French Drain Clogged vs Failed vs Bypass Decision Tree

Why this matters

An existing French drain that has stopped working can be one of three things, and the repair scope ranges from a $400 jetting to a $15000 tear-out and replace. Diagnosing the cause from above ground without invasive inspection is half experience and half methodical observation. Picking the wrong path either burns labor jetting a drain that is structurally collapsed (clears for a week, fails again) or quotes a tear-out on a drain that just needed jetting and a daylight outlet repair. This tree walks the symptoms back to one of the three states (cloggable but intact, structurally failed, or being bypassed by groundwater) and tells you the test that confirms each.

Symptom presentation

Three patterns. First, the drain was working and recently stopped: water in the basement, sump pump running constantly during rain, but the drain outlet at daylight or in the discharge pit shows no flow during rain events. Second, the drain has been gradually getting worse for years: water has crept up the wall finish, the sump pump has been increasing in run-time, performance has degraded each season. Third, the drain runs but water still enters the basement through walls or floor: the drain has capacity issues, depth issues, or is being bypassed.

Quick checks at the door

Locate the drain discharge: a daylight outlet (gravity), a sump pump pit (mechanical), or a drywell. Inspect the discharge during or shortly after a rain event; a healthy drain delivers visible flow to the outlet when groundwater is high. If no flow at the outlet during high water inside, the drain is either clogged or has lost grade. Run a sewer camera into accessible ports (cleanout caps if present); look for soil intrusion, root mass, sediment beds, or collapsed pipe sections. Probe the soil along the drain path with a soil probe; saturated soil over the drain run during rain indicates the drain is not collecting. Ask the customer about events since last working: tree growth (root intrusion is the most common slow failure), heavy equipment traffic above the drain, recent grade changes, plumbing leak above the drain run.

Isolation tree

Branch A clogged but intact: camera shows pipe is round and continuous but has sediment or root mass blocking flow. Repair is jetting (high-pressure water jetting with a root-cutting nozzle if roots), followed by camera verification of clear pipe. If roots, recur within 1 to 3 years unless source tree is removed; advise customer. Branch B structurally failed: camera shows collapsed pipe section, deformed shape, broken segment, or void in soil around the pipe. Repair is excavation and replacement of the failed section (or full run if multiple failures). Pipe replacement should use a non-perforated pipe in the soil layer at the wall and perforated drain pipe in the gravel envelope per IRC standards. Branch C lost grade: camera shows pipe is intact but holds standing water. Cause is original install grade error or soil settlement under the drain. Repair requires excavation and re-laying of the affected run to proper grade (typically 1/8 to 1/4 inch per foot downhill toward the outlet). Branch D outlet bypass or failure: drain itself is fine but the daylight outlet is buried under landscape soil, the sump pump has failed, or the drywell is saturated. Repair is restoring the outlet path. Branch E groundwater bypass: drain is working at its installed depth but groundwater is rising above the drain elevation due to hydrologic change (rising water table, new upgradient development, neighbor's drainage discharging into yard). Repair is a deeper drain, a higher-capacity drain system, or a different approach (interior perimeter drain with sump pump pit if exterior cannot reach the water table).

Confirming diagnosis

Clogged-but-intact is confirmed by camera footage showing continuous pipe with blockages that move on jet or rod testing. Structural failure is confirmed by camera footage showing pipe deformation, voids, or breaks. Lost grade is confirmed by water level in the pipe at rest; a properly graded drain holds no standing water. Outlet failure is confirmed by tracing from the drain end to daylight; if buried, blocked, or pumping failed, the outlet is the problem. Groundwater bypass is confirmed by an exterior groundwater observation well or by elevation survey showing the drain depth is above the local water table during the wet season.

Remediation

Clogged-but-intact: jet the line at appropriate PSI for the pipe diameter and material (typical 2500 to 4000 PSI with appropriate nozzle for 4 inch corrugated, lower for thin-wall pipe); use a root-cutting nozzle for roots, repeat with a clean-out nozzle; camera-verify clean throughout. Schedule annual maintenance jetting if root intrusion is identified. Structurally failed: excavate the affected section, remove failed pipe, replace with new perforated drainage pipe of equal or greater diameter (typical 4 inch perforated SDR-35 or rigid corrugated PE), bed in clean drainage gravel wrapped in non-woven geotextile filter fabric, backfill in lifts with compacted soil, restore grade. Lost grade: same excavation, re-lay pipe at 1/8 to 1/4 inch per foot slope toward outlet, verify with a transit or laser level. Outlet repair: daylight a buried outlet (excavate, install splash pad and animal screen), service or replace sump pump (verify discharge piping is intact), evaluate drywell capacity (if saturated, may need expansion or alternative discharge). Groundwater bypass: install a deeper exterior drain, install an interior perimeter drain at the footing tied to a sump pump pit, or upgrade to a larger pump and discharge.

References

• IRC R405 Foundation Drainage requirements
• ASTM F405 Standard Specification for Corrugated Polyethylene Pipe and Fittings
• ASTM F758 Standard Specification for Smooth-Wall Poly Vinyl Chloride PVC Plastic Underdrain Systems
• IRC R401.3 Drainage requirements (lot grading and surface water)
• ASCE Hydrology and Hydraulics references for groundwater assessment
• Foundation Performance Association publications on basement drainage repair