What are french drains?


French Drains 101

Photo: Helet van Blerk

Things with “French” in the title are usually fancy, right? Poodles, perfume, pastries. But a French drain is nothing more than a ditch in the ground, inset with a perforated pipe under a layer of gravel. That pipe funnels storm water away from where you don’t want it—along the foundation, for example—and deposits that water in a more desirable place, such as the municipal storm drain or a rain barrel.


It sounds elegant but in reality, it’s nothing more sophisticated than a gravel-lined ditch with an embedded pipe that carries water away from the home.

Whereas gutters collect precipitation as it runs off the roof, French drains manage water at ground level. Let’s say that after a rainstorm, water tends to pool in a particular low spot on your property. Rerouting the flow of water with a French drain would alleviate that problem.

A French drain also provides a solution for basements that admit water through the foundation. In these “wet” basements, water presses against the foundation and gradually leaks through. With a French drain, however, water near the foundation can be rerouted and deposited elsewhere.

If water continues to invade your basement despite seemingly adequate outdoor drainage, then you might need to install a French drain indoors. Installation involves cutting a trench in the basement slab along the perimeter of the foundation, laying pipe in the trench, and putting in a sump pump to move water from the interior to the exterior.

Related: 7 Ways to Avoid Basement Flooding

Photo: wikimedia.org


Whether installed in the yard or the basement, a French drain works on the same principle. First, a trench is dug with a slope in the direction you want the water to go; a slope of one inch for every eight feet in length is generally recommended. To determine the correct angle, use a level string tied between stakes, then measure the distance from that reference point to the trench bottom.

Because there is a direct relationship between the diameter of a drain pipe and its relative effectiveness, make your trench no smaller than 12 inches wide, and aim for a depth between 18 and 24 inches. If you’re installing a French drain around your foundation to prevent basement moisture, take care to position the pipe below slab or finished floor level.


After digging your trench, fill it with a few inches of crushed stone. Cover the stone with water-permeable landscaping fabric to discourage weed growth. Next, lay piping into the trench. Choose one of two types, either rigid PVC with predrilled holes or flexible drain pipe cut with slits. PVC lasts longer, and if you encounter a clog, it can be cleaned with pressure or a plumber’s snake. Flexible pipe, on the other hand, is less expensive and easier to work with.

Opting for PVC? You can attach a 45-degree angle joint to the start of your pipeline and then connect the joint to a pipe that can be left sticking out of the ground for an easy-access clean-out point. Another important thing to remember in PVC installations: Orient the pipe holes downward. Counterintuitive though it may be, French drains work by allowing water to flow into them from below.

Wrap landscaping fabric around the pipe to keep dirt and roots from obstructing the system. Finally, infill the trench with gravel to grade. Alternatively, infill with gravel to a point a few inches below grade, then add dirt to span the remaining distance. Although covering the pipe complicates future maintenance efforts, doing so allows the French drain to be completely concealed.

Further Notes on French Drains

  • Instead of wrapping pipe with landscaping fabric, you can buy a flexible perforated pipe that comes encased in water-permeable fabric.
  • If you are planning to dig a long trench, think about renting a trench digger to make quicker work of it.
  • Place a catchment barrel at the terminus of your drain as a way of harvesting rainwater for use in the garden.
  • After trenching, expect to have a large quantity of loose dirt in need of a home. Before you begin the project, decide what you will do with the dirt.

Surface Drains vs. French Drains

Surface Drains vs French Drains

Providing proper drainage for your property, whether commercial or residential, is one of the most important long term preventative maintenance measures you can implement. Insufficient drainage can cause water to build up and flood basements, garages and can cause considerable damage to building foundations. Without adequate drainage, water can build up in the soil under the foundation of your home or commercial building, causing the soil to swell. When this happens, the expanding soil can cause the foundation to buckle and fracture, leading to costly and time consuming repairs. A cracked foundation reduces the value of your home or commercial property considerably, and foundation repairs can cost up to $10,000. Considering these costs, ensuring that your property has adequate drainage is essential.

The two types of drains most commonly used to tackle excess water problems are Surface Drains and French Drains.

Surface Drains vs French Drains: Design

While designs over the years have varied, modern French drain systems usually feature a trench into which a perforated pipe is laid and then covered with gravel. The perforated piping material is often easily broken, and so care must be taken when installing and working around the drain. Installing a French drain is time consuming and requires a lot of labor and design considerations.

Surface Drains, by comparison, are drains that feature a metal grating that is flush with the surface to be drained, providing a safe and convenient drainage system. Installation is straightforward and, since they are made of metal, surface drains are resistant to corrosion and breakage. Unlike French drains, surface drains can be used in concrete areas such as parking lots and garages where water tends to accumulate.

