How French Drains Work

An overview of subsurface drains

Whether you’re trying to protect a multi-million dollar structure or just keep your basement dry, subsurface drains get the water out without getting the soil out too.

Practical Engineering is a YouTube channel about infrastructure and the human-made world around us. It is hosted, written, and produced by Grady Hillhouse. We have new videos posted regularly, so please subscribe for updates. If you enjoyed the video, hit that ‘like’ button, give us a comment, or watch another of our videos!

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DISCLAIMER
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This is not engineering advice. Everything here is for informational and entertainment purposes only. Contact an engineer licensed to practice in your area if you need professional advice or services. All non-licensed clips used for fair use commentary, criticism, and educational purposes.

SPECIAL THANKS
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This video is sponsored by Nebula.
Stock video and imagery provided by Getty Images, Shutterstock, Pond5, and Videoblocks.
Tonic and Energy by Elexive is licensed under a Creative Commons Attribution License
Video by Grady Hillhouse
Edited by Wesley Crump
Voice of Henry French by Wesley Crump
Produced by Ralph Crewe
Graphics by Nebula Studios

[Inflorescensse]

The hardest job I ever did was digging a French drain around my house built in 1780. The roots the roots!!!

[spddiesel]

I've worked construction for 30+ years, and one mantra that's served me well is "you cannot stop water; you can only direct it." Forget who told me that, but I hope I thanked them.

[austinwagner]

One of my favorite fields of study is Engineering Philosophy. Questions like "to what degree is the engineer responsible for protecting idiots from themselves?" Based on that criteria, I would absolutely call you a philosopher, Grady. You have helped shape my perspective on engineering ethics, and I teach high school engineering, so your philosophies and opinions (no matter how mild) are permeating multiple generations of engineers. Keep up the good work!

[randyhaley]

I am a Stormwater Management/Sediment Erosion Control Inspector for the state of MD, and we don't allow French drains for the exact reason Grady stated. No matter how much you try to regulate sand/gravel size, any smaller sediment particles above, and in contact with, larger stones, will enter and clog the spaces between the gravel. We typically use very similar devices like Bio-retentions/Micro-bios in residential areas between houses. There is a whole world of mildly interesting devices and structures used just for moving around the water that comes from the sky, and the ultimate goal is to have the water end up in the exact same place it was before it was developed. Its also always changing, and things we were doing 5-10years ago is already considered out of date, but awesome video as always. Absolutely nailed all the nuance in this field! Keep up the great work!

[CaravelClerihew]

Redirect water for me like one of your French drains

[cobhallagames]

You don't just explain the Science but 'Why it matters'. That's philosophy in my book.

[hjldffp]

all my life I've been confused by the phrase "weeping tile" because us non-specialists think of tiles as flat. But you showing tiles built like pipes explains a lot.

[alecrabedeau]

"It made a heck of a mess, pardon my French drain" is one of the all time great Grady jokes 😂

[whyjnot]

You have to admire the simplicity of the humble French drain. I'm from Connecticut and we have tons of these all over the place. Truly, you cannot understate the amount of rock in the soil here. So its no wonder why we have so many of them. It makes it pretty easy to see how well they work as long as you know what to look for.

[AlanTheBeast]

Just learned from this: here in Quebec we call them "Les drains Français" where we should be calling them "Les drains French".

[tonytavoularis]

I work as a geotechnical engineer in Canada, some municipal governments have made it a policy that new additions to houses or house rebuilds that are larger than the original building footprint require a soak away pit (if the sub soils are favourable enough that is). I have spent many hours watching water infiltrate into the ground when doing in-situation testing for these pits, truly riveting stuff.

[napoleonjewfro]

So during a piece of road construction I was inspecting a few summers ago in northern Michigan, we suddenly tapped into a spring that we didn't know was there late in the afternoon and needed to be ready traffic the next day for paving. I was pretty inexperienced at the time and had no idea what to do but one of the construction guys on the crew suggested using a French drain. I knew two things, water flowing underneath the road would ruin it and the guy talking about the drain knew more than I did. We were there until 4am adding stone and blanket. Let my boss know the next day and he was happy with the French drain

[AbstruseLoL]

I absolutely adore the way you convey information in your videos. From engineering handbooks to failure analysis, your use of references and practical demonstrations always leave me with a feeling of greater understanding. I'm not even a civil engineer (electrical over here), but the content you deliver is always interesting and widely accessible.

[NickCotter-bfhl]

As an environmental engineer, this is one of my favorite channels. I deal entirely with groundwater but never in the context of how it interacts with and affects structures. Thank you for your amazing content, Grady!

[keatoncampbell]

Man I helped my old man put a few French drains in his yard, some to help channel water across a dirt mountain road, and another to help drain a pond retention embankment/wall/ pile of dirt. All of them work most of the time, and the first year they even drained well during the high rain periods in the fall, with lots of leaf litter. Now, although they drain well most of the time, during the really rainy months they don't have enough flow due to clay infiltration of the geotextiles.
If only I had know to put some sand in, it wouldn't have taken any longer, we already had to transport the gravel and dig trenches with equipment.

If only we had known! The drainage guys we talked to said the drains we did should be more than enough, as they are very wide!
Now at least we can stop the seepage coming from the retention embankment before it becomes liquifaction

[thefluffymarshmallo]

When I was younger, I watched my dad build a French drain and he told me the holes had to go on the bottom. The entire thing completely baffled me and it was a burning question for _years_ about how it worked. Thanks for finally answering the question I had when I was younger, but forgot about 😅

[Mr_Feeny]

I just finished installing a French drain in my yard, and it’s working great. During the digging process, I discovered that my yard was flooding due to a large granite slab just below the surface. The combination of the slab, clay soil, and sod erosion made the water retention issue clear. Since my dig, I’ve learned not to take porous soil for ‘granite.’ 🥁

[RaindropsBleeding]

I used to install French drains. Common practice was to drill the holes in two lines at 90 degrees from each other, and orient them down at 45 degrees off center on both sides. This left a channel in the center for water to flow

[salimufari]

2:20 I would think to pay even a bit more attention to those tabs on the left. This shows how powerful the primary message of proper drainage can effect the overall system.

[zachsimsphoto]

Literally calling companies two days ago to get quotes for a french drain. This video gives me so much info on what I need to know. Thanks for posting.