How to Build 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. These systems have been used for over a hundred years to move unwanted water, and it’s a simple idea that 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 drain is straightforward.

Step 1 - Prepare and Measure

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 your local building codes and find out if you need any permits to excavate on your property. Also, call DigSafe (888-DIG-SAFE (344-7233) or go to their website, 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 slope downward along its entire length for water to flow correctly. Luckily, it doesn’t require a lot of slope (a 1% grade or one 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 has 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. Then, go to the far end and loosely tie the other end there. Attach your level and make sure the string is positioned correctly. Adjust the loosely tied end as needed and fasten it.

Now, since your drain needs to fall one foot for every 100 feet of its length, you can easily calculate how much it needs to go down along its length. For example, a 50-foot long drain needs to go down six inches in total. Simply move the string on the lower stake down the necessary amount and it will be a guide for the proper grade.

Step 2 - Build the Drain

Dig your trench six to 12 inches wide (the wider your drain, the more runoff it will able to handle) and about eight to 10 inches deep (following the grade) along your chosen route.

Roll out landscape fabric the entire length of your trench and then lay your piece of perforated drainpipe inside. In the end, you might want to attach a 45-degree elbow to angle a piece of pipe to the surface because this will allow you to flush the drain in the event of a clog without the hassle of digging it up.

Cover the pipe with a two-inch layer of gravel. The landscape fabric will help stop dirt from filling in the gaps in the gravel and blocking the water flow.

Finish your French drain by folding the rest of the landscape fabric over the top of the sand layer and filling in the drain trench with topsoil and grass seed or sod.