Digging out and installing a French drain, can help to alleviate problems associated with rainwater runoff, especially if your home and property are located downhill from other landscaped properties. Knowing some tips going into the process will make it easier and more effective. A poorly made French drain can possibly get over-saturated with draining water and can erode and get clogged over time. Avoid those occurrences as much as you can by doing the job right the first time.
Step 1: Dig the Trench
When digging the trench for a French drain, wider is better. Narrower trenches tend to cause the gravel or drainage pipe to clog up faster with silt. The wider you can make it the better. It means more digging, but it will make the drain last longer.
Step 2: Slope the Trench
The idea of a French drain is that it allows rainwater running down onto your property or towards your foundation to drain away in a desired course. Since water flows downhill, it is best to encourage it to continue to do so. A 100-foot French drain should slope 1% or 1 foot over that distance. Use a level to ensure the drain slopes downhill. Stake either end and string a line between them to guide your course.
Step 3: Lay Out Landscape Fabric
Lining the French drain with porous landscape fabric will help to keep silting soil from clogging the gravel and the pipe. A lot will be required, especially for longer trenches, but it will save you time and money in the future. The landscape fabric should be sized so that it will completely envelop the gravel once entirely filled in, so there needs to be plenty extra sticking out either side.
Step 4: Pour in 2 Inches of Gravel
Before you lay the pipe, pour in a 2-inch layer of washed gravel. Smooth out the first layer of gravel over the entire slope.
Although it’s not authentic, most French drains built today incorporate a water-permeable drainpipe at the bottom of the trench to help facilitate water draining. Slide the entire perforated pipe into a porous drainpipe sleeve and place it at the bottom of the trench. Where it exits should be at a location where the draining water will pose no threat to a foundation. The drainpipe should be rigid, not flexible. Make sure the perforations in the drainpipe face downwards in the trench. Use secure fittings where you are forced to join pieces.
With the pipe in place, you can fill in around and over it with more gravel. There should be at least 2 inches of gravel around, but 4 inches is better.
Step 5: Wrap Landscape Fabric Around Gravel
Wrap the loose edges of the landscape fabric around the gravel, making sure they overlap. You can now backfill in the trench and cover it with sod.
Done right, your French drain will act as a rainwater runoff bypass, preventing your backyard from being inundated from a higher-elevation property. It will also protect your foundation from water damage over time.