French drains should not be plumbed into rain drain discharges
I hear it all the time. I call it “voo doo” drainage. The homeowner says, “I have recieved a couple of other bids from contractors for my french drain work. One of the drainage contractors suggested that I could just put the french drain into my rain drain discharges venting my gutters. What do you think of that”? Easy answer. Do not do that!
You will plug the rain drain discharges in time. Even if you could match up the grade successfully to accomplish it, which you probably would not be able to do, it would cause problems. I have discussed this practice in other articles within this home groundwater drainage blog.
The fact that some people on the internet talk about a grade of 12″ in 100 lineal feet within a french drain being adequate for groundwater removal does not make it a sufficient or the best way to accomplish the results of groundwater removal.
Do not use the perforated pipe with the sock over it as this will plug too easily. Use weed cloth placed well above the bottom of the hand excavated french drain, with exposed rock on the surface for the best water collection.
Do not wrap perforated pipe with road cloth or weed cloth. Leave the perforated pipe as it was designed, with the slits available to collect groundwater. Protect the hand excavated french drain with weed cloth placed above the pipe to allow groundwater flow down to the perforated pipe and collection of the debris on the upper level of the hand excavated french drain prior to the debris getting down to the depth of the perforated pipe.
Do not use gravel in your hand excavated french drain construction. Use 3/4″-1 1/2″ round river rock that is clean and lacks any fine material. This is not the cheapest rock, but it is the best. Gravel, pea gravel or 3/4″ minus has too much fine material in it. The hand excavated french drains won’t work well.
Use solid 3″ abs pipe from the rain drain discharges and perhaps connected to 3″ solid ads flexible pipe at times for twisting and turning to the hand excavated drywell or daylighted vent. This is the practical standard for downspout roofwater removal.
Perforated pipe from a hand excavated french drain could perhaps be merged with a solid pipe via a silt collection box, but that is not a secure way of keeping silt and sediment out of the rain drain discharges. It also flows groundwater towards the foundation instead of away from it, and spills the water out of the perforations before it can be collected in the silt collection box. Bad home drainage practice.The chance of the rain drain dishcharges plugging and failing becomes more a certainty than a possibility. It is for that very reason that the perforated pipe is used on the bottom of the hand excavated french drain as an overflow when large amounts of groundwater build up in the bottom of the french drain. Most of the groundwater runs on the bottom of the hand excavated french drain as it is finnished with a flat surface that is clean and like a piece of pottery.
The groundwater runs at a grade of 2″ per 10 lineal feet to the hand excavated rock filled drywell or daylighted vent. That equals 20″ of grade within a 100 foot section of french drain system.
As a practical matter I split the 100 foot hand excavated french drain run into two systems that are 50 feet each in length, if the area being treated is flat, and use twohand excavated drywells instead of one and two 50 ft. long hand excavated french drains.
The hand excavated french drain is approximately 8″ deep on the upper end of the system and enters the drywell at 18″ deep. The drywell itself is usually 4 feet deep. This creates more depth within the hand excavated drywells for groundwater storage prior to perking out. If the downhill grade is sufficient to use a daylighted vent, following the example of 100 lineal feet; continuing a sufficient groundwater drainage grade, this may be the most advantageous method of venting.
Don’t let those other professed drainage contractors talk you into the quick fix soon to fail scenario as described above. Never put french drainperforated pipe on the foundation or into the rain drain discharge venting system. In most cases this could not even be physically done. The depth of the rain drain discharge near the foundation where these contractors say they can connect the perforated pipe is too shallow.
I have reconstructed many of these failed french drain systems. Sloping french drain perforated pipe towards the foundation to connect them to the solid pipe rain drain discharges conveys groundwater against the foundation and makes more of a problem as the water spills out of the perforations as it travels to the proposed vent pipe wetting the foundation area all over again.