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French Drains


What is a French Drain?

A French drain (also known as a weeping tile, perimeter drain, land drain) is a trench filled with gravel/rock or containing a perforated pipe that redirects surface water away from an area/structure. French drains often have perforated pipe(s) along the bottom to quickly and evenly vent water through the upper level of gravel or rock.

French drains are primarily used to prevent ground and surface water from penetrating or damaging building foundations.

French Drain basement waterproofing

French Drain basement waterproofing

Depending on the average amount of rain water or runoff an area gets will help determine how to dig your drain. French drains can be widened or made up of multiple pipes leading to multiple drainage areas.

Multiple pipes also provide support in case one pipe is clogged or damaged. A pipe might become overfilled if:

  • It is on a side of the drain which receives a lot of water
  • Closer to an uphill slope
  • Closer to a roof line that drips near the drain.

When a pipe becomes overfilled, water can seep, sideways, into a parallel pipe, as a form of load-balancing, so that neither pipe becomes slowed by air bubbles, as might happen in a full-pipe with no upper air space.

External French Drain

French drains are often 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 drain tile is installed around the foundation walls before the foundation soil is backfilled. It is laid on the bottom of the excavated area, and a layer of stone is laid on top. In some 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.

While an external French drain can operate for ten years or more without the need for maintenance. It can also be prone to clogging so proper service is recommended.

When there is no filter fiber, sediments can make their way through the stone. Over the years the drain can clog. But when you use a filter fabric it can help prevent the clog. It may be wise to provide cleanouts, much as is done with sanitary sewers, to provide access for inspection with a camera snake.

Also, a French drain, that is not installed with a sump pump, counts on gravity alone to drain foundation water. 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.

Interior French Drain?

Installing a French drain around the inside perimeter of the basement is most commonly done after the house has been built. This tends to be done in response to a wet basement or right before performing a basement finishing.

To install this kind of drain:

  • Perimeter of the basement floor is jackhammered down to the footing
  • Concrete is removed
  • Save for a 2 in (5.1 cm) gap around the edge – this exists to allow water in from the basement walls
  • A layer of stone is laid down
  • A perforated drain pipe is laid on top of it
  • Water is collected from the basement wall floor joint, as it enters
  • Pump is installed to remove water from the house and away from the foundation

This can be installed very quickly—one to two 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.

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 US gal (76 L) size or larger to prevent the sump from having too little water and turning on and off continuously.

For an effective, and cost efficient french drain basement waterproofing option, call us today (610) 495-9111. We can work with your needs and budget.