What is a French Drain?
A French drain is a gravel-filled trench with a perforated pipe that collects and redirects groundwater away from foundations, basements, and low-lying areas. Named after Henry French, who popularized the technique in 1859, a French drain works by gravity: water seeps through the gravel, enters the perforated pipe, and flows to a discharge point such as a storm drain, dry well, or daylight outlet at a lower elevation. French drains are one of the most common drainage solutions in residential construction and are frequently encountered during property inspections and rehab projects.
Types of French drains
Exterior foundation drains ($5,000-$15,000) run along the outside of the foundation footer, intercepting water before it reaches the foundation wall. This is the most effective type and is standard in new construction. Retrofitting an exterior drain on an existing home requires excavating around the perimeter, which adds cost but provides the best long-term protection.
Interior footer drains ($3,000-$10,000) are installed inside the basement perimeter, beneath the slab. They collect water that seeps through the foundation and direct it to a sump pump, which pumps it out of the building. Interior drains are the most common retrofit solution because they do not require exterior excavation.
Yard drains ($1,000-$5,000) address surface drainage problems in low spots, along driveways, or between structures. These are shallower trenches designed to prevent standing water and soggy ground rather than protect a foundation.
How French drains work
The system relies on a simple principle: water follows the path of least resistance. The gravel backfill provides a porous channel that collects water from the surrounding soil, and the perforated pipe at the bottom of the trench carries it away. A filter fabric (geotextile) is typically wrapped around the gravel to prevent soil and sediment from clogging the pipe over time. Proper slope is critical -- the pipe must descend at least 1% grade (one inch per eight feet) toward the discharge point for gravity flow to function.
Real estate implications
The presence of a French drain on a property is generally a positive indicator. It means a drainage issue was identified and professionally addressed rather than ignored. When evaluating a property with water concerns -- such as standing water in the yard, damp basement walls, or efflorescence on foundation surfaces -- budget for drainage installation if no system is present. Proper drainage ($3,000-$15,000) costs far less than the damage that chronic water exposure causes: mold growth, foundation deterioration, wood rot, and pest infestations.
Maintenance and lifespan
French drains last 15-30 years with minimal maintenance. Over time, silt and root intrusion can reduce flow. Annual inspection of discharge points and sump pump function (if applicable) helps catch problems early. Replacement involves excavating and re-laying the pipe and gravel, which costs roughly the same as original installation. When analyzing a rehab property, check the age and condition of existing drainage systems and factor replacement into your repair estimate if the system is more than 20 years old.