French Drains

What is a French Drain?

A French drain is a general term used to describe a drainage system placed in the ground to move water away from your home and prevent water related damage. They can be particularly helpful in regions with higher precipitation, like the Seattle-Tacoma area, where basements and crawl spaces are more at risk of flooding or leaking.

Typically the soil is removed from the affected area, exported off-site for disposal, and replaced by geo-textile soil retention fabric, a perforated PVC pipe, and washed drain rock. The perforations in the pipe allow water to flow up into the pipe as the ground is becoming more saturated and the rock prevents the trench from collapsing and the free flow of water. The soil retention fabric prevents soils from migrating creating sinkholes, erosion, and potential for settling.

French drains are installed on the outside of a building’s foundation to protect the basement or crawl space from taking on water. They are also commonly installed in wet and soggy yards to remove standing water.

Over the last several decades based on common usage of the term “French drains” can also be placed on the inside of the house to protect critical areas like basements and crawl spaces. These are also referred to as sub-slab pressure relief system of crawl space de-watering systems.

The water collected by true French drains is routed away from the structure where it can be discharged or re-distributed in an effective manner via gravity or a sump pump system. While the basics stay the same (trench, pipe, rock), French drains are built with varying widths, depths, and materials depending on the objective and needs of the property.

Most French drains will have a few common components:

• Good French drains are installed below the impervious or clay layer of soils to collect the water than runs horizontally on top that cannot saturate into the ground. French drains should be placed at the lowest possible point to lower the water table effectively around the perimeter of any structure.
• The trench should be lined with a geo-textile soil retention filter fabric to prevent dirt and sediment from getting into the rock field so the rock and pipe do not get clogged over the long term and of course to prevent settling, sinkholes and erosion.
• Large rounded or large coarse crushed drain rock is best. Small rocks or pebbles (pea gravel, 5/8” crushed rock) may compact and not allow water to flow through the system or may enter the piping system through perforations destroying the purpose of performing the work in the first place.

If you have a water problem and want to learn more about the solutions that French Drains can offer your greater Seattle, WA area home, to solve your water problem for your basement or crawl space, call Perma Dry Waterproofing at 206-233-9003 today.

Common French Drain Installations:

• Footing Drain – A perforated pipe system placed outside of the foundation just below the footing and foundation of structure, most commonly around a basement.
• Basement Drainage System – (Sub-slab Pressure Relief System) A drainage system installed inside the home under the basement concrete floor to prevent water table ascension causing Hydrostatic Pressure and ensuing flooding. Typically much more affordable and quicker to install than outside footing drains.
• Crawl Space Drainage – (Crawlspace De-watering system) A drainage system installed inside the crawl space, around the internal perimeter of the dirt crawl space to keep it free of all standing water.
• Strip / Trench Drain – (Channel Drains) – A trench is cut across a driveway usually closest to the garage door and a specialty shaped drain with an open grate across the top allows surface water to enter the pipe and route away from the house.
• Yard Drain – A standard French drain is installed in a yard or planting area to collect standing water in a yard so soils can dry out to help prevent muddy or mushy conditions. Very often installed behind, in front of, or both when retaining walls are installed.