The Situation
The narrow side yard between your house and the fence is always muddy. It is about 40 feet long and 8 feet wide (320 square feet). Roof runoff from two downspouts dumps directly into this area, and the clay soil keeps it saturated for days after every rain. You want to install a French drain to intercept and carry this water to the backyard, where the grade drops enough to daylight the pipe.
Which Calculator to Use
The French Drain Calculator helps you determine the pipe size, trench dimensions, and gravel quantity for a subsurface drainage system. It accounts for both the pipe capacity and the infiltration capacity of the gravel envelope.
Walking Through the Inputs
Design Flow (Q = 0.15 cfs)
First, we need to estimate how much water this French drain needs to handle. The contributing area includes:
- Roof area draining to two downspouts: approximately 800 sq ft (C = 0.95)
- Side yard surface area: 320 sq ft of lawn on clay (C = 0.35)
Using the Rational Method with a 10-year rainfall intensity of 4 in/hr:
Q = (0.95 x 800 + 0.35 x 320) / 43,560 x 4 = 0.08 cfs
We will use 0.15 cfs to provide a generous safety factor. French drains eventually accumulate some fine sediment over their lifetime, so oversizing by 50-100% is good practice.
Trench Length (L = 40 ft)
The drain runs the full length of the side yard, from the front corner of the house to the backyard discharge point. 40 feet.
Trench Width (W = 12 inches)
A 12-inch-wide trench is the minimum standard for a residential French drain and fits the narrow side yard. In tight spaces, 12 inches is appropriate. In open areas, 18 inches provides more storage and filtration capacity.
Trench Depth (D = 18 inches)
An 18-inch-deep trench allows for 4 inches of gravel below the pipe, the pipe itself (4 inches), and 8-10 inches of gravel above the pipe. This is the standard residential French drain depth.
Pipe Diameter (d = 4 inches)
A 4-inch perforated pipe is the standard for residential French drains. At 1% slope, a 4-inch pipe carries about 0.10 cfs, plus the gravel provides additional flow capacity through interstitial spaces.
Pipe Material (PVC)
Smooth-wall PVC is preferred over corrugated pipe for its better flow characteristics and longer lifespan. The perforations allow groundwater to enter the pipe from the surrounding gravel.
Slope (S = 1.5%)
The side yard drops about 7 inches over 40 feet, which gives us:
7 inches / (40 feet x 12 inches per foot) = 0.0146 = approximately 1.5%
This is an excellent slope for a French drain — above the 0.5% minimum and well within the 1-3% ideal range.
Soil Type (Clay)
The surrounding soil is clay, which has a very low hydraulic conductivity (about 0.01 in/hr). This means the French drain’s gravel envelope will not drain into the surrounding soil very quickly — the system must rely primarily on the pipe to convey water to the outlet.
The Results
The calculator confirms:
- 4-inch PVC pipe at 1.5% slope: Adequate pipe capacity
- Gravel quantity: Approximately 0.9 cubic yards (about 1.2 tons) of washed 3/4-inch gravel
- Filter fabric: Approximately 100 linear feet of non-woven geotextile (to line the trench walls and top)
What This Means for Your Project
This is a straightforward DIY French drain project:
- Estimated material cost: $150-250
- Estimated time: 1 full day for one person, half a day for two people
- Difficulty: Moderate (the digging is the hardest part)
- Tools needed: Shovel, level, tape measure, utility knife, PVC cement
The trench is only 18 inches deep and 12 inches wide — no trenching machine needed. Hand digging in clay is tiring but manageable for a 40-foot run.
Try This Scenario
Try this scenario in the French Drain CalculatorNext Steps
- Read the French Drain Installation Guide for step-by-step installation instructions
- Check How Much Does a French Drain Cost? for detailed cost breakdown
- Redirect the downspouts into the French drain trench with solid PVC adapters