Surface Drains vs French Drains: Purpose

The design purpose of surface drains vs French drains is an important consideration when choosing what is best suited to your drainage needs.

French Drains are primarily used to drain groundwater from gardens and around the foundations of buildings. French drains are designed to move water that is trapped in the ground away or towards an area. They are best used in soil that is prone to frequent saturation from rain or flooding.

Surface Drains are used in a variety of commercial and residential applications. They are used to collect and divert large amounts of water in a short period of time. They are often placed at the bottom of sloped driveways and other similar surfaces to catch water runoff and drain it away from buildings, garages and houses.

Surface Drains vs French Drains: Effectiveness

French drains are effective at removing moisture from over-saturated soil. However, the slope of the soil has to be considerable in order for water to slowly drain through it and into the drain basin. Designed for limited use, French drains are not typically good at removing large amounts of water in a short period of time and are often overwhelmed in flooding and high rainfall conditions.

Additionally, French drains are susceptible to frequent clogging, as they are underground, and roots from trees and shrubs often displace and damage the perforated piping. These design drawbacks usually result in the drain lasting only a couple years before having to be replaced.

Surface Drains are designed to move large amounts of water fast.

Surface drains from LTEC Drains are well suited for draining large amounts of water in small amounts of time. They have a flow rate that is considerably greater than French drains and are less prone to clogging. If clogging does occur, LTEC surface drains are resistant to clogging and are easily unclogged without having to dig up and remove the whole drain. While French drains are effective at removing water from highly sloped ground once it’s there, the design and purpose of LTEC surface drains keeps excess water from building up in the first place. In the battle to protect your property from excess water and flooding, the high flow, low maintenance surface drain from LTEC, designed to reduce water runoff into Lake Tahoe, is the most effective and long lasting solution.

Everything You Need to Know About French Drains

Table of Contents:

  1. What Is a French Drain System?
  2. How Do French Drains Work?
  3. How to Tell If the French Drain Is Working
  4. Advantages of French Drains
  5. Disadvantages of French Drains
  6. What Does a French Drain Cost?
  7. How to Install French Drains
  8. What You’ll Need
  9. How to Unclog a French Drain
  10. French Drain Maintenance
  11. Snaking the Drain
  12. Contact Mr. Rooter for Your French Drain Needs

If you’re dealing with a permanently damp area in your lawn or end up with water in your basement after a heavy rain, there’s an easy solution — a French drain outside or inside your home. It’s essential you have good drainage to ensure your house remains dry and mold-free. If you have groundwater collecting in your basement, not only is it an eyesore, but it can also cause the wood to mold and rot.

Here is a guide to French drains, so you’ll learn everything you need to know about them, including what they are, how they work, how to install them, their cost and more.

What Is a French Drain System?

An outdoor French drain is a trench you cover in rocks. It’s connected to pipes to guide water away from your home. The first part of French drains is the elevated or higher end, referred to as a drain field. The drain field is where the groundwater or excess surface water enters the drainpipes. Then, there’s the second part: the French drain exit point, which is the lowest point where the water leaves the drain.

A French drainpipe offers you the solution for if your basement is admitting water through the foundation. In a “wet” basement, the water presses against the foundation, slowly leaking through. However, a French drain reroutes the water near the foundation and releases it elsewhere.

If you’re still getting water in your basement even though you have adequate outdoor drainage, you may have to install an interior French drain. You’d cut a trench in your basement slab along the foundation’s perimeter, lay pipe in the trench and put in a sump pump to move the water out of your basement and to the outdoors.

There is also the option of a pipeless French drain. A pipeless French drain doesn’t have a pipe and has more internal resistance to the flow of water. Therefore, it can’t evacuate as much surface water.

How Do French Drains Work?

Water will always seek out the lowest point along the most straightforward path and readily move in loose soil into empty pockets. That is where a French drain comes in. It provides this accessible path and creates a sunken channel, encouraging water to drain out of the soil surrounding and flow along a smooth course.

For a French drain to function properly, it must leverage gravity as it initially forces the water down from the surface from saturated soil, pulling it along the downward-sloping pipe to the appropriate discharge point.

You should slope the trench bottom around an inch for every eight feet in the direction you’re looking to have the water flow. Depending on your particular circumstances, you can divert the water to:

  • A drainage ditch

  • A low-lying spot on your property

  • The street

  • A dry well

A simple way to decide where to install the French drain around your house is to look for areas where water pools, particularly if the water is sitting for hours or days after it rains.

