Perforated Pipe — French Drain and Foundation Drainage

A perforated pipe is a drainage pipe with rows of holes or slots along its length that allow groundwater and surface water to enter the pipe and be carried away from the surrounding soil or gravel bed.

Perforated Pipe diagram — labeled parts and installation context

What It Is

Perforated pipe is the core component of subsurface drainage systems used to intercept and redirect water that would otherwise saturate soil, undermine foundations, flood crawl spaces, or damage landscaping. The pipe is buried in a trench lined with gravel and wrapped in filter fabric, creating a drainage pathway that collects groundwater from the surrounding soil through the perforations and carries it by gravity to a discharge point such as a daylight outlet, a catch basin, or a storm drain connection.

The perforations are typically round holes or elongated slots arranged in two rows along the bottom third of the pipe, positioned downward in most installations so that water collected in the gravel bed rises and enters the pipe rather than immediately flowing along the pipe surface. The surrounding gravel acts as a filter and provides void space that stores water temporarily while the pipe conveys it away.

Filter fabric wrapping, often called sock or geotextile sleeve, prevents fine soil particles from migrating into the gravel bed and eventually clogging the perforations. Without this fabric, silt infiltration over time reduces drainage capacity significantly.

Types

Corrugated flexible perforated pipe, typically made from HDPE, is the most common residential drainage product. It is lightweight, easy to cut and connect, and available in 3-inch and 4-inch diameters for most residential uses. Rigid PVC perforated pipe provides a smoother interior for higher flow rates and greater crush resistance, making it preferred under driveways and other loaded areas. Slotted pipe uses elongated openings rather than round holes and is used where finer drainage control is needed.

Where It Is Used

Perforated pipe is installed in French drain systems around foundations, under crawl spaces, at the base of retaining walls, in swales and low-lying lawn areas, at the toe of slopes, and beneath sports fields and agricultural fields. It is a key component of any subsurface drainage system designed to manage hydrostatic pressure against below-grade structures.

How to Identify One

In a trench, perforated pipe is identifiable by the rows of holes or slots visible along the pipe body. It is typically surrounded by crushed stone or drain rock rather than compacted soil. Above grade, the outlet pipe emerging at a slope or discharging into a catch basin may be a solid continuation of the same pipe run.

Replacement

Perforated pipe rarely fails structurally but can become clogged with silt, roots, or collapsed sections over time, reducing drainage performance. Jetting or rodding the pipe can clear soft blockages. Root intrusion or crushed sections require excavation and pipe replacement. Inspect by running water into a cleanout or inlet and observing flow at the outlet to confirm the system is draining properly.

Perforated Pipe — FAQ

Which way do the holes face when installing perforated pipe?

For most residential French drain applications the perforations are oriented downward so that water accumulating in the surrounding gravel bed rises and enters the pipe from below. This prevents the pipe from filling with soil-laden surface water. Some specific applications, such as leach fields, orient perforations upward for distribution rather than collection.

Do I need filter fabric around perforated pipe?

In most soil types, yes. Filter fabric or a perforated pipe sock prevents fine silt and clay particles from migrating through the gravel and into the pipe perforations. In very clean, coarse-gravel-only applications, fabric may be omitted, but in typical yard soils, the fabric significantly extends the life and effectiveness of the drainage system.

What size perforated pipe do I need for a French drain?

A 4-inch diameter perforated pipe is standard for most residential French drain and foundation drainage applications and handles the typical volumes involved. Three-inch pipe is used in lighter drainage situations such as small planting bed swales. For large drainage areas or high groundwater conditions, a hydraulic calculation may be needed to confirm pipe size.

How deep should a perforated pipe be installed?

The required depth depends on the problem being solved. For foundation drainage the pipe is typically installed at or just below the footing elevation. For lawn drainage a depth of 12 to 18 inches is common. For crawl space drainage the pipe sits at the lowest point of the crawl space floor. Local frost depth also affects minimum depth in cold climates.

How do I know if my perforated pipe is clogged?

Standing water in areas that the drain system was designed to control, wet soil around the outlet area, or water backing up at cleanouts are common signs of a clogged system. Camera inspection or a water flush test from a cleanout inlet can confirm whether blockage or a collapsed section is preventing flow to the outlet.