A French drain that has stopped working has almost always silted up, and where the silt is determines whether you can flush it or have to dig it. A properly built drain — perforated pipe, clean gravel, filter fabric, real fall, real outlet — should run for decades with occasional attention. One built without all five elements starts failing within a couple of seasons.
The three ways a French drain silts up
From the outside in. Soil fines migrate into the gravel and fill the voids. This is what filter fabric exists to prevent, and it is why a fabric-free installation on clay has a short life. Once the gravel is packed with silt, water can no longer reach the pipe.
From the inside. Sediment enters the pipe — often through an inlet or a poorly protected outlet — and settles wherever the fall flattens. A low spot in the pipe becomes a sediment trap, and the trap grows until it blocks.
From the ends. The outlet is the most common single failure point and the easiest to overlook. Mulch, leaf litter, soil wash, an animal nest, or grass growing over the mouth will close it off, and once the outlet is blocked the whole run backs up regardless of how well it was built.
Roots are the fourth cause
Trees and large shrubs planted near a drain line find it. Roots enter through the perforations or at joints, seeking exactly the water the pipe is carrying, and then grow into a mat that catches everything else.
Willow, maple and any vigorous shade tree are the usual culprits. This is why planting over a French drain should be limited to shallow-rooted groundcover and perennials.
What can be flushed and what has to be dug
Start at the outlet. Clear it fully, then run a hose into the upstream inlet or cleanout and see whether water arrives. This one check resolves a surprising share of 'failed' drains.
If water flows but slowly, the pipe interior is likely partly silted. Smooth-wall pipe can usually be flushed with a hose or cleared with a drain snake, and a professional jetter handles heavier deposits. Corrugated pipe is much harder to clear, which is one of several reasons to avoid it for conveyance.
If the outlet is clear and water still will not move, the problem is either a collapsed or back-pitched section of pipe, or gravel that has silted up so completely that water cannot reach the pipe in the first place. Neither can be flushed. Both mean excavation.
Clogged gravel is the one that hurts, because it means rebuilding the trench rather than repairing it.
Routine maintenance that prevents most of this
Keep the outlet clear and visible. Check it seasonally and after any storm that moves debris around. A stone apron and a rodent grate at the mouth help considerably.
Rinse out catch basins connected to the system once or twice a year. The basin is designed to trap sediment below the outlet pipe, which only works while there is room in the sump.
Keep mulch, soil and turf from creeping over inlet grates. Never mulch across a grate.
Flush the line with a hose annually if the system has a cleanout, and watch how quickly water reaches the outlet compared with previous years.
Signs the original install was the problem
Some drains never had a chance, and it is worth knowing before you pay to maintain one.
- It failed within two or three years. Properly built systems do not.
- There is no filter fabric visible when you excavate. On clay this is decisive.
- There is no pipe at all — just gravel in a trench.
- The outlet is higher than parts of the run, or there is no real outlet at all.
- The whole run is perforated pipe, including the section that should have been solid conveyance to the discharge point.
- Corrugated pipe with no cleanout anywhere along a long run.
Repair or replace
Blocked outlet, silted pipe, root intrusion at one point: repair. A localised dig, a clean section of pipe and a proper outlet often restores a system entirely.
Missing fabric, no pipe, no fall, no outlet: replace. Money spent maintaining a system that was built wrong is money spent twice.
Southern Style Turf & Design inspects existing drainage as well as installing new systems across Lake Norman, and will say plainly which of the two you are looking at.
Frequently asked questions
Why did my French drain stop working?
Almost always silt. Either soil fines have clogged the gravel, sediment has settled in a flat section of pipe, or the outlet has become blocked by mulch, leaf litter or grass growth.
Can a French drain be flushed out?
A silted pipe interior can often be cleared with a hose, a snake or a jetter, especially in smooth-wall pipe. Gravel that has clogged with soil fines cannot be flushed and has to be dug out.
How long should a French drain last?
Decades, when built with perforated pipe, clean gravel, filter fabric, consistent fall and a genuine outlet. Systems missing any of those often fail within a few years.
What maintenance does a French drain need?
Keep the outlet clear, rinse out any connected catch basins once or twice a year, keep mulch and turf off inlet grates, and flush the line annually if there is a cleanout.
Want this looked at on your property?
We give free, no-pressure on-site estimates across Denver, Mooresville, Cornelius, Davidson, Huntersville and Troutman.
