All you need to know about - French Drains - November 17th 2010

All you need to know about - French Drains

For many of us who aren't familiar with a French Drain, a French Drain is a simple and effective sloping trench dug in the ground, filled in with gravel or rock which will redirect rain water and ground water from one area to another.

French drains commonly contain hollow pipes that run along the bottom of the trench that allow water that seeps through the gravel to drain away quickly. French drains are primarily used to prevent ground and surface water from penetrating or damaging building foundations.

Alternatively, the French drain technique may be used to distribute water, such as that which flows from the outlet of a typical septic tank sewage treatment system. French drains are also used behind retaining walls to relieve ground water pressure.

What makes the French Drain ideal is that it requires little to no maintenance. Variations on the French drain model include:

- Filter drain to drain the ground water
- Collector drain (or interceptor drain) combines groundwater drainage with the interception of surface water or run-off. A collector drain could connect directly into the underground pipes (to rapidly divert surface water); however, a collector drain should have a cleanable filter to avoid sending surface debris, to the underground area, to clog the buried pipes.
- Dispersal drain distributes the waste water from a septic tank
- Fin drain uses a perforated pipe with a thin vertical section (the fin) of drainage composite above. The advantage is that the fin drain is narrower (200mm or 7 inches) than a traditional French drain (450mm or 17 inches and up), and cheaper to build.

French drains can lead to a downhill slope, or to dry wells or environmentally-friendly rain gardens where the extra water is held and absorbed by plants, when city water systems, or other waste water areas cannot be used.

Use in foundations

French drains are installed around a home foundation in two different ways:
- Buried around the foundation wall on the external side of the foundation
- Installed underneath the basement floor on the inside perimeter of the basement

In most homes, an external French drain or perimeter drain is installed around the foundation walls before the foundation soil is backfilled. It's laid on the bottom of the excavated area, and a layer of stone is laid on top. In many cases, a filter fabric is then laid on top of the stone to keep fine sediments and particles from entering. Once the drain is installed, the area is backfilled and the system is left alone unless it clogs.

Disadvantages: While an external French drain can operate for ten years or more without the need for maintenance, it's prone to clogging without any warning and can eventually lead to a flooded basement. When there is no filter fiber, sediments can make their way through the stone as years pass and clog the drain, and when the filter fabric is present, that can instead clog with sediments.

Also, a French drain that is not installed with a sump pump counts on gravity alone to drain foundation water, and if the house is not located on a hill or near a steep incline, finding this slope can be problematic. Additionally, maintenance on an external French drain involves expensive exterior excavation, which includes removal of walkways, shrubberies, porches, gardens, and anything else along the perimeter.

Installing a French drain around the inside perimeter is most commonly done after the house has been built. Most commonly, this is done in response to a wet basement or right before performing a basement finishing. To install this kind of drain, the perimeter of the basement floor is jack hammered down to the footing and the cement is removed. A layer of stone is laid down, and the drain is laid on top of it. Water is collected from the basement wall floor joint as it enters, and a sump pump is installed to pump the water out of the house and away from the foundation.

Once completed, the area, save for a 2" gap around the edge, is cemented over. This gap exists in order to allow water in from the basement walls. This can be installed very quickly- 1-2 days by an experienced crew. The system is easy to maintain once installed, and the sump pump will need annual maintenance to perform properly. An interior French drain is much less likely to clog than an exterior, partially due to the fact that it is not sitting underneath several feet of soil.

Disadvantages: Interior French drain installation is an effective way to waterproof a basement but requires the use of a sump pump. Many contractors will install plastic sump pumps that can quickly break down or neglect to install a battery backup sump pump, making the basement vulnerable to flooding during power outages. Sump pumps should be installed with a battery backup system in a proper sump liner of 20 gallon size or larger to prevent the sump from having too little water and turning on and off continuously.

Even though the French drain in all of its applications is under the ground, it's best not to think "out of sight, out of mind" and have your drainage inspected every 2 years by a professional.

Regular inspections and maintenance will prevent any unwanted surprises such as a flooded basement or soggy yard, especially during the spring melt or run-off season when ground water levels are at their highest

If you haven't had at your homes drainage inspected in the last 2 years, doing so will not only give you peace of mind; it will also ensure that future problems don't arise when you least expect them.

Call Shell Busey's Home Service Referral Network at 604-542-2236 or toll free 1-888-266-8806 to find a qualified drainage contractor in your area.