WHAT IS A
FRENCH DRAIN?

Although a French drain may sound exotic, the reality is that it is a very common and important tool. French drains are a drainage system invented by American Henry Flagg French in the nineteenth century. As a claim to fame, French was also the first president of what is now UMass Amherst (formerly the Massachusetts Agricultural College).

A French drain is a system for removing surface-level water from fields and is more commonly now used to remove water either from inside homes or in yards. Essentially, a French drain is an underground guttering system, moving water from where it’s not wanted. 

This guide will walk you through all the key aspects of a French drain – from the basics, to cost, to installation – so you can see if a French drain is the right fit for your home.

HOW DO FRENCH
DRAINS WORK?

At its simplest, a French drain is simply a trench – usually in your yard, but sometimes in your basement – that moves water away from your home. The initial entry point is a drain at a higher point than the rest of the piping system. Water enters the drain and then is piped down to an exit point far away from your home.

French drains work based on the simplest principle around: gravity. They create a channel that heads downhill, away from your home.

As water enters the drain, it will move away from your home, preventing water from either saturating the ground around your home or from entering your home through weaknesses in the building envelope, such as foundation cracks.

The exit point for a French drain needs to be:

Low-lying

Far from your home

For that reason, the most common places to vent the water out are:

  • Drainage ditches
  • The street
  • A dry well
  • A storm drain
  • Any low-lying spot on your property

WHEN DO I
NEED A FRENCH
DRAIN?

French drains solve a multitude of problems, so there is a variety of different circumstances in which you might need one. If any of the following apply, then a French drain may be an option for you:

There is water entering your basement

There is water in your crawl space

Your yard or driveway has standing water

You are building a retaining wall

In each of these cases, water is pooling – or likely to pool – in the wrong place. This can lead to further problems, such as mold in your home, or damage to the structure of your foundation. In these instances, a French drain is a quick (and cost-effective – particularly in the long-term) solution to fix the problem.

PROS AND CONS
OF FRENCH
DRAINS

Before installing a French drain, you need to decide if it’s right for you and your home. Use the following to weigh up the pros and cons and see what’s best in your situation:

PROs:
  • Efficient. French drains are highly efficient ways of piping water away from your home.
  • Affordable. Installing a French drain will cost around $500 (less, if you do the work yourself), considerably less than fixing structural damage to your home.
  • Speed of installation. Putting in a full French drain will usually take a weekend (less, if you have a friend or family member help).
  • Saves your lawn or basement. French drains are very much a ‘stitch in time that saves nine.’ Putting one in will help prevent the need for expensive work to your lawn or basement at a later date.
CONs:
  • Difficulty of installation. If you don’t have a decent level of experience, installing a French drain may be too tricky a task. If you have buried pipes or tree roots then there is an extra difficulty that an amateur may not be able to overcome. In addition, if you’re not physically able, the concept of digging a ten-foot (at least) trench may not be possible. In that instance, you will need to hire professionals.
  • Potential clogging. If stones, mud, or other debris get into a French drain it can quickly clog up the whole system. This leads to backups which can make the problem even worse than it was before the drain was installed. It may even involve digging up the whole structure to find and flush out the blockage.

HOW MUCH DOES
A FRENCH
DRAIN COST?

As mentioned above, the cost of a French drain is lower than the potential cost of not installing one. Water can be extremely damaging to a home, especially if left unchecked, and could cause damage totaling thousands of dollars.

The cost of installing a French drain depends on whether it’s an interior drain or an exterior one.

Interior
French Drain

If you install a French drain in the basement of your home, it can be labor-intensive, which drives up costs. To calculate the cost of internal installation, work out the perimeter of your basement and then multiply that by $50 to $60 (the price per linear foot). For a standard 20-foot by 30-foot basement, the perimeter is 100 feet, meaning the final cost will be between $5,000 and $6,000.

Exterior
French Drain

Exterior French drains are cheaper (primarily because they don’t involve digging up a basement floor). The average cost of installation for a system in your yard is likely to cost between $10 and $15 per foot – meaning an average total cost of $1,000 to $1,500.

Adding a
Sump Pump

Adding in a sump pump will increase your bill by around $200. For basements with a perimeter more than 150 feet, you may require an additional sump pump.

HOW TO
INSTALL A
FRENCH DRAIN

Because of French drain systems are relatively simple, you can install them yourself without too much difficulty. If you are planning on installing a long drainage system (more than 15 feet) you may want to hire an excavator to help dig the trench but in general, installing a French drain yourself is a project you can do over a weekend.

THE STEPS BELOW WILL GIVE YOU AN OVERVIEW OF WHAT IS INVOLVED:

1

Find the right spot

The best way to find where you need to put the drain is to wait until a heavy rain. Head out into your yard and see where the water is pooling. Place markers where the puddles are and that’s where you’re going to need to dig the trench or trenches.

Before you start digging, you’ll need to check your city’s regulations to see if there are rules on where you can and can’t pipe water.

Assuming you’re not violating code, you’ll want to plan out a trench that runs from the waterlogged areas to a storm drain, the curb, or other outlet. Once you know the start and endpoint of your drain, mark the route with spray paint or stakes.

2

Dig a trench

Following the path you have marked out, dig a trench about 18 inches deep and a foot wide. The trench should ideally run down a slope, letting gravity do the hard work. If not, you will need to dig it with a gradient of 1% (to help with the math, it should drop 1 inch for every 10 feet it travels).

3

Line the trench

Once you’ve dug the entirety of the trench, you line it with a water-permeable fabric filter. These usually come in rolls; unfurl the role along the length of the trench, leaving at least 10 inches of fabric either side of the trench.

4

Pour the gravel

Push the filter fabric down into the base of the trench, creating a depression. Then pour in gravel to a depth of about 3 inches in the base of the trench.

5

Build an inlet

At the place where the trench begins, install an inlet grate. This is where water will enter the drain. You can create your own inlet pipe with PVC pipe, just drill holes every six inches along the pipe to allow water to enter.

6

Lay pipe in the trench

Connect the pipe to the inlet you have created, and then lay the pipe in the trench, on top of the gravel. Once you’ve laid it down, test it by pouring water into the inlet. It should flow out of the other end without too much difficulty.

7

Cover the pipe

If you’re happy with the flow of water through the pipe, you can cover the pipe with another 3 inches of gravel. This should not raise it above ground level. Then, fold the filter fabric over the top of the pipe.

8

Fill in the trench

Now you can cover the entire trench with topsoil until it is at the same level as the surrounding ground. Be warned, when you are filling in the area around the inlet, place a tarp or other cover over the grate so that loose stones and rocks don’t enter the drain (potentially leading to blockages).

Twice a year, you should inspect the inlet and outlet of the grate to ensure there are no blockages. Spray water from a hose into the grate to flush out any debris that has got stuck in the drain.

Ultimately, a French drain is a simple solution for preventing water buildup. Such is the simplicity of the system that you can usually install it yourself without the need for too much expertise. Not only will a French drain help keep your yard looking tidy, but it can also have major long-term benefits in preventing structural damage to your home. And that alone is a good enough reason to install one.