Do you need a French drain next to a slab or patio?

What a French drain is

A French drain is a trench lined with filter fabric, filled with gravel, with a perforated plastic pipe inside. It collects water from the soil and routes it away from the pooling area to a place where it can drain, such as a stream, swale, ditch or slope^[3]. The fabric keeps soil out of the gravel so water keeps moving.

It is not a surface drain. Water running across a patio or driveway needs a slope, a swale or a grated channel to catch it. A French drain deals with water that has already soaked in.

Fix the surface first

Most water problems at slabs start above ground. Before you pay for a buried drain, check these:

Check	What the guidance says
Ground slope	The IRC calls for at least 6 inches of fall within the first 10 feet from the foundation[2]
Hard surfaces by the house	Patios, walks and drives within 10 feet should slope at least 2% away from the building[2]
Tight lots	Where walls or lot lines block that fall, the IRC calls for drains or swales to move water away[2]
Gutters	EPA advises cleaning and repairing gutters regularly[1]
Downspouts	EPA says redirecting downspouts may not suit every site; consider local rules and where the water goes, to avoid property damage[6]

EPA's moisture guidance says the ground should slope away from the foundation so water does not collect around it^[1]. Those code numbers are model code. Your city may have adopted a different edition, so check your local code. See our pages on yard grading and downspout extensions.

When a French drain makes sense near a slab

A French drain is worth a look when:

• The yard stays soggy for days after rain, uphill of a patio, walk or driveway.
• Water seeps out of a slope onto a slab.
• Grading cannot be fixed because of a wall, a neighbor's lot or a tight side yard. The IRC names drains as an option there^[2].
• Water keeps washing out soil under the edge of a walk or patio, so panels sink. See why concrete sinks.

It is a poor fit when the yard is flat with nowhere to send the water. Delaware's environmental agency says French drains are not recommended where frequent flooding comes from a lack of slope, and that a poorly designed drain can increase the risk of water reaching the foundation^[3].

Why distance from the slab matters

Digging right along a slab edge can do harm. A trench next to a house slab or a patio can remove soil support at the edge. A gravel trench can also collect water and hold it near the slab if the outlet is poor.

This matters most on expansive clay. The Foundation Performance Association says repairs on clay depend on keeping soil moisture under and around the foundation steady and moving stormwater away. It also says continuously saturated soil harms the foundation and repair piers^[5]. A drain that feeds water into clay next to a slab works against that. On clay soils, ask a structural or geotechnical engineer where the trench should go before anyone digs. Our expansive clay page explains why.

Delaware's guidance draws the same line in plain terms: a simple, shallow yard drain can be a DIY project, but deeper drains, or drains close to the building's foundation, call for a drainage contractor^[3].

How a yard French drain is built

These are the basic parts. Local soil, depth and outlet rules change the details.

1. Plan the outlet first. The pipe must fall the whole way to a place where water can leave: daylight on a slope, a swale, a ditch or another approved spot^[3]. Do not aim it at a neighbor's yard. Delaware's guidance says to check each year that the drain is not causing flooding or pooling on neighboring property^[3].
2. Contact 811. See the next section.
3. Dig the trench. Delaware describes simple yard drains as 8 to 24 inches deep^[3].
4. Line it with filter fabric, add gravel, set the perforated pipe, cover with more gravel, and wrap the fabric over the top^[3].
5. Backfill and restore the surface.
6. Maintain it. Delaware lists monthly clearing of debris, leaf litter and rodent nests, and a yearly check that the drain still works^[3].

Contact 811 before you dig

Every dig needs a call, even a shallow trench by a patio. The national 811 campaign says to contact 811, or your state 811 center's website, a few business days before you dig. It also says some utilities are buried just inches below the surface, and that shovels are the second most common cause of damage after backhoes^[4]. The service is free^[4].

Wait until the utilities mark their lines, then dig carefully around the marks^[4]. Your state sets the exact notice period.

What it costs

Cost data for yard French drains is thin. Delaware's environmental agency published these 2022 planning estimates for a 60-foot run^[3]:

Type (60 linear feet)	Estimate (2022 dollars)
Materials for a simple DIY drain, 8 to 24 inches deep	$600 to $2,000
Installed deep, complex drain	$1,000 to $4,000
Range across both	$600 to $4,000

Treat these as one agency's estimate, not a quote. Depth, outlet distance, concrete to cut and restore, and access all move the price.

When to call a pro

• The slab is a house foundation on clay soil: ask a structural engineer where any drain should go.
• Panels of a walk or patio keep sinking after rain: find the water source before lifting or replacing them.
• There is no obvious low spot to drain to, or the outlet would cross a property line.
• The trench would run within a few feet of the house or under existing concrete: hire a drainage contractor.