How to Tell If the French Drain Is Working

The easiest way to tell if your French drain is operating properly is to look at your lawn or basement. If there’s water, chances are there could be a problem and your drain isn’t working correctly. This issue could be due to clogging.

When water seeps through soil to get to the French drain, it often brings along sediment into the drain, where it dumps the sediment in the drain on the gravel as it flows through the ditch. The sediment gradually builds up over time and slows the water flow through the French drain. It could even block it up completely. When this happens, the water can no longer pass correctly through the drain and causes homes and other property areas to flood.

Another way of telling if your French drain is working effectively is to call in a professional to inspect it. They can let you know if there’s any buildup that could turn into a clog.

Advantages of French Drains

A French drain can be a blessing for your land or home. They have several advantages.

  1. Efficient drainage: A significant benefit of a French drain is its ability to move water away from your home or foundation. You can choose where you want the water to go with either an interior or exterior French drain system. Whether you opt to send the water to a sump pump, your septic tank or as part of a French drain system combo, a French drain helps keep your basement dry to prevent potential foundation water damage.
  2. Affordable: A French drain is surprisingly affordable when you compare it with other basement waterproofing techniques.
  3. Transforms your lawn: You can change what would otherwise be a damp, spongy lawn into a garden or a play area for your children.
  4. Fast: Installing a French drain isn’t complicated, and the installation process is relatively quick, depending on the complexity and size of the French drain you require.

Disadvantages of French Drains

While there are several benefits of French drains, there are also some disadvantages of French drains you may want to consider before installing one. Some downsides include:

1. Difficulty of Installation

It takes a lot of know-how to install a French drainpipe underground, and there are several safety concerns. You need to consider factors like:

  • Water lines

  • Gas lines

  • Other underground utilities

Installation might involve temporary removal of structures like sidewalks and porches, and it could be dangerous work. Because of this, it is probably a good idea to let a professional handle the installation.

2. External Pipe Clogs

Even an above-ground French pipe can become clogged and more frequently than one buried underground. When the pipe clogs, you could unexpectedly wind up with a flooded basement.

3. Mud-Related Clogging

There could be soil erosion causing muddy water to enter the French drain, which can cause it to clog. While you need to bury the drain deep enough, the groundwater passing through it can contain sediment that can build up. And, while at first the amount of this sediment won’t be too much, over time it can clog the drain.

What Does a French Drain Cost?

You usually install a sump pump in the basement with an interior French drain to prevent or fix flooding. These types of drains are deeper in the ground. When a contractor handles the installation process, it’s labor-intensive, requiring the digging of a large moat around the foundation of your building. You’re looking at paying around $50 to $60 per linear foot.

If you have a 20′ x 30′ basement, that’s a perimeter of 100 linear feet, meaning the entire installed cost could be around $5,000 to $6,000. If your basement perimeter is less than 130 to 150 linear feet and only requires a single sump pump, you may pay around $175 to $225. A bigger basement usually requires two sump pumps.

A professional can install an exterior French drain around two to three feet underground so it can move surface water away from your home and property. The drain length will depend on how far away you would like the water released. On average, an exterior “surface” French drain will cost between $10 to $15 per linear foot for all labor and parts, and therefore, you’re looking at paying approximately $1,000 to $1,500.

How to Install French Drains

You can save yourself some money by installing the French drain yourself. Below are the steps you need to take to install your French drain.

What You’ll Need

First, you’ll need to gather your tools, equipment and materials.

Tools and equipment include:

  • Line level

  • Hammer

  • Digging shovel

  • Mason’s line

  • Square spade

  • Bow rake

  • Tape measure

  • Utility knife

Materials include:

  • Landscape fabric staples

  • Landscape fabric

  • Wood stakes

  • Crushed granite

Now you’re ready to start the install.

Here are the step-by-step French drain installation instructions.

Step 1: Decide Where You Want the Trench Located

Evaluate the areas of your yard that are prone to flooding to determine a general route and location for the French drain. You’ll want to decide where you want the water to flow and confirm the trench’s drainage end or exit is in an ideal location.

Step 2: Obtain Approval

Ensure your yard drainage won’t negatively affect any public areas or anyone else’s land, as this could cause legal issues. Ensure your plans comply with local law by checking with the building authority of your city.

Step 3: Assess the Slope

You need to slope the French drain to carry the water down to the desired spot. You should go with at least a 1% slope, which is a one-foot drop for every 100 feet in length.

If your yard has a steeper natural slope, this is OK, but the steepness increases the speed of the water, potentially leading to more erosion in the area the water exits.

