Case file — 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 County, MD — a school building, not a house |
| The problem | Water reaching a long run of below-grade wall on a building that cannot simply be closed for a season |
| The fix | About 110 linear feet of exterior French drain, with drain board and termination bar across the full run, plus two buried downspouts |
| Scale | The longest single drain run in this series |
The same system, four times the length
Most of the drainage work in this series is residential: fifteen feet here, forty feet there, matched to the stretch of wall where water was arriving.
This one is a school building, and the run is about 110 feet.
The physics do not change with the size of the building. What changes is that every decision gets multiplied by 110 — every foot of trench that is dug a little too shallow, every joint in the pipe, every section where the backfill is not properly compacted. On a fifteen-foot drain a mistake is a mistake. On a 110-foot drain it is a pattern.
What went in
The trench — one foot wide, one foot deep, run the full length of the affected wall, lined with filter fabric.
The pipe — perforated drain pipe set in the fabric-lined trench and surrounded by gravel, so water in the soil reaches the pipe and silt does not.
Drain board and termination bar across the whole 110 feet — this is the line item that separates this job from a residential trench, and it is worth explaining.
Gravel backfill, compacted from a foot below grade up to the finished surface.
Two buried downspouts, so the roof water on that elevation goes underground and away rather than onto the ground above the new drain.
What the drain board and termination bar actually do
A trench with a pipe in it collects water that is already in the soil. It does nothing about water running down the face of the wall itself.
Drain board is a dimpled sheet fixed against the below-grade wall. It gives water a vertical path straight down to the pipe, held off the wall by the dimples, instead of soaking into the wall on its way past. Water that touches the board goes down; it does not go in.
The termination bar is the strip that fastens and seals the top edge of that board to the wall. Without it the top of the board is an open lip — a funnel, collecting surface water and delivering it behind the board, which is precisely the opposite of the point.
Drain board without a termination bar is a very expensive way to make the problem worse. It is also, unfortunately, not rare.
Why a building like this gets the full treatment
On a house you can often solve the problem with the trench alone, because the water is arriving through the ground and the wall itself is sound.
On a longer, older, institutional wall you are usually dealing with both at once: groundwater in the soil and surface water running down the face. Splitting those into two jobs and doing one of them is how a building ends up with a drain that works and a wall that still gets wet.
The part that matters for a homeowner reading this
The line between “exterior French drain” and exterior waterproofing is not as clean as the two names suggest, and some companies use the gap to their advantage.
If you are quoted a drain, ask whether anything is going against the wall, and what holds its top edge. If the answer is nothing, that may be entirely correct for your house — but it should be a decision somebody made, not a detail that went missing.