting up, and it comes from skipping details:
Where the problem turns out to be surface runoff instead, a trench drain is the cheaper and better answer, and you’ll be told that.
Practically, they’re the same family. The distinction people use:
Describe the symptom and Voyager will tell you which you need. Often the answer is a combination.
Named after Henry French, an American who wrote about the method in the 1850s. Nothing to do with France.
Not strictly: a gravel trench alone will move some water. But a perforated pipe substantially increases capacity and is standard for anything beyond a very small run.
Properly built with filter fabric, many years. Without it, fines migrate into the aggregate and the drain silts up, sometimes within a couple of seasons. The fabric is the difference.
Drainage removes the cause, but a wall that has already moved is a structural question and should be assessed by an engineer. Voyager will say so rather than install a drain behind a wall that needs rebuilding.
To a legal discharge point, usually a stormwater connection or a suitable lower outlet. Establishing that is part of the assessment, and it’s the part that gets skipped in bad installs.
Usually. The common causes are silting, a collapsed line, roots, or an outlet that was never connected properly. Which one determines whether it’s a clean-out or a rebuild.
Free quote, a fixed price agreed before the work starts, and someone on the end of the phone at any hour.
Voyager Plumbing · NSW Contractor Licence 476657C
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Page reviewed and updated 2 October 2026
