Why Your French Drain Isn't Working (And How to Actually Fix It)

A French drain that does not work usually has one of four problems: clay soil packed back on top of the stone, fabric that is clogged or the wrong type, too little slope, or no real place for the water to go. Fix those in that order. Check the outlet first, because a perfect trench with a blocked exit just fills up and sits there.

A homeowner in southeast Pennsylvania posted a good example on r/landscaping. He dug a trench the day before a big storm, laid two 4-inch pipes, connected one to a 12-inch basin, and lined the trench with landscape fabric. His slope was 1 inch of drop for every 8 feet over a 40-foot run. After the storm, water was still sitting on the lawn. He asked if he should move the basin or use a different fabric.

What the commenters found

The first question was about the soil on top. He had spread the clay from the trench over the drain, because that was easier than hauling it away.

Several commenters said that is the problem. Compacted clay acts like a plastic sheet, so water on the surface cannot get down to the stone. One suggested a test: pull the clay off the top of the drain and see if the water gets away. If it does, replace the clay with sandy soil or gravel. A second commenter said the same thing in simpler terms: you cannot drain through something the water cannot enter.

Other points came up too.

• One person asked where the drain exits. A drain with no outlet fills and stops, and the water has to go somewhere.
• Another said the yard was clay on all sides, so the water might have nowhere to soak in. The fix there is an outfall such as a ditch or a low point, or digging down to a layer that drains.
• Someone pointed out the fabric. They advised non-woven geotextile, which lets water pass through, rather than a plastic-like sheet.
• One person said the drain was brand new and might need a month to work, since it takes time to dry out the soil around it.
• A few suggested a rain garden at the outlet, or a gravel walkway over the drain.

That is a long list, and the answer is that the thread did not settle on one cause. The clay cap was the most likely. Here is how to check each item.

The four main failures

1. A clay cap or clay backfill

Water enters a French drain from above and from the sides. If the top is dense clay, the stone below never sees the water.

Landscape Drains says not to refill the trench with the dirt you dug out over the drain. LawnStarter’s method backfills with gravel to a few inches below the surface, folds the fabric over the top, and then adds topsoil or sod. The surface layer is thin and the stone is close to it, so rain gets in.

2. The wrong fabric, or fabric that clogged

Without fabric, soil gets into the stone and fills the gaps. With the wrong fabric, water cannot get in. Stormwater magazine explains the slow failure: clay and silt particles get caught in the geotextile and eventually stop water from passing, and fine particles that get past the fabric fill the voids in the stone.

Landscape Drains recommends non-woven fabric for drains, and notes that woven fabric is less permeable and suits ground cover or driveways. The homeowner’s drainage supplier told him his fabric was fine, but he never said which type it was. Check the label.

3. Too little slope

Sources disagree on the minimum. LawnStarter says about 1%, which is roughly 1 inch of drop per 8 feet. Landscape Drains also gives 1%. Stormwater magazine says at least 2 feet of fall per 100 feet of pipe, or 0.25 inch per foot, which is 2%.

The Pennsylvania owner had 1 inch per 8 feet. That is 1/8 inch per foot, or about 1%. It passes the first rule and fails the second. Over 40 feet it comes to 5 inches of fall, where the stricter rule wants 10 inches.

Too much slope has its own problem. LawnStarter says a steep run lets debris settle and clog the pipe, and can erode the outlet.

4. No outlet, or a blocked one

Stormwater magazine is blunt: to operate properly, the drain must have adequate slope and a place to open onto the ground. A drain that ends in a buried stone pit only works until the pit fills.

Landscape Drains adds that downspouts should stay out of the drain, and that a catch basin ahead of the drain field helps. If roof water is loading the drain, it may be overwhelmed in a storm.

How to tell which failure you have

The symptoms point to different causes.

Water sits on top of the ground right over the trench, and the soil there is hard clay. That is the cap. The water cannot get down to the stone.

The drain worked at first and slowly got worse over months or years. That fits clogging. Stormwater magazine describes fine clay and silt building up on the fabric and in the stone until water stops passing.

Water collects at one low spot along the run. That points to a sag in the pipe or a stretch with too little fall. LawnStarter lists standing water from improper slope and collapsed trenches from poor backfill among the usual causes.

The pipe fills in a storm and drains slowly afterward, or never. Look at the exit.

Worked example: the 40-foot drain

Say you are redoing the Pennsylvania drain with a trench 1.5 feet wide and 2 feet deep, in line with LawnStarter’s guideline of 1 to 1.5 feet wide and 1.5 to 2 feet deep, and deep enough to clear your frost line.

Trench volume: 40 feet times 1.5 feet times 2 feet is 120 cubic feet. Subtract about 3.5 cubic feet for a 4-inch pipe and you have roughly 116 cubic feet of stone. Divide by 27 and you get 4.3 cubic yards. At about 1.4 tons per cubic yard that is about 6 tons. The French Drain Gravel Calculator does this for any length and width.

Slope: 1% on 40 feet is 0.4 feet, or 4.8 inches. At 2%, it is 9.6 inches. The Drainage Slope Calculator gives the drop for your length.

Pipe: buy about 10% extra, as LawnStarter advises, so 44 feet.

Professional cost: LawnStarter’s average is $5,000, with most people paying $1,650 to $12,250. The French Drain Cost Calculator gives a quick ballpark.

The fix, in order

1. Find the outlet. Walk the line. Is the pipe open at the far end, and does water come out when you run a hose into the high end or the basin? If not, clear it or extend it to a ditch, a low spot or a dry well.
2. Check the slope. Stretch a string line with a level along the pipe or measure the fall between the two ends.
3. Pull the cap. Dig off the top 6 to 8 inches of soil over the stone. If it is clay, take it away. Replace it with gravel or sandy soil.
4. Test the pipe. If the pipe is crushed or full of mud, you may need to dig out that section and replace it.
5. Check the fabric. If it is woven or plastic-like, replace it with non-woven geotextile, or wrap the pipe in a sock if you use crushed stone.
6. Look at the holes. LawnStarter lays the pipe with the holes pointing down.
7. Keep downspouts out of the drain.

If the whole yard is clay and the lot has no outlet, Stormwater magazine suggests another option: surface drainage with a slope of at least 1.5% to 2%.

How to decide

Wait a few weeks after a new install. Then check after the next big rain. If water still stands, work the list above before paying for a new drain.

Call a pro if the yard is clay with no outlet, if the drain is under 8 to 10 years old and already clogging, or if water is reaching the house. Stormwater magazine says you may have to dig up and reinstall a French drain every 8 to 10 years, so a drain that has lasted a decade has done its job.

Sources

• Taking drainage solutions after I installed a French drain (r/landscaping thread)
• Stormwater Solutions: French Drains and Their Downfall (published 2010, updated 2019)
• Landscape Drains: 10 French Drain Mistakes
• LawnStarter: How to Install a French Drain in Your Yard (updated September 2025)