French Drain Design

French Drain Design with PVC Pipe
Our 100 year old barn was in bad shape when we bought the property. Here's a link to my barn foundation repair article. Many errors were made when the original stone foundation was replaced by this concrete foundation in the 80s, but the main problem with the foundation was poor drainage around it.


Before, looking alone side of the barn from the back. Not sure if can see it here well, but the grade (slope) is running toward the foundation ("negative slope"). (Note: click on photos to enlarge them)


Before, looking along the side of the barn from the front. Perhaps this shows the negative slope towards the foundation better. You can also see the drip line cut in the soil, from the huge amount of rain coming off the barn roof, which would then permeate the ground, right next to the foundation.

The area next to this side of the barn foundation acted like a catch basin for about 2,000 square feet of the property next to it, including a poorly designed driveway and a standalone garage placed next to and on higher ground than the barn. Add to that another 900+ square feet of rain running off the barn roof, just on this side. Every year the ground would freeze and thaw, pushing the foundation in even further. And with that, the ground around the foundation slumped even more, making the drainage problem even worse.

French Drain Design
I decided against installing gutters on the barn, as they would take away from the original, historic design. A french drain design with PVC pipe was used for the drip edge, and a swale was created to divert water away from the barn foundation.

French Drain Design Diagram



Two sticks, and a string keep my outside line straight. I dug out from the foundation about a foot past the drip edge (in hindsight a bit more might have been better). I dug down about 18". While I dug, I used a long level to check the french drain slope, which should be 1/8" per foot, or of course more downward slope is fine too.


I then laid down think plastic, as this was a drip edge, and wanted to catch the massive amount of rain water coming off the barn (Note: I've read that you don't want the soil too dry next to the foundation, as that can cause problems too, but I didn't do much beyond the swale to divert ground water, so hopefully I've struck a good balance of removing rain from the roof, and some ground water coming under the plastic to keep the soil from completely drying out).

I then added about an inch of rock to act as a solid foundation for the french drain PVC pipe. On top of that I started to put in the PVC pipe, with the holes on the bottom side.

Why the Pipe Holes Should be Down and Not Up on Drainage Pipe

Yes, they go on the bottom side. Imagine the drain filling in from the bottom up, and when it reaches the holes, the water enters the pipe and runs away. If the holes were at the top, the water level would have to get to that height before it entered the pipe. The trench would fill with a lot more water, before it was drained away.

The roll of white fabric (weed barrier) goes over the pipe to keep weeds, roots and other debris from getting into the pipe. (Note: After I filled the french drain with rock, I cut the vertical PVC pipe down to size and added a cap to it, which is my access point, should they (there's a pipe like this on both sides of the barn) ever get clogged.)


At the other end french drain design, I stopped putting down the plastic and dug a simple trench for solid PVC pipe. The solid pipe goes to daylight after its well around the backside of the barn, where the water can run away.


Before I added the top layer of rock over the PVC, I check the entire french drain slope (1/8" per foot) with a level, adding rock below the PVC pipe where needed. I then cut off the excess plastic on both sides of the drain with a utility knife.


French drain materials and tools.


After the french drain was done, I went to work creating a swale further out from the barn foundation. Yes I did this with a shovel. Lots of work. You might want to consider a Bobcat rental. I didn't as I wasn't sure I could do what I wanted as I don't have Bobcat experience. You can see also that I covered my nice new french drain with clear plastic to keep dirt out of it while I worked.

Before Shown Again for Comparison to Below


After

Here's how the surface runoff water moves down into the swale and away from the barn foundation, or into the french drain. You can also see that I've cut down the vertical PVC pipe (on the right), leaving about 2" exposed above the rocks, in order to add a access cap that can be removed should I have to remedy a clog. You might wonder why I made the swale contour hump up right next to the drain, well, it's not ideal you are right. The reason is that I broke my back digging the 60 ft. of swale by hand, and I just couldn't bare digging any deeper in the dense clay, so I decided a few inches of surface water could run off to the drain side instead of going into the swale. Granted some will permeate into the ground and raise the water table next to the foundation, but I've read that you actually don't what the soil next to the foundation to be too dry, as it will crack and become less stable, so perhaps I've struck a happy balance between too wet and too dry. Time will tell.

The Results
This project was done in Nov. of 2005, and it's now Dec. 2008. The system has worked very well as far as I can tell. During heavy rains a small stream moves down the swale, while a heavy flow of water comes out the daylight ends of the drainage pipes on both sides of the barn. After three snowy Winters and three extremely wet Springs, the barn foundation has not shown any signs of movement, or damage.

After (not the best shot, there are some fence pieces leaning there, part of another project...)


All Worth it to Save this Barn


Materials
PVC Drainage Pipe
PVC Drainage Pipe Cap
PVC Drainage Pipe Outlet Cover
PVC Primer and Cement
Sandpaper
Drainage Pipe Weed Barrier
Plastic Sheeting

Tools
Hacksaw
Miter Box
Level

Resources
Ask the Builder: Drainage Articles
Ask the Builder: French Drain
Ask the Builder: French Drain Design (slope and depth for ground water)
DDA Construction: Design for to catch downspout water and ground water
How to glue PVC pipes
How to cut PVC pipes

My Standard Disclaimer: This is what I did. This does not mean this is the best way, the right way, to building code, or even safe for your needs. So you are on your own with your project. I make no promises about the information presented here. I'm just a do-it-yourselfer, not a professional at all, sharing my story. So if something goes wrong with your project, you are on your own. Good luck, and have fun!

All content and photos, copyright 2008, Dover Projects.

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