What is a french drain and how does it work
I am asked this question almost every day by someone. What is a french drain and how does it work? A french drain is a hand excavated groundwater removal system that starts with the hand excavation of a gravity flow aquaduct with straight sides and a flat hard bottom.
The excavation is plumbed with a flow enhancing perforated pipe laid flat on the bottom of the aquaduct, usually a 3″ perforated pipe is adequate for residential home drainage solutions.
The french drain aquaduct is filled with 3/4″-1 1/2″ river rock and a layer of weed cloth laid shiny side up, placed 4″ from the top layer of the french drain.
The weed cloth protects the french drain from weeds invading your home drainage system, and prevents dirt and silt from accumulating in the layers of the river rock.
A layer of weed cloth is placed towards the top of the french drain to allow air passage and better groundwater passage to the bottom of the french drain on which 90% or more of the groundwater will actually run.
Never use a perforated pipe with a sock on it. This is for foundation footing drains, and clogs easily, as it also prevents the perforated pipe from collecting the rainwater fast enough in heavy rains. Just don’t go there. The pipe with the sock is not a good method in my opinion.
There are many ways that self professed french drain installers who are not educated to the fine points of home drainage can mess up the concept and performance of a french drain. There are literally dozens of convoluted concepts created in the name of home groundwater removal that are called french drains.
A true french drain is a hand excavated french drain as described above. French drains were always hand excavated, from the beginning, hundreds of years ago to the present. No exceptions.
A french drain must be engineered to cover no more lineal footage than would allow for the creation of a hard finished bottom within the french drain, a grade of 2″ per 10 lineal feet or more sloping to the vent, which is either a daylighted vent, sump well, or a hand excavated drywell.
If a drywell approximately 4 feet deep is used, the typical length for each french drain is approximately 50 lineal feet if the depth is 18″ at the drywell, and 8″ deep at the top. This would only apply to a french drain excavated on a flat surface.
If you have a continuous downhill grade which is in itself 2″ per 10 lineal feet or greater, you could follow the grade maintaining the same depth to a daylighted vent hundreds of feet away theoretically. If you are working with a flat surface over 50 feet long, create two drywells and split the grade with the french drain sloping from the middle out to both sides to drywells with the depths advised above.
If a hand excavated drywell that is 4 feet deep by 4 feet in diameter is used, which is most common, as OSHA will not allow workers in a hole over 4 feet deep without supported sides during excavation, the french drain groundwater removal system is hand excavated to a depth of 18″ at the drywell and slopes upwards from the hand excavated drywell as it is being excavated to the top of the hand excavated french drain system at a depth of 8″, which is a 2″ per 10 foot slope.
Read other articles on this website for instructions in the order of work accomplished and other fine points of hand excavated french drain installation. A french drain is a hand excavated french drain, 100% of the time. No exceptions.