Author Options:

I have to put in a "French Drain" from my backyard to the front. So what do I need to do? Answered

I've dug the trench but I'm having problems get the right depth for gravity to take over the water flow.  What am I doing wrong?



 I generally go with 1"  per  10'  depending on the width of the ditch and the amount of surface water I'm trying to get rid of.   I use a transit and eyeball to set my slope whenever I have to dig one.   If you don't have access to a transit or a good level, try using a garden hose as a water level.  
  Pound a stake in the ground at the start and end points of your drain.  Once you've gotten the hose completely filled with water and one end attached to one of the stakes, you can use the water level to find the point that is the same elevation on you other stake.  Then just stretch a string line between the two points, and voila, you've just created a reference line from which to measure.
It seems like I saw a commercially available waterlevel attachment for garden hoses a few years back.  All it did was let air out of the hose instead of water so that you didn't  get it all over you as you were tromping around.
  I also always check my slope by letting the water hose run for a few minutes so I can find little dips and pockets.
  Be sure to use a good geo textile liner to completely line the ditch and cover the rocks; otherwise, it'll only work for a couple years before it plugs up.  I also use a "sock" over my pipe just for added protection.
Have fun.


8 years ago

This can be difficult to achieve by eye especially if you have a long run, on shorter runs a half bubble over 8-10' should be sufficent (use a long level)

 You can of course dig the entire trench deeper than needed, lay the pipe and support/backfill to achieve your slope. Its more physical work but less aggravation

if you don't have a long level, use a 12 or 24" one on a straight piece of lumber

ok, both steveastrouk and ravingmadstudios have it right. I'm combining thier answers and tossing in a sprinkle of my own.

As you know, a french drain is a fancy name for a drain trench filled with rocks, and all drains act to collect and control water. Your trench will need to slope from the highest point to the lowest to ensure drainage, I'm going say beef up the slope to 2% minimum to allow for proper drainage and prevent any problems associated with long runs and low slopes. Ravingmad has the right equation.

It's important to know that the low point of your trench should meet up at level with your perimeter drainage or your connection with the city and should not be any lower unless you plan on using a sump pump (which costs more if you can avoid it).

As for finishing your trench, line with textile, and cover with more textile if you plan on covering the rocks (crushed gravel, etc.)

Hope that helps, good luck.

+1 on the step up to 2% slope.

Take the time to do it 1% and you'll be angry if you've gotta do it over again.

2% is standard around here for earth, sidewalk, and pavement grading. There's a good reason for that. (everything settles)

Also line the trench with a suitable geotextile, fill the drain with rocks, and then fold the geotex back over the top of the rocks before you back fill with soil.

The geotex will significantly help keep the drain clear of silts and blocking


The trench needs to be sloped from the back yard to the front at a 1% grade. Pound a stake into the ground at each end of the trench, and tie a string between the stakes. Level the string using a line level, and use the string to measure your grade. A 1% grade drops 1 foot for every 100 feet of horizontal length.