Filter fabric is used in a French Drain system to keep soil from getting into your trench and mixing with the round rock. This is important, because keeping the round rock clean is essential to keeping the void space in your trench open, and to allow the water in your trench to move freely and to the discharge station quickly.
Your trench needs void space to allow air to move openly around the trench, therefore creating an underground drying effect.
Proper fabric allows all of this to happen. Not all fabric that is used for French Drains are equal, so if your system has fabric but is still not performing well, it could be that your fabric has not been tested for a high-water flow.
Is It Bad If My French Drain Does Not Have Fabric In It?
If your system does not have any fabric, you are probably running into an issue of your pipe being clogged with dirt and debris.
This dirt clogs up your pipe, and water will have a harder time getting to its discharge area. Putting in a tested filter fabric will protect your pipe from getting clogged, and move the water in your trench faster.
Double punched filter fabric:
The reason fabric is used in the best French Drain system is so that soil will not be able to get into the system and mix with the gravel. Keeping the gravel clean is essential in keeping the void spaces open, which avoids clogging and allows the water to runoff quickly. It also allows air to freely move around which creates an underground drying effect. The double punched filter fabric in this system is tested, and has a high-water flow. This fabric allows water to get into the pipe, and taken to its discharge station. While MOST fabrics claim to have a high flow rate they usually do not, and this can lead to drainage problems. Most contractors that have had a bad experience with fabric will tell you not to use any because it will plug up. The facts are: using tested, DOUBLE punch filter fabric will not plug up, and will allow over 100 gal/Min/sqft to pass through it, even in heavy clay situations.
Using Untested Fabric, And Not Encapsulating The Entire System
One of the reasons that fabric is used in a French drain install is so that the soil does not get into the system and mix with the gravel. One problem with using some kinds of landscape fabric is that the water pools on top of it if it.
When this happens, the water never has a chance to get into the drain pipe, and be taken to its discharge.
Before using any fabric in your French drain pipe system, it is important to know if the manufacturer specifications of the fabric are correct.
We tested filter fabric that stated that it could move up to 95 gallons of water per min, per square ft. This fabric did not do what the specifications stated, and the flow of water pooled in our test pail. Some people choose not to use fabric for this reason. Luckily, there is fabric out there that can keep up with the water flow, you just need to make sure to test it first.
The reason it is important for fabric to encapsulate the entire trench is that once done, no soil can get into the gravel, and your trench will last centuries.
If the fabric used does not line the bottom of the trench, as well as the sides, and top, the soil will mix with the gravel. Once this happens, you are putting an expiration date on the system, and it will eventually clog.
For a french drain to truly last, tested filter fabric should encapsulate your entire french drain trench.