Drive a stake at the starting point and ending point of your planned trench route to check the natural slope. Next, you’ll:

  • Use a mason’s line to tightly tie on one stake.

  • Run the line over to another stake where you’ll loosely tie it off.

  • Take your line level and attach it to the line.

  • Untie the line’s loose end.

  • Pull the line tight.

  • Adjust the line so it’s level.

  • Tie the line to its stake securely.

Then, at regular intervals — around every four feet — measure straight down to the ground from the line to see the way the slope changes. If needed, adjust the trench’s depth until you reach the desired slope.

Step 4: Dig the Trench

Now, if it’s necessary, you’ll reset the level line and stakes so the line will run down the center of the trench you’ll dig. Using a square garden spade, cut a line through the sod, ensuring it’s straight, three inches to one side of the line to start digging your trench. On the other side, you’ll repeat the same process three inches from the line to create a six-inch-wide trench. Remove the sod and start digging, creating a sloped bottom and vertical sides. Your trench can be as deep as you want it. As you work, compact and smooth the trench’s bottom.

Step 5: Use Fabric to Line the Trench

Use landscape fabric to line the trench, using a continuous swath, if you can. If you can’t, overlap fabric pieces by a minimum of 12 inches, and use fabric staples you’ll drive into the ground with a hammer to secure the ends. Use the staples to secure both fabric ends. If there’s excess fabric, fold it back to both sides of the trench, which you’ll trim later on to fit it.

Step 6: Fill Your Trench

Use coarse drainage gravel like crushed granite to fill the trench, so it’s flush with the surrounding sod or ground. Begin raking the top of the gravel to make it smooth and even it out with the top of your trench. Use a utility knife to trim the extra fabric along the edges.

How to Unclog a French Drain

To keep dirt from building up in the pipe which will prevent water from flowing in and out, you need to clean the French drain. A clogged interior French drain can result in water in your basement. If you have a clogged exterior drain, water will start backing up and won’t drain properly out of your yard. If there is a clog, you’ll need to know the steps to fix it.

Step 1: Locate an end of the French drain. You’ll see a hole in the ground where the French drain end begins. If you’re uncertain where the drain starts, try looking at the highest point of your yard, since the design of the French drain is to help water run downhill.

Step 2: Use a garden hose to run water down the French drain. If you notice the water backing up instead of running straight through, it’s clogged.

Step 3: Use a pressure washer to unclog the drain, which will use high pressure to send water down the drain. When you do this, stand to the side, because the water could end up coming back out at you once it hits the clog. Aim the high-pressure water at the bottom of the French drain to eject the clog. Moving the spray around, clean all sides of the pipe.

Step 4: Use a sewer snake for stubborn clogs. A snake is a long piece of piping you will stick down the French drain to reach the clog. Gently push the clog with the snake to try and free it up. If it breaks free or even feels looser, switch back to using the pressure washer to be sure you break up and dissipate the entire clog down the drain. You’ll continue using both the pressure washer and sewer snake until you see the water flowing freely down the drain.

French Drain Maintenance

Fortunately, there’s not much regular maintenance needed for a French drain to function. However, over time, debris and leaves can build up in the drainpipe’s holes, compromising its effectiveness. To prevent this, clean out and snake the French drain once a year.

Whether you’re performing interior French drain maintenance, basement French drain cleanout or exterior French drain maintenance, you’ll need an electric sewer snake. You can easily rent one if you don’t want to buy it. You can find them for rent at hardware stores, home improvement centers and tool rental facilities.

Snaking the Drain

Here are the steps to snake and clean a French drain.

  1. Before you begin, put on some tough leather gloves. The gloves help protect your hands from becoming torn up as you feed the snake’s cable into the drain.

  2. Uncover the drain’s lid. If the French drain attached to a gutter downspout, you’ll have to remove the downspout from the drain’s entry point.

  3. Turn the snake on and feed its cable into the French drain.

  4. When the snake cable hits an obstruction or a turn in the drain, hold it back as the snake is turning in the pipe. Once it overcomes the turn in the drain or obstruction, feed it forward gently into the drain.

  5. Continue feeding the cable into the drain until it gets to the other side. Allow the snake to turn inside the drain when it hits resistance from tree roots, clogs and other obstructions.

  6. When finished, retract the snake and feed it back into the device.

  7. Using a hose in power jet mode, spray water into the drain using a garden hose to push out debris or other obstructions.

