What is a French Drain Really?
Monday, February 4, 2013 • 10:50am
A French drain is a trench covered with gravel or rock that redirects surface and groundwater away from an area.
This is an outside drainage system.
The earliest forms of French drains were simple ditches, pitched from a high area to a lower one and filled with gravel. These were described and popularized by Henry French (1813-1885) a lawyer and Assistant US Treasury Secretary from Concord, Massachusetts in his book Farm Drainage.
A French drain can have perforated hollow pipes along the bottom to quickly vent water that seeps down through the upper gravel or rock. French drains are common drainage systems, primarily used to prevent ground and surface water from penetrating or damaging building foundations. French drains are also used behind retaining walls to relieve ground water pressure
French drains aka Perimeter Drains are often installed around a home foundation during the initial construction of the structure. They are buried around the foundation wall on the external side of the foundation. In most homes, an external French drain or drain tile is installed around the foundation walls before the foundation soil is backfilled.
Then we come to the inside drainage system.
Installed underneath the basement floor on the inside perimeter of the basement and while commonly referred to as a French Drain it is technically called a Bonded Deep Channel Pressure Relief System. This is because it relieves the hydrostatic pressure (water pressure) from the footing area.
The outside system is based on grading and gravity; the inside system uses a mechanical pump (sump pump) to remove water from the basement. The water enters the sump pit from the perimeter drains of a basement waterproofing system, and then the sump pump pumps it up and out of the basement.
Installing a French drain around the inside perimeter is most commonly done after the house has been built. Most often, this is done in response to a wet basement or right before performing a basement finishing.
To install this kind of drain:
- The inside perimeter of the floor is cut approximately one foot from the wall. A trench is excavated and pitched.
- Where possible, the lowest course of block is tapped and bled (weep holes) in order to drain the walls.
- Fully slotted polyethylene drainage pipe is installed and covered with gravel. This pipe meets the requirements for drainage pipe as required by the State Code and the standards for ASTM-F405-74.
- Polydrain wallboard is installed over the bottom of the wall and over the entire trench in the basement.
- The basement floor is re-cemented to its original level.
- To discharge the water, we will install a submersible pump in a covered polyethylene liner.
The Bonded Deep Channel Pressure Relief System (French Drains) is usually installed in 1 day by A-1 Basement Solutions experienced staff. The system is maintenance free once installed. 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.
The Bonded Deep Channel Pressure Relief System carries a transferrable lifetime guarantee. There is no tax of any kind as this is a Capital Improvement to the home.
If you see any signs of water in the basement you should seek the advice of a professional basement waterproofing contractor.
Doug Lynch is a basement expert from Westfield, NJ where he lives with his wife Kelli and their dogs Sammy and Angie. Doug teaches a class at Westfield Adult School “Keeping Your Basement Dry” and is the Author of Basements 101 Everything You Need to Know About Your Basement. Doug has testified in NJ Superior Court as an Expert Witness in Basement Waterproofing and does seminars for local Real Estate Agents to educate them about basements. Doug is the owner of A-1 Basement Solutions in Scotch Plains and authors the popular BASEMENT SCHOOL Blog.
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