The Ultimate Guide to French Drains

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Have you been dealing with wet spots in your yard, pooling water around your foundation, icy walkways or worse, water in your basement? 

If so, you have probably searched the internet for solutions to the ongoing headache. 

Am I right? 

Well, a french drain could just possibly be the answer to your prayers. 

You see, there are times when a perfectly implemented groundwater management system including gutters, downspouts, extension, proper grading, and positive slope around the foundation just aren’t getting the job done. Perhaps your neighbor’s drainage pours directly into your backyard. 

If this is the case, an external french drain may be right for your property. 

Read on to learn the in’s and out’s on french drains, how to install one, and a thorough explanation about whether it’s right for you. 

What is a French Drain

rock filled

Contrary to what the title implies, external french drains are simple enough to install yourself (typically). The hardest part is going to be the design phase. This is where you can talk to a professional to ensure your plans are on track. 

 So, what are they?

A french drain is a trench containing perforated pipe and gravel that leads water away from a building or property and towards a given destination such as a swale, dry well, or other suitable drainage areas. 

Say what? 

Don’t worry, I said it was simple and it is. Essentially, it’s a pipe set into the ground and backfilled with gravel. And to back up, sometimes there is no pipe at all. 

dig out

Let’s just go over some anatomy a bit before we get into the juicy stuff. 

The components of a french drain are: 

  • Gravel: The stones should be 1) no smaller than ¾ inches and 2) dense. Some examples could be granite or river gravel. These types of gravel actually allow for water flow versus something like a lime rock, crushed rock or small pea gravel should be avoided since it can hinder water movement.
  • Perforated pipe: Rigid PVC (with pre drilled holes on one side) or corrugated pipe with slits will work. Most professionals will use a 4” pipe but each situation is unique. If there is an extraordinarily large volume of water, it may be more suitable to use a 6” or 8” pipe. If you are unsure, get a professional’s opinion. We prefer PVC for a few reasons: 1) Corrugated pipes are structurally more likely to hold standing water in both the pipe itself and the corrugations and 2) They can be more prone to clogging in some scenarios. Ultimately, the corrugated are easier to work with, but functionally we prefer the Rigid PVC. 
PVC pipe and corrugated pipe
  • Catch basin: You may choose to use a catch basin or two in your French drain system. Typically this will be a 1 foot by 1 foot plastic or concrete box with a grate at the top and an outlet that your pipe can attach to. 
catch basin
  • Landscape fabric: Preferably use a non-woven geotextile drainage fabric in order to prevent the subgrade from moving into the drain rock and causing problems like clogging. Tip-Use 4 to 6 ounce weight non-woven drainage fabric for best results. 
  • Slope: French drain’s work on gravity, so there must be at least a 1% slope in the soil for water to be carried successfully to it’s drainage point. Many contractors actually recommend that the drain slopes 1 inch for every 8 feet in length. 
french drain trench
  • Trench: Your trench should be about 12 inches wide (no smaller) and 18-24 inches deep. 

How do French Drains Work

drain system downhill

*This client’s driveway used to wash out yearly. Complete Building Solution’s French drain design and geotechnical water management plan now keeps the driveway intact. 

There’s nothing worse than a swampy yard or basement water intrusion issues stemming from poor drainage. 

Sometimes, proper drainage basics aren’t enough to deal with the amount of water that is ending up on a property. This may be particularly true if you live at the bottom of a hill or on a low spot. 

The solution? Well, a French drain could be one option.

A traditional French drain works by using gravity to funnel water to a desired location…hopefully away from your yard. 

Rainwater will soak into the soil, move into the gravel-filled trench (the French drain) and then flow upwards into the perforated pipe located at the bottom of the trench. From here, the water will travel with the sloping grade via gravity to the terminus. 

Watch this awesome video to see what we mean. 

Keep in mind that every single scenario is unique. There may be times when a pipe is not necessary. A landscape professional or geotechnical structural engineer can help. 

Why is Gravel Used in a French Drain?

building a french drain

The point of the French drain is to provide a channel for water to flow, correct? Well, gravel does not hold onto water the way that some soils, like clay, do. Because of this, water easily flows through the gravel and into the perforated pipe.

If clay were surrounding the pipe, the soil would soak up as much water as possible, preventing it from efficiently flowing into the French drain. This is the opposite of what we are trying to accomplish. 

When is a French Drain Necessary?

grass with puddles forming

*Yard with standing water

French drains become necessary when typical drainage solutions are not working. 

When it comes to drainage solutions there are various basics that must be covered before going into specialty solutions. 

Every home should consider the following basic drainage strategies before looking into French drains or other options. 

  • Gutters, downspouts and extensions 
  • Positive slope away from the foundation 
  • Eliminating water traps around the foundation such as  landscape borders or large tree roots 

To learn more about these basics principles click here

If these factors have all been addressed and you are still having drainage issues, it may be time to move to phase 2. 

It’s important to understand why you are having drainage issues in the first place. Was your home built on poor soils? Soils like organic and alluvial clay soak up water like a sponge and never seem to dry out. This can cause tremendous amounts of frost heave in the winter. 

