How To Build a French Drain

By Murray Anderson

On to Part 1 - Planning a French Drain

If you have a wet soggy area on your property or perhaps your basement is subject to periodic flooding, a French drain may be the solution to your problem. French drains have been used for over a hundred years to move unwanted water and the simple idea still works well.  Basically, a French drain is a small downward sloping ditch filled with rocks or gravel to prevent soil erosion, that channels unwanted water from one area to another. Other than the fact that actually digging the trench requires a lot of physical effort, building a French drain is straightforward. Here’s how

Thins you’ll need

Shovel or spade
¾” aggregate gravel
Sand
Landscape fabric
Wooden stakes
Line level
String
Tape measure
Sod

Building your French Drain

• Start by figuring out where your excess water can safely be routed. You don’t want to have your runoff water ending up on your neighbor’s land or you could be creating a larger problem than just having a wet area on your property.
• Check you local building codes and find out if you need any permits to excavate on your property. Also, call DigSafe (888-DIG-SAFE (344-7233) http://www.digsafe.com/ and have your local utilities come out and mark the routing of their facilities on your property.
• Once you know the location of the utilities, you can determine the best route for your drain, running from the wet area down to the outlet point. For ease of digging you want the route to be free of trees, shrubs, large rocks and of course any buildings like a garden shed.
• Your drain needs to be sloping downward along its entire length for water to flow. Luckily, it doesn’t require a lot of slope (a 1% grade or 1 foot down for every 100 feet in length is sufficient) but the grade must be consistent or you will end up with water pooling in your drain.
• You can easily measure the grade your drain using two wooden stakes, some string, a string level and a tape measure. First, drive one stake in at the top end of your drain and fasten the string to it. Next go to the far end and loosely tie the string there. Now, attach your string level and level the string, adjust the loosely tied end and fasten it. Now, since your drain needs to fall 1 foot for every 100 feet of it length, you can easily calculate how much it needs to go down along its length. (For example a 50 foot long drain need to go down 6” in total over its length).  Simply move the string on the lower stake down the necessary amount and it will be a guide for the proper grade.
• Dig your drain 6” to 12” wide (the wider your drain, the more runoff it will able to handle) and about 8” to 10” deep (follow the grade) along your chosen route.
• Roll out landscape fabric the entire length of your trench then cover it with a 2” layer of gravel topped by a 2” thick layer of sand. The landscape fabric will help stop dirt from over time filling in the gaps in the gravel and blocking the water flow.
• Finally, finish your French drain by folding the rest of the landscape fabric over top of the sand layer and fill in the drain trench with topsoil and grass seed or sod.

Murray Anderson is an experienced freelance writer with over 500 articles published on the web as well as in print magazines and newspapers in both the United States and Canada. He writes on a wide range of topics and is a regular contributor to DoItYourself.com. He can be contacted at murand@lycos.com.