Ideal Slope Range
The slope of the French drain should be at least 5 percent, while the maximum recommended slope is 3 percent. Washington State University recommends, for systems with 4-inch perforated PVC piping, the slope of the French drain be between 5 and 1 percent.
The Colorado State University Extension suggests a slope of at least 1 to 3 percent for a French drain with only gravel or rock. This means that for every 100 feet of horizontal distance the trench goes, the bottom of the trench should drop between 5 and 3 feet in elevation.
Implementing the Slope
Once the trench has been roughly shaped, use a shovel, mattocks or other digging tools to finish the bottom. Pound wooden stakes into the bottom on either end of the trench and tie a string onto the stake where the upper end of the trench bottom will be.
Stretch out the string with a string level and locate the point on the other stake where the string is level. Measure down on the stake and mark where the bottom of the trench should be.
For example, if the planned slope is 1 percent and the trench is 50 feet long, make a mark 6 inches down from where the level string hits the stake. Once enough soil has been moved, tie the string at this spot and move soil around so the string touches the bottom of the trench uniformly.
Under certain circumstances, it may not be feasible or practical to run a properly sloped French drain to an outlet. In this case, the sloped French drain can lead to a sump pump or dry well.
These options may be most suitable where there is limited space in the landscape, no outlets for nearby drainage, or if the drain is very deep and the amount of earth-moving that would be required to dig a progressively deeper trench does not outweigh the installation of a pump near the foundation.
Additional French Drain Specifications
Several other aspects of French drain design and construction will influence the drain's ability to function effectively. In silty or unstable soils, consider lining the trench and topping the gravel with a filter fabric that will allow the passage of water while blocking sediments.
This will typically prolong the working life of the drain. A 4-inch perforated PVC or corrugated plastic will encourage faster draining.