Contact Mr. Rooter for Your French Drain Needs

Now that you know everything you need to know about French drains, you can make an informed decision about installation, maintenance and cleaning. Remember, if your French drain is underground, you still will need to have a professional inspection every two years. The same goes for an interior French drain. Proactive maintenance will prevent clogs and other unwanted disasters like a soggy yard or flooded basement.

If it’s been more than a couple of years since you’ve had the drainage of your greater Syracuse home inspected, schedule a French drain inspection appointment with Mr. Rooter. We offer installation, cleaning, repair and maintenance of French drains. Call us at 315-472-1203 or request a job estimate via our online form.

Generally, French drains direct water to the street, a drainage ditch, or a dry well. If you have a low-lying pond on your property, this also could be a destination.

How much does a French drain cost?

Above: An unobtrusive perimeter drain runs alongside the foundation of a house in Mill Valley, California.

Hiring someone to install an outdoor French drain costs on average $25 per linear foot in a trench that is one to two feet deep and from six to 18 inches wide, according to Costhelper.

If you install your own outdoor French drain (which as you already know I do not recommend), the labor is free and the cost of construction materials will include crushed rock for about $2 per square foot; PVC pipe and fittings, and rental fees for equipment: a compactor rents for $65 to $85 per day. A trencher costs $50 to $80 to rent for half a day, and a small backhoe costs $150 to $200 a day to rent, according to Doityourself.
If installing a French drain indoors in a basement, prices will vary widely depending on the size of the basement; expect to pay $2,000 to $15,000 to cut a channel around the perimeter and fill it with gravel, says Costhelper.

Feeling drained? Or are you inspired to tackle more projects? See our curated design guides to Hardscape 101, including Decks & Patios and Fences & Gates. Read more:

  • Hardscaping 101: Sump Pumps
  • Garden Envy: 10 Dramatic Drainage Ideas to Steal
  • Hardscaping 101: Rain Gutters

How to dig and install a French Drain

On this page we will dig into how to install a French drain system to reduce drainage problems in your lawn, yard or garden. Officially a French drain is a gravel drain with no pipe. The water just collects in and travels through in a gravel or stone filled channel that starts from the surface or just below it. But in modern times, and for practical purposes, we will consider a French drain to include a drainage pipe in the gravel.

Another difference you will notice here is that while other drainage guides recommend fairly narrow trenches for their French Drains – often only 6 inches wide – my recommendation is that your trenches be between 8 and 12 inches wide for the following reasons:

  • A wider French drain will last longer
  • The capacity to collect and disperse water is better when wider
  • It is easier to dig and grade a wide French drain, especially when it gets deep

We will look at each of these points in detail…

A wider French drain will last longer because the primary cause of failure in a French drain is silting up of the spaces between the gravel by clay and soil – and a wider system simply has more gravel in it and takes longer to clog up. It is also best if the French drain trench is wrapped in permeable landscape cloth (the type water flows through) and the pipes are covered in drain sleeve fabric to further slow down any clogging by roots or soil particles. Digging a drainage system is a big job – be sure to do all you can to make your hard work last as long as possible.

The capacity to collect and disperse water is better in a wide French drain. This is a common sense type thing. Think about how much more water can flow through a 10 inch channel compared to a 6 inch channel. Also remember that we want to allow as much of the collected water as possible to soak into the subsoil in dryer parts of the yard, and a French drain that is 12 inches wide has twice the drainage area as a 6 inch wide trench. There is also the potential for small drywells that you can add to the bottom of the French drain trench – it is much easier to dig these in a wider trench.

It is easier to dig and grade a wide French drain, especially when deep, because you can get your digging tools, hands, and even feet into a deep 12 inch trench – but not a 6 inch one. Grading the bottom of trench to obtain the proper slope is also much easier in a wider French drain, especially when digging by hand. There is also the issue of using a posthole digger in the bottom of the French drain trench to add drywells, as mentioned in the paragraph above. For those using a trencher, many of skidsteer type trencher attachments, and a few walk-behind trenchers, will handle a 10″ or 12″ wide trenching chain – ask at the rental yard.

So plan on doing the job well, and therefore only doing it once in your lifetime. Take a little extra time to dig the wider width and spend a little extra for the additional gravel that will be needed. It will be well worth it!

French Drain and Drain Tile Installation

A few days before you begin to dig contact your local utility company to have all the underground utility (gas, water, sewage, electric, phone, etc) lines on your property located and marked.

In your yard, stake out the drainage system you designed in Step 3. Read the instructions on our Determining Drainage System Slope page to figure out how deep to dig and what slope the different parts of your system should have.

