French Drain Fabric

What Is a French Drain?

Despite its sophisticated name, a French drain is simply a gravel-lined ditch with a pipe installed that drains water away from areas that tend to pool water or areas where you don’t want any excess water such as near house foundations.

Unlike gutters, which collect rainwater falling from rooftops, French drains handle water at ground level. When water collects in a low area in your yard after a downpour, you can solve that issue by diverting the water flow using a French drain.

How a French Drain Works

French drains are a convenient way for water to move through a structure. At the bottom of the trench, water flows into a gravel-filled channel and into a perforated pipe.

The water flows freely through the pipe, which drains a safe distance from the home and into the ground or can be carried by a closed pipe once the water is captured and moved even further.

The trench bottom should be slanted about 1 inch for every 8 feet in the direction you want the water to flow.

Dry Well vs. French Drain: What's the Difference?

Dry wells and French drains are two distinctly different types of drainage management that may be used in conjunction. A dry well is a hole that is covered and filled with stones or similar material. It is the terminus of your drainage system. Water is channeled to the dry well, where it trickles down through the gravel and distributes itself deeply into the soil. 

A French drain is a drainage trench that gathers and channels water downward. To discharge all of the water collected by a French drain, it must have a termination point. A dry well is an excellent place to end your French drain.

When You Need a French Drain

If You Have a Problem With Surface Water

Install a French drain that is just a few inches deep. You can run the drain through the area that pools water and increase the depth of the drain as needed to insure it runs away from the affected area. It is also known as a curtain drain. To put it simply it collects water and directs it away from the puddle.

This drain does not need to be particularly deep; a typical dimension is 2 feet deep and 1.5 feet wide. Switch to solid pipe (not perforated) if the drain travels through areas with trees or bushes to lessen the possibility of roots entering the piping and obstructing it.

If You're Putting Up a Retaining Wall on a Hill

Install a French drain behind the first course of stones or blocks while constructing a retaining wall.

Otherwise, water flowing down the slope would accumulate behind the wall and weaken it. The pipe should be supported by the same compacted gravel foundation or concrete footing as the wall.

Drape landscaping fabric over the base or foundation and up the slope before installing the pipe and drain gravel to keep the drain from clogging with silt. Fold the cloth over the top of the gravel at the top of the wall, and cover it with several inches of dirt.  You always want to use a good quality fabric to separate gravel from soil particles to keep your drain active.

If Water Is Penetrating Your Basement

Even if your basement's foundation is porous, putting in a French drain may help to keep it dry. So-called "wet" basements occur when water slowly seeps in via cracks in the foundation. However, in many cases, you can use a French drain to redirect water away from the foundation.

If water continues to enter your basement despite what seems to be sufficient outside drainage, you may need to install an internal French drain. Cutting a trench around the perimeter of the foundation in the basement slab, laying piping in the trench, and installing a sump pump to transport water from the inside to the outside are all part of the installation.  But this is generally a more complicated process and you should seek the advice of an expert in waterproofing basements or foundations.

Landscape Fabric vs. Drainage Fabric

Landscape fabric serves as a physical barrier (a weed barrier) in gardens and planting beds. The material is laid on the ground between plants/crops, producing a barrier that enables air and water to flow through so that plants may develop. It also acts as a barrier against weed development, preventing them from taking root. The material also prevents weeds from developing by blocking off light.  Landscape fabric is generally lighter and thinner when compared to civil or environmental filter fabric.  Some of the most common landscape fabric is called spunbond non woven fabric.

Drainage or civil filter fabric is generally made using the needle punched non woven filter fabric process and results in a variety of fabric thicknesses that act as a separation fabric as well as a filter to allow water to pass through.  The most common thicknesses of filter fabric will depend on the application but generally you see a 4 oz, 6 oz, or 8 oz filter fabric specified in projects with 4 oz or 4.2 oz being the most common in French drain applications.

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