Drainage & Excavation · Nashville, TN

What a French Drain Should Actually Be

A French drain is a simple thing built wrong remarkably often, and the failures are all predictable.

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August 25, 2026 · Michael Blair

A French drain is a gravel-filled trench with a perforated pipe, collecting subsurface water and carrying it somewhere lower. Each element has a specification, and skipping any of them shortens its life considerably.

Clean angular gravel

Angular stone locks together and leaves large voids for water. Rounded pea gravel packs tighter and drains worse, and fines mixed in clog it.

Washed and angular is the specification, and it is the most common substitution.

Geotextile

Fabric separates the gravel from the surrounding soil so silt does not migrate in and fill the voids. Without it a drain silts up over a few years and stops working.

It should wrap the whole gravel envelope rather than only lining the trench bottom.

Clean angular gravel, not rounded pea gravel
Geotextile wrapping the whole gravel envelope
Perforations down, contrary to intuition
Continuous fall to a daylight outfall
Cleanouts so it can be maintained

Perforations face down

Counterintuitive and correct. Water rises into the pipe from the gravel below, and holes facing down let it enter at the lowest point rather than requiring the trench to fill first.

Pipe laid holes-up is a common error and it means the system only works once the gravel is saturated.

The outfall

Continuous fall to somewhere lower, daylighting where water can leave. A drain discharging into a low point that does not drain has simply relocated the puddle.

Cleanouts at the ends let it be flushed, which is what keeps it working in the long term.

FAQ

What a French Drain Should Actually Be — questions

Should French drain pipe have holes facing up or down?

Down. It is counterintuitive but water rises into the pipe from the gravel beneath, and holes facing down let it enter at the lowest point. Laid holes-up, the trench has to fill before the pipe does anything, which delays and reduces its effect.

Why do French drains stop working?

Silt migrating into the gravel and filling the voids, which happens within a few years without geotextile wrapping the whole envelope. After that: rounded rather than angular gravel, insufficient fall, or an outfall that does not actually drain. All four are avoidable at installation.

Do I need cleanouts on a French drain?

They are worth having. A drain that can be flushed can be maintained; one that cannot has a fixed life. Cleanouts at the ends and at any change of direction cost very little at installation and are impossible to add afterwards without digging it up.