Before you begin to dig think about how you will handle all the leftover dirt. Installing a French drain is much different than normal trenching in that much of the dirt will not go back into the trench (because of the gravel). Most often people load the extra subsoil into a wheelbarrow and move it to the road for pickup and removal, or move it to another part of their property for fill. Be sure to plan to pile the subsoil so that you do not have to cross the open trench with a wheelbarrow. The topsoil is usually piled on the side opposite the subsoil since it will be spread back over the top of the trench.

Often it is best to start digging at the farthest end of the drainage system, usually a ditch, the street, a drywell, or other low or dry area you are draining to. This way you lock in the fixed end and if troubles occur further upstream in the system (like a boulder or major roots) you can sometimes just increase the slope of the French drain piping to go over the top of the obstruction.

So gather your friends and some good trench digging tools, and get to it.

Installing the Drainage Pipe and Gravel

Congratulations, you have finished the hard part (designing and digging). It was a lot of work and you do not want to have to do it again, right? To make your French drain last a long, long time you need to use these following items:

  • Clean or washed gravel (not crushed limestone – it becomes cement like)
  • Permeable landscape fabric (they kind water passes through)
  • Rigid plastic perforated drain pipe (not flexible corrugated pipe – read why not)
  • Correct drain pipe fittings (to allow Roto-Rooter cleaning – read how to choose)
  • Fabric drain pipe sleeve or sock – (keeps roots and soil out of the pipes)
  • Downspout leaf seperators: either plain or fancy (to keep leaves out of the pipes)

French drains only fail when the gravel becomes full of clay or soil particles or when the drainage pipe becomes full of soil or roots. By using the above items you will maximize the life of your French drain system and make it possible to easily clean it out with a Roto-Rooter or sewer snake if it does become plugged.

Here is what your French drain trenches should look like…

Note that there is always at least 2″ of gravel all around the 4″ drain pipe. Also note the fabric drain sleeve and that the landscape fabric is overlapped and completely surrounds the gravel. Remember that the rigid drain pipe is always installed with the holes on the bottom.

For information on how to select and install the right pipe and fittings for a French drain system that will work well and be easy to maintain, go to the next page:
How to install French drain pipe and fittings for easy Roto-Rooter cleaning in the future

A French Drain is a low environmental–impact solution to installing drainage for areas around walls, driveways and gardens.

Installing a French Drain is fairly easy and you should be able to manage to build a drainage system like this with a few materials, and either a hired mini digger, or some serious hard work with a shovel and wheelbarrow.

Here we introduce you to our 10 step guide – How to Make a French Drain.

Why Install a French Drain?

The surface of this wall is damaged below the DPC where water has not drained away

You can build a French drain to relieve a build–up of water against a house wall which would otherwise lead to damaged external masonry and damp internal walls (see picture above).

You may decide to use Geotextile sheet to protect the wall of the building where you are installing the French Drain. Find out more about using Geotextile Sheet in our French Drains and how they work Project.

You can also use it to drain areas of the garden, or at the edges of hard landscaping and drives to take water away from the site and into a suitable Soakaway or sewer.

How to Make a French Drain – A Step By Step Guide

French drain – detail showing a cross section of the drainage pipe and graded stones

Before you start work

  1. Plan where the water will drain off to – For more information see our French Drains How they Work project and our Guide to Foul and Surface Water Drainage. If you are going to connect your French Drain to a Soakaway you might also want to check out our project that deals with Soakaways.
  2. Plan and mark out the route of your trench – When installing any drainage, try to plan the trench so it will avoid obstructions such as a shed or trees. Use line marker spray or a string stretched between two small battens in the ground to mark the route you have chosen for your trench.
  3. Decide where you will place the waste soil – there is no point in piling it up against a wall as that can cause damp. You could make it part of a garden landscape ideas project or, if you want to dispose of it, you need to hire a skip.
  4. Check the depth of the buildings foundations – The French drain should be no deeper than the foundations of the adjacent buildings and where foundations are shallow you should dig the trench 1m away from the wall.
  5. Plan Your French Drain – Generally a trench of about 200-300mm wide is suitable in most cases, but if you have a large amount of water to drain you may need a trench of up to 450mm wide. The trench should be about 300-500mm deep, and dug so that it slopes at a ratio of 1:50. That means that for every 2 meters of trench length, it should fall 40mm.
  6. How to Make a French Drain – Dimensions

  7. Digging your trench – You can do this using manual labour, or you could hire a mini digger to make the job much easier. The sides of the trench should slope away from the building at no less than a 45 degree angle to ensure the surrounding ground remains stable.
  8. Building a French Drain – Filling the Trench

