Scenario: Drying Out a Soggy Side Yard

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.

Is a French Drain the Right Fix?

Before you dig, check that a French drain actually matches your problem. A French drain intercepts and conveys water to an outlet — it is not a magic sponge. Use this table to confirm:

For this side yard the problem is the third and second rows combined: two downspouts plus a soggy clay strip with a downhill outlet available. That is a textbook French-drain application. See common drainage mistakes for the errors that make a drain fail.

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, composition shingle (C = 0.90, HEC-22)
• Side yard surface area: 320 sq ft of lawn on flat clay soil (C = 0.15, HEC-22)

These are standard HEC-22 values — you can look them up in our runoff coefficient table. Using the Rational Method with a 10-year rainfall intensity of 4 in/hr:

Q = (0.90 x 800 + 0.15 x 320) / 43,560 x 4 = (720 + 48) / 43,560 x 4 = 0.07 cfs

We will design for 0.15 cfs to provide a generous safety factor (about 2x). French drains accumulate fine sediment over their lifetime, so oversizing by 50-100% is good practice. You can reproduce this design flow in the Rational Method Calculator.

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

Frequently Asked Questions

What size French drain do I need for a soggy side yard?

For a typical residential side yard collecting one or two downspouts, a 4-inch perforated PVC pipe in a 12-inch-wide by 18-inch-deep gravel trench handles the flow. In this example the design flow is about 0.07 cfs, and a 4-inch pipe at 1.5% slope carries well over that before you even count the gravel envelope, so it has a comfortable safety factor.

How much slope does a French drain need?

Aim for at least 0.5% (about 1 inch of fall per 16 feet). The 1-3% range is ideal. In this example the side yard drops 7 inches over 40 feet, which is about 1.5% — well within the ideal range and enough to keep the pipe self-cleaning.

Will a French drain work in clay soil?

Yes, but it works differently. Clay has a very low infiltration rate (about 0.01 in/hr), so the gravel envelope will not drain into the surrounding soil. In clay you must size the pipe to carry the water to a daylight outlet or dry well — the drain acts as a collector and conveyance system, not an infiltration system.

How much does this side-yard French drain cost to build?

As a DIY project the materials for a 40-foot residential French drain run about $150-250. A landscaper typically charges $1,500-3,000 for the same run installed. See our full French drain cost breakdown for regional ranges.

Next Steps

1. Read the French Drain Installation Guide for step-by-step installation instructions
2. Check How Much Does a French Drain Cost? for detailed cost breakdown
3. If the neighbor’s runoff is part of the problem, compare with the property-line swale scenario
4. Redirect the downspouts into the French drain trench with solid PVC adapters