Solving Yard Drainage with a French Drain
If your yard turns into a swamp after every rainstorm, or worse, if hydrostatic pressure is forcing water through your foundation walls into your basement, a French drain is the most effective permanent solution.
Despite the name, a French drain has nothing to do with France (it was popularized by an American farmer named Henry French). It is simply a trench filled with gravel containing a perforated pipe. Water always follows the path of least resistance. Surface water naturally sinks through the highly porous gravel, enters the holes in the pipe, and is safely channeled via gravity away from your house to a lower discharge point.
While digging the trench is the hardest physical labor, accurately calculating the massive volume of gravel required is the most critical logistical step.
The Core Components of a French Drain
A properly built French drain consists of three elements:
- The Trench: Dug at a continuous downward slope of at least 1% (a 1-inch drop for every 10 feet of length).
- The Perforated Pipe: Usually 4-inch corrugated PVC or rigid SDR-35 pipe, laid at the bottom of the trench with the holes facing down (so water rises into the pipe from the bottom).
- The Washed Gravel: Clean, round rock (usually 1-inch to 1.5-inch diameter) that fills the trench around and above the pipe. Never use "crusher run" or gravel with limestone dust; the dust will turn to cement and clog the drain permanently.
(Note: The entire trench should be lined with non-woven geotextile landscape fabric before the gravel goes in, to prevent surrounding dirt from washing into and clogging the gravel).
How to Calculate French Drain Gravel Volume
Because gravel is ordered and delivered by the Cubic Yard, we must calculate the volume of the rectangular trench in feet, subtract the volume of the empty pipe inside it, and convert to yards.
The Formula
- Convert the Trench Width from inches to feet (divide by 12).
- Convert the Trench Depth from inches to feet (divide by 12).
- Multiply Length × Width × Depth to find the Total Trench Volume in Cubic Feet.
- Calculate the volume of the pipe. (Pipe Radius in feet squared × 3.14 × Trench Length).
- Subtract the Pipe Volume from the Total Trench Volume to get the Required Gravel in Cubic Feet.
- Divide by 27 to convert Cubic Feet into Cubic Yards.
Cubic Yards = ((Trench L×W×H) - Pipe Volume) ÷ 27
Example Calculation
You are hand-digging a French drain to protect your patio. The trench is 50 feet long, 12 inches (1 ft) wide, and 18 inches (1.5 ft) deep. You are laying a standard 4-inch corrugated pipe at the bottom.
- Total Trench Volume:
50 ft × 1 ft × 1.5 ft = 75 cubic feet - Pipe Radius in feet:
2 inches ÷ 12 = 0.166 ft - Pipe Volume:
3.14 × (0.166)² × 50 = 4.3 cubic feet - Subtract Pipe from Trench:
75 - 4.3 = 70.7 cubic feet of gravel needed - Convert to Yards:
70.7 ÷ 27 = 2.61 cubic yards
You should call your local quarry or landscape supply yard and order 3 cubic yards of washed drainage rock.
Bagged vs. Bulk Gravel
Can you just buy bags of gravel at the hardware store? A standard bag of drainage rock weighs 50 lbs and contains roughly 0.5 cubic feet of stone.
In our example above requiring 70.7 cubic feet of gravel, you would have to buy, load, and transport 141 bags of rock! Not only would this require dozens of trips in a heavy-duty pickup truck, but bagged rock is exceptionally expensive. For any French drain longer than 10 feet, it is always cheaper and easier to have a dump truck deliver bulk gravel by the cubic yard.