  9. Filling the French Drain – To prevent the pipe being clogged with ‘fines’, you can line the trench with weed fabric, and if you are using it allow an overlap of some 300mm on either side of the trench to fold in before the top layer goes on.
  10. If you are using a perforated land drain pipe in the bottom of the trench, lay some coarse gravel in the trench first, then lay the pipe, making sure it is straight and has a 1:50 fall down to the end of the drain.
  11. Half fill the trench with a layer of coarse rubble made up of recycled crushed brick, or railway ballast, without fines, followed by a layer of coarse gravel to just below the top of the trench. Fold over the layer of weed fabric to prevent soil making its way into the drain and then finish off the French drain.
  12. Finishing the French Drain – To finish the drain you can top the trench off with fine ‘pea’ shingle which is quite decorative. Alternatively you can happily turf over the French drain on top of the weed fabric, although you may find that that the grass dries out very quickly in dry weather.

Ensure your Soakaway stays within building regulations

Please check with the local authority that your soakaway stays within the building regulations and is 5 metres away from your property.

All project content written and produced by Mike Edwards, founder of DIY Doctor and industry expert in building technology.

Why Should You Install a French Drain

If you have water drainage problems on your property, a French drain is an effective and straightforward drainage system that can help rid your property of excess water. Knowing what a French drain is and how it works can help you decide whether or not this type of drainage system is right for your home.

What is a French drain?

A French drain is a pipe placed in the ground to collect rain water. French drains are often used in yards and in basements that have drainage problems. French drains are used to transport excess water to low lying areas where the water will do no damage.

How does a French drain work?

A French drain usually consists of a wide pipe perforated with holes that has been buried in the ground. Usually French drains are angled downward to keep water rolling through the pipe. To enable water to flow through the ground to the drain, gravel is placed on top of the pipe, because water flows through gravel more easily than soil.

Who can benefit from a French drain?

French drains are an effective solution on properties that have chronic water drainage problems. Your property may benefit from a French drain if:

  • the soil is often soggy and standing water is common.
  • the basement floods regularly.
  • your yard includes a retaining wall on a hillside.

What are the advantages of a French drain?

If you’re thinking about getting a French drain installed on your property, you’ll want to know what makes a French drain better than other drainage options. Some of the advantages of this type of drain include:

  • French drains have no moving parts, which can make them less likely to break down or stop working.
  • If installed properly by a qualified plumber, a French drain can last for a very long time.
  • In some circumstances, shallow French drains can be easy to install as a DIY project.

What are the disadvantages of French drains?

Although French drains have many good qualities, they are not entirely without issues. For example:

  • Because they’re buried in the ground, French drains can be difficult to repair when they develop problems.
  • French drain installation, if placed deep in the ground around pre-existing structures, can be expensive.

If you’re thinking about installing a French drain, what should you do to get started?

Before a French drain can be installed, the exact path for the drain must be planned. The drain must run through the areas where water is the most problematic while avoiding exterior water pipes and sewer channels. The exit point of the French drain should be a sunny, low-lying area. The exit should be directed away from any neighboring properties to avoid flooding someone else’s property. Work with your plumber to plan a sensible path for your French drain.

If you live in the Pittsburgh area, contact Terry’s Plumbing. We’ll be happy to help you plan your French drain, and then we’ll perform the installation when you’re ready. With over 30 years in the plumbing business, we’ve got the experience and skill to install a French drain that will last for many years.

French Drains 101: How to Eliminate Standing Water in Your Yard

DrDrainageFollow May 26, 2016 · 6 min read

As summer arrives and amidst ongoing public concerns about Zika Virus and its transmission by mosquitoes, the single most effective thing a homeowner can do to minimize the presence of mosquitoes is eliminate standing water wherever it occurs.

Because mosquitoes lay their eggs in still water, puddles and standing water can become a breeding ground after just one week, according to the Centers for Disease Control and Prevention. That’s why that low spot in the backyard where water always collects is now much more troublesome than just being an eyesore.

Homeowners may think solving their drainage problems will be difficult and costly, or that they’ll end up with an ugly drainage ditch on their property. But, it is possible to eliminate standing water cost-effectively and attractively by building one of the most popular and common solutions for getting rid of standing water, a French drain.

What is a French Drain?

A French drain is a gravel-filled trench that includes a perforated or slotted pipe. It is used to direct surface water or groundwater away from a specific area, such as a home’s foundation. French drains direct surface level water toward the lowest point and allow it to seep through the surface level gravel into the drain. This gravel also blocks the passage of excess debris. The water is then collected in the perforated pipe, running at the base of the drain, and directed away from the home and toward a more suitable area for daylighting or infiltration. French drains differ from typical surface drains because they collect water over the entire length of the drain instead of one particular spot.