Or, 

Do you live at the bottom of a hill or somewhere with low elevation? In a case like this, you may need to put some effort into developing a good drainage plan. 

bottom of a hill drain

*This client’s home was located at the bottom of a hill. Water naturally flowed right to the home’s foundation causing a plethora of issues including foundation shifting and cracking. 

Hiring a professional is always best because they have years of experience working with different soils, topography and the water intrusion issues they present. A French drain may be a part of their solution. 

So, what issues exactly could indicate the need for a french drain? 

crack in foundation

*Shifting and cracking garage apron due to moisture intrusion

Here’s a list:

  • Wet basements 
  • Frost heave issues 
  • Shifting garage aprons and driveways
  • Standing water in your yard 
  • Low or wet spots 
  • Water pooling in your driveway or walkways 
  • Icy driveways and walkways
  • Faulty retaining wall 
  • Flooded sports fields 
  • A cracking patio

As a structural engineering firm, customers call us regularly with concerns about how poor drainage can affect their foundation. Their concerns are very valid.

When water builds up in soils, a large amount of pressure can be put upon the foundation walls. Overtime, cracking, shifting or moisture intrusion can occur. This obviously de-values the home significantly and affects the structural integrity. 

crack from water damage

*A garage block wall showing serious signs of cracking and shifting due to frost heave from excess moisture and water run-off. 

Click here to learn about the dangers frost heave can impose upon your foundation. 

Sometimes, French drains are the answer to eliminating the drainage issues that are negatively impacting your home’s foundation. 

At the very least, a French drain may be able to fix the nuisance that excess water imposes on a property. 

Will a French Drain Stop Water from Coming into my Basement?

wet floor

We couldn’t write this blog post without addressing this specific question. As mentioned above, water in the basement is a huge concern for many people. 

Think of it like this, French drains aren’t specifically stopping water from infiltrating your basement, but rather creating a new pathway that drives water away from the home and towards an exit point. By channeling the water away from the foundation, water doesn’t have a chance to find a route inside the house. 

Another factor to consider is hydrostatic pressure caused by large amounts of water sitting around foundation walls. Over time, this pressure can cause foundation damage leading to water infiltration in the basement. 

3D model of draining

You see, certain types of soils can absorb and hold more water. If your home was backfilled with poor soils and you have drainage issues, your foundation is in danger. 

Again, in this situation, a French drain may be helpful. It could prevent excess water from building up in the soils and therefore relieve pressure on the foundation walls. 

Voila! Your home’s structural integrity is safe. 

To learn more about foundation repair options click here. 

How to Install a French Drain

pipe installation

Before you go ahead and install a french drain, consider hiring a professional such as a structural engineer or landscape expert to give their two cents.

Ensuring that you know the root cause of your drainage issues is essential. 

For instance, if you happen to live on top of underground springs or have undiagnosed leaky pipes causing havoc under your property, a simple French drain will NOT even begin to fix your issues. 

leaking pipe

Or, maybe your landscape issues aren’t as bad as you thought and the solution is a proper groundwater management plan. 

Easy peasy.

Remember, every single French drain install will be individualized to the project at hand. If you live in Florida, your install will be different than if you live in northern Minnesota. Even in areas with similar soils, installs will be unique to the property and the specific issues presenting. 

So, the French drain install instructions we are listing below are simply an example of what an install could look like. If you have the know-how to design and install a French drain system yourself, then by all means, please use this for your reference. 

Step by step instructions for installing a French drain: 

Step 1) Check your local building codes- This may come as a shock, but many cities have rules when it comes to drainage. They may not allow you to run water out to the road, for instance. If this is the case, you may need to consider a dry well or another drainage alternative. 

Step 2) Location, Location, Location!

Knowing where to install your french drain is crucial. If you put it in the wrong place, your drainage issues will remain. Here are some tips to determine the best location. 

Here are some tips:

  • Elevation- Look for areas on your property where standing water seems to be a problem. Offering this standing water a new path via French drain is the goal. Since French drains work by gravity, the floor of the trench needs to continuously drop in elevation by 1% as a rule of thumb.
  • Outlets- Where you will drain excess water is also important. Do NOT drain into your neighbors yard. I know it’s tempting, but it won’t turn out well ha. If your city allows it, you can drain to the street. Otherwise, retention ponds, bodies of water, rain gardens, dry wells or other drainage pathways will work. 
  • Mark your trench- Use stakes to mark out your french drain’s starting and stopping points. 

Step 3) Dig your trench- Before you start digging, have your local utility company mark underground lines. You don’t want to hit anything. Start digging, your trench should be somewhere between 18 inches to 2 feet deep and at least 12 inches wide.

dig out for drain

Step 4) Figure out slope-  Remember we want a 1% slope to ensure water flows smoothly to the outlet. 

Here’s an easy way to do this for DIYers. 

