Drainage
How Deep Should a French Drain Be?
Standard yard French drains run 18-24 inches deep; foundation drains must reach footing depth (6-10 ft); interceptor drains go below the water path. Depth rules by drain type, with width and cost impact.
Depth is the French drain question with the most conflicting answers online, because there is no single right depth: there are three different drains that share a name. Identify which one you are building and the depth chooses itself.
Depth by drain type
| Drain type | Problem it solves | Depth | Width |
|---|---|---|---|
| Yard / lawn drain | Soggy areas, shallow subsurface water | 18-24 in | 12 in |
| Curtain / interceptor drain | Water flowing downhill toward a structure | 2-4 ft, below the flow path | 12-18 in |
| Foundation perimeter drain | Basement or crawlspace seepage | At footing: 6-10 ft (basement), 2-4 ft (slab/crawl) | 12-18 in |
| Downspout collector | Roof water routed away underground | 12-18 in (solid pipe, not a true French drain) | 8-12 in |
The principle: intercept the water, not the surface
Water moves through soil along the easiest path, usually the interface between topsoil and denser subsoil, or along a clay layer. The drain must sit at or below that path to catch anything. A soggy lawn usually means water perched in the top 18 inches, hence the standard depth. A wet basement means water at 8 feet, and no 18-inch trench will touch it. Digging the standard depth for the wrong problem is how homeowners end up with a working drain and a still-wet basement.
What extra depth costs
Each additional foot of depth on a 50-foot drain adds roughly 1.9 cubic yards of digging and spoil, several tons of stone if you fill full-depth, and, critically, 12 inches of required outlet elevation: a deeper pipe needs a lower outlet, which flat lots often cannot provide. Run your dimensions through the excavation calculator for spoil volume and the French drain calculator for stone tonnage before committing to depth you may not need.
Past 4 feet, OSHA-style trench safety applies to you too, not just contractors: soil collapses kill, and a 5-foot trench wall is genuinely dangerous. Foundation-depth work belongs with an insured excavation contractor; pricing is in the cost guide.
Depth mistakes to avoid
Averaging the depth. The trench bottom must fall continuously at ~1% (see the slope rule). "18-24 inches" means the inlet end can start at 18 and the outlet end reach 24 on a 50-foot run; it does not mean depth can wander.
Measuring from grass height. Measure from finished grade. A lawn being regraded needs the drain set to the new grade, not the old one.
Going deep on a flat lot with no outlet. If the yard cannot give the pipe somewhere lower to discharge, a deep drain is a buried bathtub. Consider a dry well terminus or a shallower interceptor placement instead.
Full installation sequence, fabric and stone specs included, is in the step-by-step install guide.
Sources & standards: quantities and factors in this guide follow the references documented in our methodology, including ACI 302/318 (concrete practice), OSHA 29 CFR 1926 Subpart P (excavation safety), ASTM aggregate designations, and the International Residential Code. Local codes, climate, and site conditions govern.
Frequently Asked Questions
How deep should a French drain be for yard water?
18 to 24 inches for surface and shallow subsurface water. Deep enough to sit below the root zone and intercept lateral flow, shallow enough to dig by hand and outlet easily.
How deep is a French drain around a foundation?
At or slightly below the bottom of the footing, which is typically 6-10 feet on a full basement and 2-4 feet on a crawlspace or slab. Protecting a basement with an 18-inch drain does nothing; the water is entering at footing level.
Can a French drain be too deep?
Yes. Every extra foot costs digging, stone, and outlet elevation. A drain deeper than the water it intercepts wastes money, and past 4 feet you enter shoring territory and should involve a contractor.
How wide should a French drain be?
12 inches is the workhorse width: wide enough for stone capacity and easy pipe bedding, narrow enough to dig efficiently. Go to 18 inches for heavy flows or where you want more storage in the stone.
Does a deeper drain need a bigger pipe?
Not usually. Depth and pipe size solve different problems: depth determines what water you intercept, pipe size determines flow capacity. A 4-inch pipe handles most residential runs; upsize to 6 inches for long runs collecting multiple downspouts.