How Does a French Drain Work?

Remember that liquid always seeks out the lowest point it can reach along the easiest path, readily moving into empty pockets in loose soil. That’s the secret to a French drain: It provides a reliably easy path, creating a sunken channel that encourages water to percolate out of the surrounding soil and flow along a smooth course. Leveraging gravity is essential for a French drain to function properly, as it first forces water down from the surface and out of saturated soil, then pulls it along the downward-sloping pipe to the desired discharge point.

Where Should I Install a French Drain?

An easy way to site a French drain is to watch where water pools — especially if it sits for hours or days — after a rain. Some of the most common drainage problems homeowners face are:

Flooding in a backyard. If heavy rains have left your yard with an unwanted water feature or the spring thaw has saturated your yard, a French drain can help. Placing a French drain in this wet region allows the drain to collect unwanted water and redirect it to a safer location, giving you back your green space.

Damage to an outdoor patio. Your patio is a great source of pride and a meeting place for family and friends, but excess water can damage the area, deteriorate the pavers, and also create a breeding ground for mosquitoes, ruining your ability to spend time outdoors. A French drain can work as a shield, collecting water before it reaches the patio and diverting it away. This will eliminate the standing water that mosquitoes need.

Damage to a home’s foundation and low-level areas like a basement. Check the walls in your basement. If you notice a musty smell or wet floor, you need a French drain immediately. A French drain can stop this water from ever reaching your home, protecting your basement from flooding and your home’s foundation from incurring additional damage.

What Do I Need to Know Beforehand?

There are many factors to consider before installing a French drain. Slope is essential, as a downhill course must be downhill enough to keep water running along to its intended destination. Generally, a French drain should drop at least one percent in depth for every 100 feet of length.

Other key issues to consider include:

French drain depth: About 8 inches to 2 feet deep should be sufficient for many water-diverting projects, though related systems, such as those built around foundations and sub-ground living spaces, as well as the bases of retaining walls, may be deeper.

French drain aggregate: The size of the gravel used can vary from pea gravel to larger pieces of river rock. If aggregate of different sizes is used, smaller pieces usually go closer to the pipe, while larger pieces sit closer to and on the surface.

French drain pipe length: This really depends on a project’s specific elements. Of course, the pipe should be long enough to carry water from the underground areas where it collects to an end point, where it daylights.

French drain flow: Slope is a big factor in maintaining a free flow of water, as is the aggregate placement and surrounding fabric to prevent debris from clogging the pipe.

How Do I Install A French Drain?

Here’s a step-by-step guide to installing a French drain on your property.

1. Decide on the best location. Determining the best location depends on where the problem areas are located, nearby elevation, and the condition of the soil. Use the design tips above to help with siting to ensure proper slope, depth, and more. When it comes to the soil, sandy soil located in an out-of-the way area is preferred.

2. Prepare to dig. It’s your French drain, but it doesn’t affect only you. City codes must be considered, and you need to be aware of how your newfound drainage solution will affect your neighbors. This is also an ideal time to contact the local utility company to have any underground lines marked before digging begins. Once you start digging your drain depth will need to be anywhere from eight inches to two feet, depending on your needs and available options.

3. Measure the grading. This process begins by pounding two stakes into the ground to mark the dimensions of the trench. A taut string is tied between them, allowing for easy grade measurements as the digging process begins. Regular measuring checks will be performed throughout the process.

4. Dig a trench across the slope. Digging the trench is the most labor-intensive portion of the project and as it is being dug, constant grading measurements are necessary. The width of the trench will depend on how large the drainage problem is. Smaller drainage problems can be handled with a trench that is 5 to 6 inches wide, while larger problems will require a larger trench.

5. Add the fabric. Once the trench has been dug, it will be lined with landscape fabric, and then additional gravel is added. This fabric prevents dirt from mixing with the gravel and promotes water percolation. Once the fabric has been applied, a light load of gravel is shoveled over the top so the corners of the fabric can be wrapped around this gravel, holding it in place.

6. Add the pipe. Place a slotted or perforated pipe at the bottom of the trench. The pipe needs to be directed towards an outlet to drain away from the home.

7. Fill with gravel. The drain is nearly complete. Now the gravel of whatever size you choose will be shoveled in. Once the gravel is in, top it with another layer of landscape fabric and then cover it with topsoil and new sod.

For more information about how to use French drains to solve many of the most common water problems, visit the NDS Home Drainage Center.

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