Stomp a shovel (or stake) into the start of your trench. Stomp a second shovel (or stake) 100 inches down the trench. Run a line from the first shovel to the second with a line level attached. Make sure it’s level. Then measure the height of the line at the first shovel. Measure the height of the line at the second level. For example, if the height of the line at shovel 1 was 50 inches and the height of the line at shovel 2 was 51 inches, that means that in a 100 inch run, we have a 1% slope. Keep in mind the slope can be greater, 1% is a minimum. The more slope you have, the faster water will move which may not alway be great for your specific issues. 

Check out this video (watch minutes 1:04-3:00) for a great example on how to create the right slope for french drains. 

Step 5) Add landscape fabric if needed- Line the trench with landscape fabric and add a few inches of gravel to the trench. Industry standards for French drains are 4 to 6 oz non-woven drainage fabric. The fabric is responsible for preventing dirt from mixing with the gravel. 

Step 6) Lay piping- Next, lay your perforated piping of choice into the trench. When using PVC, make sure the holes are facing downwards. This is important. Rain water will seep into the soil and then raise upwards where it can enter the pipe holes and be moved out to the terminus.

Holes have to face down

Step 7) Fill the trench with gravel- Finally, fill the trench to grade with gravel. Close your landscape fabric up. Once the landscape fabric is laid, you can cover it with topsoil and add a layer of sod if desired.

Here’s a few final thoughts on the french drain install process: 

  • Install catch basins through the French drain if desired. Catch basins help to drain surface water directly into the French drain where it can effectively be moved off the property. 
  • Never connect your french drain directly to a gutter downspout. This is a remedy for disaster. Instead, you could install a catch basin below a downspout. 
  • Consider saving your back and renting a trencher for larger drains. 
  • Interested in using rainwater for your garden? Place a catchment barrel at the end of your drain. 
  • Consider the large amount of dirt that will be remaining post install. Plan ahead. 
drain under gutter

*Example of catch basin under a downspout

Can a French Drain Become Clogged?

Puddle on grass

Unfortunately, like any form of drain, a French drain can become clogged. This can happen if dirt or other debris builds up in the pipe, blocking the flow of water. 

So, what can be done?         

Well, the first step is to actually test the drain and make sure it is blocked. 

Here are the steps to check French drain function: 

  • Locate the access point to the French drain. This could be a catch basin or possibly a pipe sticking out of the ground. 
  • Remove the cap or fitting and run a hose into the drain
  • Notice if the system backs up. If so, there is an issue…probably a blockage. 

If you determine that your French drain is blocked, it’s important to deal with the situation. You may never know how beneficial your French drain is until you no longer have it. 

Here’s how to fix a clogged French drain:

power washer for French Drains
  • You may first want to try using a pressure washer to unclog the drain. Be cautious because water may end up shooting back at you. 
  • If a pressure washer was unsuccessful at removing the blockage, try a sewer snake. If you’ve never used one, it’s essentially a piece of piping that can be pushed down the French drain to clear blockages. Once you get some movement with the snake, try finishing the job off with the pressure washer again. 
  • If these options don’t work, call a professional for assistance. 

Can a French Drain Go Uphill?

uphill french drain

We get asked this question a lot. “I live in a valley, can my french drain go uphill?” 

Our answer is, yes, there are always solutions. 

One simple solution is to dig a deeper trench in order to keep the 1% or greater grade necessary for gravity to channel water downhill. 

Click here for a video showing this. 

A project such as this will be a bit more complicated than most and we do not recommend DIY on this since additional drainage solutions may be necessary for the terminus of the French drain. 

Reach out to a landscape professional or a geotechnical structural engineer for more questions on this. 

What does a French Drain Cost?

French drains cost

This is a very tricky question to answer because every single project is different. However, it’s a general rule of thumb that a professional external French drain install will cost about $25-$40 per linear foot. Of course costs will vary based on the difficulty of the project. 

Some factors that could impact costs are: 

  • Length of drain
  • Depth of drain 
  • Location of drain 

Overall, the materials needed to make an external french drain are fairly inexpensive. The bulk of your French drain bill will be labor. 

So, if you decide to do it yourself, you can plan on paying about $10-$15/linear foot plus your time. 

Wondering how to accurately calculate your costs? Click here to use this French drain calculator. 

Note: Internal French drains are much more expensive and should be professionally done. We are referring to external yard French drains here. 

Who can I Hire to Install my French Drain?

home page of site for french drains blog

If you decide you’d rather hire out your French drain project, it’s time to either grab a referral or hop on the internet. Many landscape or drainage contractors are available to assist you. 

Local contractors will be familiar with the type of soils and common drainage issues in your area. This means they will probably know how to handle your situation. 

However, feel free to get several opinions. Ask your contractor up front if they offer free assessments. You can also ask them about the amount of experience they have dealing with your particular issues. 

If possible, try to call the contractors directly instead of using a platform such as Angie’s List or Home Advisor. Small family run businesses get crushed by the huge fees these conglomerates place upon their customers. 

Well, there you have it. We hope this guide on French Drains was helpful. If you have any additional questions about your drainage issues, please call us at 763-544-3355.

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