French Drains: A Step-by-Step Guide

French drains are gravel-filled trenches used to keep water from collecting or flowing in places where it can cause damage. You can use them to carry storm water or groundwater away from the foundation of a house or dry out a soggy yard. Some French drains have pipes laid in them, others do not.

Why do I need a French drain?

You can use a French drain to correct a variety of problems. If water collects or flows where it does not belong, it may cause damage to your foundation or seep into your basement. Perhaps your yard turns into a mire or a huge shallow pond after a hard rain. If, during heavy rains, water runs under your house or very close to your foundation or if water flowing across your property damages landscaping or buildings, a French drain may be the solution. Finally, if your lot is located where the water table is very close to the surface, a French drain can help with seepage problems and protect your foundation.

Before beginning

Check with local authorities to see if you need a building permit and find out about how and where you may discharge storm water or seepage. Be sure that directing the water away from your property does not direct it towards a neighbor's property.

Materials

Landscaping fabric prevents soil and roots from clogging the drain after it is constructed. Landscape fabric comes in 3, 4, 5, and 6 foot widths. Determine the width of the fabric you need as follows: 2 × (width + depth) of the drain. For example, if you are building a French drain 1.5 feet wide and 3 feet deep, you will need 2 × (1.5 +3.0) = 9 feet wide. In this case, use two 5-foot wide lengths of landscape fabric to line the drain, overlapping them on the bottom.

Pipe. Perforated pipe is used to carry away surface runoff and groundwater. Use perforated clay tile drain pipe or PVC. PVC drain pipe comes in 10-foot joints. If using PVC, remember to buy PVC primer and glue to connect the joints. Use non-perforated pipe is used to collect water from downspouts. For this type of French drain, use flexible PVC fittings to connect the downspouts to the drain pipe.

Gravel should be hard, non-porous and coarse, usually 1-1/2 to 2-1/2 inches. When ordering gravel be sure to specify that it is to be used for a French drain. Sandstone and limestone gravels are unsuitable for use in French drains. Calculate the volume of gravel needed as follows:

Cubic yards of gravel = (width × depth × length of French drain, in feet) ÷ 27.

Siting a French Drain

The drain should be at least 3 feet from any building. For drains deeper than 3 feet, the distance from any building should be the same as the depth of the drain. Keeping this distance protects the foundation and keeps the trench from collapsing.

Decide where the water comes from (the high end of the French drain) and where you want to send it (the low end). It's always easiest to direct water to the downhill side of your house. You may want the water to empty into a dry well, a large cavity filled with gravel where it can leach into the ground, or you may want the drain to empty into natural drainage, such as a creek.

There should be at least 1/8 inch fall for every foot of drain. This is about 1 foot fall for every 100 feet of length. Use marking paint, the kind in a can that sprays upside down, to mark the path for the French drain.

Setting the Grade for a French Drain

Use the following steps to set the grade for the drain, or use a surveyor's transit:

1. Drive stakes into the ground beside the path of the ditch at both the high and low ends of the French drain.

2. Tie a piece of twine between the two stakes and pull it taut.

3. Hang a torpedo level (the kind that hangs on a string) from the string in the middle.

4. Adjust the string to make it level.

5. At the low end of the drain, lower the string until the line in the level cuts the bubble approximately in half on the high side. This is called "breaking the bubble."

6. At the high end, dig a hole about 12-18 inches wide in the path of the French drain and deep enough that water in the area will flow into it.

7. Take another stake and hold it on the bottom of the hole. Mark the stake with tape or a pencil at the height of the string.

Digging the Drain

1. Dig for about 3 feet, and then check the depth of the ditch by comparing the mark on the stake to the string. Adjust the depth of the ditch so that when the stake rests in the bottom of the ditch the mark is level with the string.

2. If you are not using pipe, the ditch should be several inches deeper than the bare minimum to collect the water from the surrounding ground. Dig the ditch, keeping the fall even with the string.

3. Trim roots from the bottom and sides of the ditch.

Constructing the Drain

1. Line ditch with landscaping fabric to prevent roots from invading the drain and soil from clogging it. Hold the edges of the fabric down on the bank of the ditch with rocks.

2. If you are not using pipe, fill ditch with at least 6 inches of gravel. If you are using perforated pipe, put 2 or 3 inches of gravel into the bottom of the ditch along its length. Rake smooth and lay the pipe in the ditch with the holes facing down. If the French drain is intended to collect water from downspouts, connect the downspouts with flexible fittings to the non-perforated pipe in the drain.

3. Cover the pipe with gravel nearly to the top of the ditch.

4. If necessary, grade the surrounding ground toward the drain.

Finishing the French Drain

There are a number of ways to finish a French drain. Make a border of rock or bricks along the banks of the drain to hold the landscape fabric in place. You can make the drain invisible by folding the fabric over the top of the gravel, covering with an inch of topsoil, and laying sod.