Two pipe french drain systems separate groundwater from roof water.
French drains can serve at least two purposes, if not more, at the same time.
One of the beautiful things about professionally installed french drains is that they often contain two pipes, installed side by side on the bottom of the french drain aqua duct excavation, which serves at least two purposes right away.
The solid 3″ black abs glued pipe, available at any Home Depot or Lowes, for example, is plumbed to accept the roof water from your downspouts above grade. This is called a rain drain discharge.
The water from your roof, while running on the same downhill grade as your perforated pipe is laid, on the bottom of the french drain, never mixes with the groundwater in the french drain. This increases the capacity and speeds the flow of the french drain that is professionally installed and engineered to fit the particular home site it needs to serve.
The home roof water, channeled through the gutters, downspouts, and rain drain discharges, runs in solid pipe all the way to the dry well, or day lighted vent.
It never reduces the french drain capacity for groundwater removal, by adding more water to the flow on the bottom of the french drain.
The groundwater removal is another issue, handled by a professionally installed grade excavation and a perforated pipe system.
The 3″ perforated pipe is laid next to the solid 3″ rain drain discharge pipe, on the bottom of the flat clean finished french drain aqua duct bottom.
The perforated pipe speeds the flow of the groundwater through the system during heavy rain events, as groundwater already flowing quickly away, additionally builds in height within the french drain aqua duct.
Real french drains are always hand excavated, for results, safety, and long lasting quality.
A quality professionally installed french drain is most often 12″ in width, and will have a minimum grade of approximately 2 inches per ten lineal feet, down hill away from the home.
This means if your design limits the length of your proposed french drain to 50 feet or less, your engineering on a flat surface, for a french drain installed around 18 inches away from the homes foundation, should be as I have stated above.
A french drain should be approximately 6-8 inches deep at the top shallow end of the french drain groundwater removal system, and 18″ deep at the dry well end of the system. The dry well is a round flat bottom river rock filled, hand excavated tank, basically, carved into the earth, with a hand excavated french drain feeding it from the side, where the groundwater will perk into the earth on the bottom of the dry well faster and faster as it breaks in, usually days to months, depending on the amount of groundwater that is put into it.
Hence the name, “dry well. A well always going dry, is the context for that word.
This is your quality, professionally engineered french drain. It is a natural perk system.
A professionally engineered french drain will insure that the groundwater will flow on the bottom of the french drain first. The french drain will run groundwater on that bottom most often, except during days of hard rains, without rising into the perforated pipe at all.
When it rains hard, the pipe speeds the flow.
This cannot be accomplished with any machine, or by excavating a sloppy mud laden ditch that has a grade of half what is required, or no grade at all. People call those french drains too. Caveat emptor, let the buyer beware.
Those failed systems are called ditches, not french drains. They come to be as a result of ignorance and greed, in many cases. Homeowners are famous for installing sump pumps as well as buying into contractors who feed their supposition, while all the time, these homeowners are simply shooting themselves in the foot.
When the groundwater entry into the french drain aqua duct is great, the groundwater flows faster because of the perforated pipe, due to the perforations in the pipe, which pull the rain created groundwater into the perforated pipe, as it builds up within the french drain aqua duct, only during heavy rains.
The largest amount of groundwater in your french drain should already be flowing like a river on the bottom of your french drain, before any groundwater even starts building up in your perforated pipe.
Excavating a clean hard bottom and sides for your french drain will determine your failure or success with groundwater removal.
The choice of installation area, method, order of work, how to proceed safely, and many other considerations weigh into a successful home drainage french drain installation.
Results, or lack of results removing groundwater is the standard. Not whether some dude cons you into thinking you have a “guarantee” for something.
You either stopped the groundwater, or you did not. Nothing in between really.
It will take months to longer for below grade areas, such as a crawl space or basement, to dry out once saturated.
Once saturated, and covered with 6 mil plastic under your home, these areas do not take much to become somewhat less of a problem again, only weeks later once saturated, even after french drains are installed, because it does not take much groundwater at all to kick it off again once saturated.
When homeowners install french drains before the crawl space or basement is flooded, they are at least a year or more ahead of the curve, when many of their neighbors are swimming in their crawl space looking around, wondering what to do. Installing perhaps the wrong solution to the problem, and living over groundwater for months to forever, as long as they own the home.
Hand excavating straight sides and a flat bottom to form the french drain is important for aqua duct stability and long lasting quality, as well as the speed with which the groundwater drainage system removes the rainwater.
As soils vary in compaction, composition and wetness, your challenges multiply, requiring additional experience, like having been there before, if at all possible.
Knowing what to do safely is your objective as a do it yourself french drain installer.
Read articles within this site on the installation specifics of french drains and how to maximize your groundwater removal, and save your hard earned cash, with a groundwater removal system that will last decades, and out perform the rest on each rain thereafter, long after the other systems have failed and been replaced with some other dumb method, solving nothing.