New foundations where a full basement is present, requires that a coating of waterproofing be applied to the wall areas below grade to keep water from penetrating through the wall. If the foundation is constructed of concrete masonry units, a coat of cement plaster is also required before the waterproofing is applied.
At the exterior bottom of the foundation wall a “cant” is installed using either mortar or a pre-made fiber material. A cant is shaped as a triangle and helps shed water that runs down the foundation wall, and forces it away from the wall and into the footing drains. The contractors excavator must dig a “tail” ditch from a foundation corner to a point lower than the footing where the footing drain can exit at daylight.
This distance may be only a few feet or perhaps a hundred or more feet. The tail ditch slopes downhill the entire way to assure good water flow. The excavator will now place a three inch or so layer of three quarter inch gravel all the way around the foundations exterior approximately twelve inches wide. Once the gravel is flatted out by hand, the finished height should be just below the interior basement floor elevation by about five inches. It is not necessary to compact this gravel.
The contractor then installs four inch PVC piping laying it against the side of the concrete footings with the necessary ninety degree elbows to complete a closed circuit until it reaches the tail ditch outlet corner. There, a Tee fitting is installed to allow the pipes to be joined coming from both directions and also leaves an opening for a third pipe to exit the tail ditch.
The PVC pipe used around the foundation is perforated with two rows of holes on one side. These holes face downward when finished. Do not let anyone talk you into leaving the holes face up for the water “to fall into”. It also allows all the sand, silt and earth that washes down from above to fall into the pipe and thereby quickly plug it solid.
Water when building up around the foundation will find the holes on the bottom of the pipe and will easily enter into the pipe void. Since water seeks it own level, with the drainage piping installed at a slight downward slope, the water will flow to the lowest exit point. Even if the footing drain piping is installed dead level, the tail ditch which is on a down slope, will allow the water to flow to the daylight end of the pipe. It is the path of least resistance. It is much easier for the water to follow the pipe then to push through a small crack in a foundation wall or floor.
Once all the piping is installed, a second layer of gravel is placed to completely cover the piping by four or five inches. You cannot see the pipe when finished. Once the gravel work is complete a layer of felt paper, hay or perhaps red rosin paper is placed atop the gravel to prevent penetration by the earth as much as possible.
The tail ditch piping is solid PVC and requires no holes. An alternate to PVC solid pipe is perforated flex pipe. It is a little easier to handle and comes in one hundred foot rolls but is harder to keep in shape around the walls. Flex pipe also comes in a solid version as well for the tail ditch. As foundation back filling progresses make sure no large rocks are placed on top of the footing drains as over time they will sink down and crush the pipe making it useless.
If an existing foundation has a water problem the work can be quite intensive to install new drains. Excavating around an existing building requires a good deal of skill by the hoe operator.
The ground is usually unstable with wet soils making safety a larger concern and in many cases requires over excavation to provide safe earthen slopes for people to work nearby. The foundation must be scraped clean and power washed if no waterproofing is present on the walls.
Once all the cracks are patched and new waterproofing is installed, the drain piping installation is the same as with a new basement.