Case file — Silver Spring, Montgomery County, MD. One of the 25 jobs in our 250-home series. New here? Start with our open letter.
At a glance
| Location | Silver Spring, Montgomery County, MD |
| The problem | Water in the soil against one elevation of the foundation |
| The fix | About 40 feet of exterior French drain — trench, fabric, perforated pipe, gravel — and a buried downspout |
| Why it is in this series | Because it is completely ordinary, and you cannot recognise an unusual quote without knowing what a normal one looks like |
Nothing went wrong on this job
No mold. No standing water. No structural surprise, no second trade, no change order. One wall, about forty feet, water in the soil, a drain to take it away.
We put it in the series deliberately. Every other case here is about something unusual — a crack that came back, a drain that needed a pump, a crawl space we deliberately did not seal. Those are only useful if you know what the baseline is.
This is the baseline.
Layer one: the trench
Dug alongside the foundation, at a depth and distance decided on site rather than in the driveway. How far out the trench sits depends on where the water is arriving from — which is why the scope for this item literally says the distance is assessed on site by looking at where the gutters put water.
The trench also has to fall. A level trench is a tank.
Layer two: the fabric — the one that decides everything
The trench is lined with non-woven geotextile before anything goes in.
This is the cheapest material in the whole job and the one that determines whether the drain is still working in ten years, so it is worth being specific about what it does.
Soil is not just dirt, it is dirt plus fine silt, and moving water carries that silt with it. Without a fabric barrier, silt migrates into the gravel, fills the voids between the stones, and eventually plugs the holes in the pipe. The drain does not break. It slowly stops draining, over three to seven years, and by then the company that installed it is a name on an old invoice.
The fabric lets water through and holds silt out. Non-woven matters: it is the type that filters rather than merely separating.
A drain installed without it will pass any inspection on the day it is finished.
Layer three: the pipe
A 4-inch perforated pipe, set in the fabric-lined trench. Holes down, laid to the fall of the trench, so water in the surrounding ground reaches the pipe and then goes where the pipe goes.
Layer four: the gravel, and the compaction
Gravel backfilled around and over the pipe, then compacted up to finished grade.
The gravel is not filler. It is the reservoir and the path: it holds voids for water to move through, far faster than soil could, so water reaching the trench gets to the pipe instead of sitting against the wall waiting.
The compaction matters for a reason nobody mentions at quote time: uncompacted backfill settles, and settled backfill makes a trough along the side of your house that collects surface water and points it at the foundation. A drain that creates its own puddle is a drain that was finished in a hurry.
And the downspout
One buried downspout, carrying roof water underground.
It is on almost every drainage job in this series, and we are going to keep saying it: the single most common water source at a DMV foundation is the roof, not the ground.
How to use this page
Take any exterior drainage quote you have been given and check it against those four layers, plus a discharge point.
If a layer is missing, ask why. There are real answers — some walls do not need drain board, some soils are clean enough that people will argue about the fabric. But the answer should exist. What you are testing for is whether the person who wrote the quote knows what each layer is for, or is selling a trench by the foot.