What are the downsides of a French drain?

A French drain is disruptive to install, it needs somewhere legal and downhill to send water, and it silts up if it was built against soil rather than in clean stone behind fabric. The fourth downside is the largest: it is often the wrong fix, bought because it was the fix that was offered.

The one that binds here

The largest mapped soil under Madison is hydrologic group B, and the shallowest seasonal high water table the survey records is 0. That second figure is the one that decides which downside arrives first. Where the water table rises into the depth a drain is cut to, the pipe stops collecting rainfall and starts carrying ground water, so it runs in weather that looks dry from the street. That is not a fault, but it is a surprise if nobody mentioned it, and it changes what the outfall has to handle. Ask what depth is planned and what the seasonal high is at that depth. A quote naming neither has not been measured against the ground it is being dug into.

The reasoning behind that

Disruption is unavoidable. An exterior run means a trench the length of the affected wall, and whatever is on top of it — lawn, patio, planting, driveway — comes up. Reinstatement is a real line on the quote and is sometimes larger than the drainage.

The outfall is the constraint people discover late. A drain does not consume water; it moves it. Somewhere downhill has to accept it, and where nothing does, the design becomes a dry well or a pump and the price changes accordingly.

Silting is the failure mode. A perforated pipe laid against soil fills with fines over a few seasons and stops working with nothing visible on the surface to say so. Washed stone and fabric are what prevent it.

And a drain solves ground water. It does nothing about a downspout at the wall, a slope running toward the house, or a leaking pipe. Those get checked first or the drain is treating a symptom.

More on this: French Drain and Sump Pump Repair in Madison, WI.

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