How to Build a French Drain

I use only a pipe that is designed for yard drainage and french drains. Unlike the black pipe with the tiny perforations, you see at the big box store. The blue pipe I use has 17sqin of inlet per ft. and has 4 perforations or slots per valley plus a much higher crush rating than even double wall corrugated pipe. Not sure exactly if the crush rating is better than double wall corrugated.

I also use no less than 2 perforated pipes at the bottom of my trench which brings me to how a French drain is constructed. There are many different ways of building a French drain and there’s still a lot of old school thought out there. Probably one of the most incorrect and unfortunately common methods constructing a French drain is to First, dig your trench with the desired slope and end up with a messy trench bottom. Second, add 4 to 6 inches of gravel or pea gravel or even septic rock. Then install a 4-in corrugated perforated black pipe on top of your A inches of rocks; some may use the 4-in PVC with the holes in it. They’re both just as useless and worse from one another.

When it comes to a yard drain with high demand. It’s at this point a lot of people will add a sock over the pipe or even “tie in” your downspouts, but both are incorrect. After that, you would install more gravel on top of the pipe and most would finally cover up all of their mistakes with a layer of dirt so no one could see what they had done. If any contractor is installing a French drain in this manner or any similar fashion then they are doing their client a disservice and quite frankly mistreating them in many cases because the contractor knows there are better ways of constructing a French drain and if he doesn’t know a better practice then he would be doing himself a disservice as well does any contractor that’s worth anything is doing some sort of research on the subject.

So by now he must be wondering how to construct this French drain that can last a century or more with little to no maintenance at all and that actually performs better as time goes by. Well, first, and obviously you dig your Trench and the desired location which is in the low point of your yard some people will want to add soil to the low spots in their yard to improve drainage which some cases is okay, but in reality it just moves water standing in one place to standing in another place so you use the low points in your yard as a collection point and that’s where your trench should go but not just any Trench.

This trench needs to be flat on all three sides starting with at least 1% grade at the beginning and ending with hopefully 2% grade. Now achieving a flat bottom with even a trencher can be difficult for some. If you’re using an excavator, you want a spotter in the trench and it’s his job to watch for buried cables which sometimes can become quite technical if you’re not able to pull Slack and end up having to cut the cable to achieve proper slope and or pipes most notably irrigation after you have graded your Trench to 1 to 2% slope.

This is when you line your trench with 4 oz non-woven geotextile Filter Fabric and it’s best to opt for filter Fabric that has been needle punched and bonus points if you can find some that has been double needle punched which increases the flow rating tremendously. In this step a lot of contractors love to leave out because good Filter Fabric can be expensive and it takes time to master the technique of keeping the fabric from wrinkling so it can be time consuming but it’s necessary as this is your soil separator.

After you’ve gotten your non woven goes textile Filter fabric pinned into place you go ahead and install your purpose built yard drain pipe and any other pipes you’d like to add. Just don’t make the mistake of “tieing in” your gutter downspouts. You’ll need a separate pipe for this if not then any debris from your gutters will clog and contaminate your french drain pipe.

We remove failed systems often because of this common mistake and I have used both PVC schedule 40 and yellow corrugated as a separate solid pipe that are run from deep catch basins coming from the downspout. I won’t get into incorporating catch basins into a French drain here; you can contact me for more information regarding gutter downspout.

The corrugated pipe is easiest to work with so it’s what I usually will use. Just make sure you have good slope on the corrugated pipe so that it works properly:

Once you’ve made all your connections and you’re happy with your design and layout you add your rocks. Now here is where a lot of homeowners and contractors alike will mess up a french drain. You don’t wanna use just any rocks, you only want washed clean round river rock. Round rocks will not compact like pea gravel or crushed limestone round washed river rocks will always maintain large voids for more capacity in your system.

So when installing your clean round river rocks you’ll want to leave about 2-3 inches for either your sod to go back on tip or if it’s possible 1 to add more Rock’s which in this case you’ll have an open French drain. An open French drain is beneficial if you have a lot of sheet water coming off of a hill or driveway and would have trouble getting that water into a surface or yard drain.

Most of what I build here in west Tennessee will be an open French drain due to the flow of water being uninterrupted by a layer of soil with a high clay content and grass. Okay, so now that you have your nice neat flat bottom drench with Filter fabric yard drain pipe and your clean river rocks it’s time to pin the fabric at the top encasing all your hard work so that the system is clean and functional for decades to come.

Now, this is a brief description of how foundation Drainage by Design and how I custom design complex drainage systems for our clients. To find out the rest of the little known trade secrets that go into building a custom French drain or yard drainage system that will last forever with no maintenance then you’ll have to call and schedule an assessment to have your custom drainage system designed for your situation.This can help you end the drainage problems you are experiencing and help with drying out your soggy yard.

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