How deep should a French drain be?

People ask us this question more than almost any other. The short answer is a range, not a single number. Below we explain what sets that range on a real property, what you can check yourself, and when to call someone in.

The plain answer

For a typical yard drain that collects surface and shallow ground water, a trench of about 18 to 24 inches deep and roughly a foot wide is the usual starting point. The perforated pipe sits near the bottom on a bed of washed gravel, with more gravel over it and fabric wrapped around the stone to keep silt out. A drain that is there to protect a foundation or a basement wall may need to go deeper, down near the level of the footing. A shallow drain that only catches runoff along a patio edge can be less. Depth on its own is not the goal. The drain has to sit low enough to catch the water where it moves, and it has to slope steadily downhill to somewhere the water can leave. A deep trench with no fall, or one that ends at a dead spot, will hold water instead of moving it.

What changes the answer on a real property

Soil matters first. Much of Roseville and the surrounding Placer County area has heavy clay, and in some neighbourhoods there is a hard, compacted layer below the topsoil that water struggles to pass through. When water sits on top of that layer, the drain usually needs to reach it so the gravel can collect what is perched there. In looser soil, a shallower trench often does the job. The source of the water matters too. Runoff from a neighbour's yard or a slope behind the house moves near the surface, so a moderate depth along the high side of the yard can intercept it. Water that shows up as a damp crawlspace or seepage along a foundation wall is moving lower, and the drain has to go lower to catch it. Then there is the outlet. Every French drain has to end somewhere: a street curb outlet where allowed, a lower part of the property, or a dry well. The outlet sets a hard limit. If the exit point is only a foot below grade, the trench cannot drop much below that without the water having nowhere to go. On flat lots, which are common in newer Roseville subdivisions, getting enough fall over a long run is often the real constraint, not how deep you can dig. Finally, there are things already in the ground. Sprinkler lines, gas and electric services, cable runs and tree roots all limit where and how deep a trench can go. Mature oaks deserve particular care, since cutting their roots can harm the tree.

What you can check yourself

Start by watching the yard during and after a real winter storm. Note where water pools, how long it stays and which direction it comes from. Puddles that drain within a few hours after rain stops are rarely a problem. Water that stands for a day or more, soggy turf in summer near an irrigation zone, or a white mineral line on a foundation wall tells you more. You can dig a simple test hole with a post-hole digger, about two feet deep, in the wettest area. Fill it with water and see how fast it drains. If it barely drops overnight, you are likely sitting on dense clay or hardpan. If you hit a very hard layer as you dig, note how far down it starts, since that depth is useful information for whoever designs the drain. Check your downspouts as well. A surprising number of wet yards are caused by roof water dumping right at the foundation. Before you call 811 to have utilities marked, which is free and should happen before anyone digs, look for an obvious low point where a drain could empty.

When it needs a professional

Some situations go past a weekend project. If water is getting into a crawlspace or against the foundation, the drain usually needs to run deeper and closer to the house, and a mistake there can make things worse. Long runs on a flat lot need careful grading of the trench so the pipe keeps a steady fall, and that is hard to get right by eye. If the only practical outlet is a neighbour's side of the property or the street, local rules may apply. And if the yard is wet because the ground itself was graded toward the house, a drain may only treat the symptom. On a visit we look at where the water comes from, test the soil, find a workable outlet and mark utilities and roots before we suggest a depth and a route. Sometimes the answer is a deeper drain. Sometimes it is a shallower one combined with regrading or better downspout routing. We will tell you which, and why.

How depth fits with the rest of a drainage plan

A French drain rarely works alone. Our French Drain Installation work often goes together with Downspout Drainage, because roof water is one of the biggest loads a yard sees in a Roseville winter, and piping it away in solid pipe keeps it from saturating the gravel trench. Where there is no good place for the water to exit, a dry well at the low end can give it somewhere to soak in, sized to the soil we find. If the yard slopes toward the house, Grading and Regrading may need to come first so the surface sends water away before it ever reaches the drain. Getting the depth right is part of the job, but it only works when the source, the route and the outlet all make sense together. If you are unsure which of those is causing your wet spot, a visit is the easiest way to find out before anything gets dug.