How to Build a French Drain Trench

Pooling water

The aqueducts of Ancient Rome exemplify transporting water using gravity and a slight decline. The French drain, invented by Henry French (1873-1885) applies this technique on a smaller scale. Moving excess water underground from one area to another is how this drainage system deals with structure flooding or pooling water.

Things You'll Need

• 2 stakes
• String
• String level
• Tape measure
• Shovel
• Pick axe
• Grub hoe
• Permeable landscape fabric
• Clean coarse 1 inch gravel
• Perforated PVC pipe and joints
• Joint glue or waterproof tape

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Instructions

1. 

   Cross section of working French drain, as it finds "daylight"

   Observe the landscape during rain to identify drainage patterns. Design a linear trench from the structure or pooling area to the lowest part of the lot. According to Ask the Builder Tim Carter, the system provides the water a way to "daylight," or where the water will be dumped.

2. 

   Use a level.

   Calculate trench dimensions based on the size PVC pipe needed for the drainage problem and type of landscape. Minimum trench dimensions for a 4-inch pipe are 10 inches deep and 6 inches wide. This allows for 2 inches of gravel below the pipe, 2 inches above, with 1 inch on both sides. As the size of the pipe goes up, so will the trench dimensions.

3. 

   The shovel is an indispensible tool for trench digging.

   Place a stake at each end of the designed trench. Tie the string tight to one of the stakes. Run the string to the other stake. Tie this end loosely so it is easier to adjust. Using the string level, adjust the string until it is level with the ground, then tighten the loose string to its stake. The string acts as guide while digging and allows you to measure the grade.

4. Dig the trench using hand tools, or a power trencher. Renting a trencher will cut the work and dig time significantly. Continue to check the grade with the tape measure. The declining grade should continue at 1 percent per 10 feet; at 10 feet the drop will be at 2 1/2 inches; at 20 feet it will be at 5 inches.

5. 

   Clean course gravel 1 inch or smaller

   Line the trench with permeable landscape fabric. Prepare enough to lap over the gravel and PVC pipe. The fabric will keep the gravel and pipe free of dirt and debris, extending the longevity of the drain.

6. 

   Cross section of PVC pipe

   Lay 2 inches of gravel on the fabric. Use only clean coarse gravel. Gravel maximizes the flow of water.

7. Lay PVC pipe with holes facing down, gluing or taping connecting pieces as needed. At the top end of the pipe, attach an elbow joint with extension pipe and cap that angles up to the surface of the trench. This is the clean out access. Drainage.net indicates, "Although a French drain ... can work well for a number of years, it more likely will become clogged and cause flooding at some point." Even with a clean-out access, design the trench 4 to 6 feet away from foundations to avoid back seepage.

8. Cover the pipe with 2 inches of gravel then wrap the fabric over. Fill in with topsoil.

Tips & Warnings

• Only use rigid pipe. Corrugated plastic tears when cleaning with a sewer snake.

• Roots and debris work through pipe joints. Applying glue or tape generously when making connections will reduce intrusions.

• If the landscape has large rocks or tree roots, begin digging at the downgrade end. It is easier to increase the grade, laying gravel and pipe over the obstruction, rather than digging it out.

• Never use limestone. It breaks down quickly, turning rock hard.

• Contact Dig Safe (Dial 811) before digging to locate dangerous underground gas and utility lines.

• Trenchers are dangerous equipment. Use with respect and with someone standing by.

References

• Ask the Builder: French Drain Design
• All About Drainage Systems: French Drains

Resources

• Installing a French Drain