Case file — Montgomery Village, Montgomery County, MD. One of the 25 jobs in our 250-home series. New here? Start with our open letter.
At a glance
| Location | Montgomery Village, Montgomery County, MD |
| The problem | Water collecting along one stretch of foundation, with no clear route away from the house |
| The fix | About 25 feet of perforated drain along the wall, a buried downspout, and about 15 feet of solid pipe carrying everything to a pop-up emitter |
| The detail that matters | More than half as much pipe carrying water away as collecting it |
Two kinds of pipe, and most people only hear about one
A French drain quote usually talks about one number: how many feet of trench.
But there are two entirely different pipes in a working drainage system, and mixing them up is one of the most common ways a drain fails within a few years.
Perforated pipe collects. It is drilled with holes along its length and laid in gravel inside a fabric-lined trench. Water in the surrounding soil finds its way through the gravel, through the holes, into the pipe. That is the 25 feet along the wall on this job.
Solid pipe carries. No holes. It takes the water the perforated pipe collected and moves it somewhere else without letting any of it back out along the way. That is the 15 feet leaving this house, ending in a pop-up emitter that opens under water pressure and closes flat when the rain stops.
What happens when the second pipe is too short
Everything the drain collected gets released fifteen feet closer to the house than it should have been.
Which, on a property where the ground falls back toward the foundation — and that is a great many DMV properties — means the drain becomes a loop. It collects water at the wall, carries it a short distance, lets it out, and the water walks back. The system works perfectly and achieves nothing.
This is not a rare failure. It is the single most common thing we find when we are called to look at somebody else’s drain that “never worked.”
Why this house needed fifteen feet
Because that is how far away the first place was that water could safely come out and keep going away from the building.
You do not decide that from a catalogue. You walk the ground, you look at where it falls, and you find the point where water released there will continue downhill rather than pool or return. Sometimes it is five feet past the corner. Sometimes it is forty feet across a yard. On this house it was about fifteen.
The downspout, again
A buried downspout was part of the same job, taking roof water underground from the gutter and into the same carrying line.
We keep finding this. The most expensive drainage problems on a property very often have the cheapest possible contributing cause: a downspout emptying at the foundation, adding hundreds of gallons at exactly the spot you are trying to keep dry, during exactly the storm when the ground is already saturated.
If you take one thing from this whole series: go outside next time it rains hard and watch where your downspouts actually put the water.
The question to ask
When somebody quotes you a French drain, ask them to walk you to the spot where the water will come out, and stand there with you.
If they can point at it and explain why water released there keeps going, you are talking to somebody who has looked at your property. If the discharge is described as “away from the house” without a location, the drain has not been designed yet.