Determine which areas require improved drainage next to your home or building. Next you will need to locate the "high" and "low" points of your proposed trench or drain. The high point will be the beginning of the french drain and the low point will be where the water will exit.
Use the slope of the property to your advantage. Select the exit point to be a location that is naturally lower in elevation relative to the beginning of the drain. This will make for much less digging and save lots of time, especially if you're digging with shovels.
Measure the linear distance between those two points. This will be the total length of the drain (including bends or turns, if you have any). If you do have bends, keep them to 45 and 90 degree angles. This will make your life so much easier when you buy the connectors for your drain pipe.
Keep the drain 4-6 feet from the side of your home/building. If the drain is closer than 4 feet, you could actually pull water into your basement and/or crawlspace.
Draw a topographical illustration to get a better idea of how your French drain will look. If drawn to scale, your illustration may help you discover problems before you begin digging.
Check with the authorities and codes to find out if there are any obstructions that lie in the path of the newly proposed drain. You definitely don't want to cut any lines or break any pipes that are critical to your local infrastructure. The easiest way to do this is by dialing 811 (similar to dialing 411 for information) to notify local utility companies of your intent to dig and they will mark buried lines for you.
Begin at the high point of the trench and start digging. About every 3 linear feet, check your level and make sure the ditch is sloped properly. In order to obtain the proper drainage, the bottom of the drain will need to be sloped to allow water to flow freely and carry dirt, mud, leaves, or other debris through the pipe without clogging. Typically the slope of the drain will need to be one quarter inch per linear foot of drain.
Using a 4'x4' board, a tamp tool, or your feet make sure to pack the dirt down at the bottom of the drain and recheck the slope of the trench. To ensure that the drain is sloped properly, turn on the water hose and stick it in the drain at the beginning and watch the water to see how well it flows to the exit. After turning off the water, watch as the drain dries up. If you have a really low point in the trench, water will sit and pool. You may have to dig out the dams that are creating the pools.
Once satisfied with the drainage, install garden fabric to line the drain to help prevent roots from growing into your trench. After you have filled the entire length of the ditch with 1 to 2 inches of pea gravel, pack the gravel and recheck the slope. Its good to make sure the gravel follows the slope of the trench.
Perforate the thin wall PVC drainage pipe (3 or 4 inches in diameter). In order to allow the water to drain properly, the pipe will need to have holes (1/2 to 3/4 inches in diameter) drilled the length of the drain. Keep in mind that the holes need only be on the bottom of the pipe -- not on the sides or the top. This may seem counter intuitive, but this is critical to proper drainage and clog prevention. If the holes are drilled into the top of the pipe, roots can grow into them or dirt/debris can fall into the pipe causing clogs.
You can purchase drain or pipe sleeves or socks that you can install over your perforated pipe that will filter out rock, dirt, and debris from entering the holes and clogging your drainage system. You can find these at any local gardening store or depot.
Once holes are drilled and facing down, its time to connect and install the thin wall PVC drainage pipe on top of the pea gravel. Gluing the pipe pieces together is not required, but can prevent headaches if the pipe pieces separate once you have installed the pipe sock/sleeve. Pushing the pipes back together in those pipe socks can be aggravating.
If you are still afraid of clogs, install a cleanout point or two at convenient locations in the system. This will allow you to snake the drain pipe if you suspect clogging is an issue.
Fill in the trench with rock (preferably sized to 1 inch in diameter) over the pipe up to a level that is about 2 to 4 inches below the surface of the lawn.
With the remaining soil that's been excavated, fill in and level off the area over the drain.
To further improve drainage, move the remaining/leftover soil to the area between your newly installed french drain and your house or building. Slope the dirt up towards the house. This will allow the water to slope towards the drain and away from your home or building.
Things You Will Need
- Trencher and/or Shovel
- 36" or Longer Level
- 3" or 4" PVC Drainage Pipe and Associated Connectors
- Pea Gravel
- Approx. 1" diameter sized rock (#57)
- Call 811 before you start digging to avoid inadvertently disturbing any buried utility lines. Utility companies will come to your house to mark buried lines.
- Try to dig after a recent rain. The soil will be easier to dig up.
- Rent a trench digger -- it's much easier than digging by hand.