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Published by Mo's Plumbing and Heating
What Is a French Drain?
A french drain is a gravel-filled trench with a perforated pipe in the bottom, laid to intercept water moving through the ground and carry it somewhere else. It is not a pipe for rainwater off a roof and it is not a soakaway, and confusing it with either is how people end up with one that does nothing.
The Short Answer
A trench, a perforated pipe at the bottom, a permeable membrane lining it, and clean stone filling the rest. Groundwater soaks in through the stone, reaches the pipe, and runs off along it to a discharge point. It works on water in the ground rather than water on the surface, which is why it suits a persistently soggy lawn or a damp wall at ground level and does not suit a puddle on a patio.
Why the Gravel Is There
The counterintuitive part is that the gravel is not there to soak water up. It is there to be empty.
Clean, single-sized stone has a lot of space between the pieces, and water moves through those spaces far more easily than it moves through soil. So a trench of stone acts as a path of least resistance: water travelling sideways through wet ground meets the trench and flows into it because that is the easiest route available.
Once in the trench, water falls to the bottom and finds the perforated pipe, which carries it away along the fall of the trench to wherever it is being discharged. Without the pipe you have a trench that fills up and stops working; without the stone you have a pipe nothing can reach.
The membrane matters more than it looks like it should. Wrapped around the stone, it lets water through and keeps fine soil particles out. Without it, silt migrates into the gaps between the stones over a few years, the spaces fill, and the drain quietly stops working while looking exactly the same from above. A french drain that failed after five years almost always failed for this reason.
And it needs somewhere to go. A french drain does not consume water; it moves it. The discharge point is a design decision rather than a detail, and it is where most of the problems on this page originate.
What It Is Not
Three things it gets confused with, and the confusion produces most of the disappointment.
It is not a soakaway. A soakaway is a pit or crate structure that holds water and lets it disperse into the surrounding ground over hours. A french drain moves water along to somewhere else. They are sometimes used together, with a french drain feeding a soakaway, but they do different jobs.
It is not a surface water drain. Water lying on a hard surface needs a channel or a gully to collect it. A french drain intercepts water within the ground and does very little for a puddle on a patio, because the water has to get into the soil before the drain can catch it. Surface water drainage has its own page in this cluster.
It is not a damp proof course. A french drain alongside a wall can reduce the volume of water sitting against it, which helps in some situations. It cannot fix a bridged or failed damp proof course, and it will not stop rising damp on its own. Where damp is the problem, establish the cause before digging a trench in the hope it helps.
When It Is the Right Answer
Genuinely useful in a narrow set of situations, and oversold well beyond them.
A lawn or garden that stays saturated for days after rain, where the water is sitting in the soil rather than pooling on top, and where there is somewhere lower to send it.
Ground water running toward a building across a sloping site. A french drain across the slope, above the building, intercepts it before it arrives, which is one of the situations where it earns its keep completely.
A retaining wall, where water building up behind the structure creates pressure. Drainage behind a retaining wall is standard practice rather than an optional extra.
A persistently wet strip along the base of a wall, where reducing the standing water helps alongside dealing with whatever the actual damp cause is.
Where it is not the answer: a puddle on a hard surface, a garden that is wet because the soil is compacted rather than because water is moving through it, and any situation where there is nowhere lower for the water to be discharged to.
Where the Water Is Allowed to Go
The part that decides whether the whole thing is legal as well as whether it works, and it is worth settling before any digging.
Surface water and ground water must not be discharged into the foul sewer. Foul drainage carries waste to treatment; adding rainwater and ground water to it overloads the system, and in most places it is not permitted. The two systems are separate for a reason and that separation has its own page in this cluster.
So the legitimate destinations are a surface water drain if the property has one, a soakaway sized for the ground conditions, or a watercourse where that is permitted, which generally needs consent.
Discharging onto a neighbour's land is not an option, and doing it accidentally by sending water downhill to a boundary is a genuine dispute risk rather than a technicality.
In practice, on a domestic property, the question is usually whether there is a surface water drain to connect to and whether the ground will take a soakaway. Both are worth establishing at the start, because a trench with nowhere to discharge to is just a long hole that fills up.
Why They Stop Working
A properly built french drain lasts a long time. A poorly built one fails in a predictable way and usually for one of four reasons.
No membrane, or the wrong one. Silt fills the voids between the stones over a few years and the drain becomes an underground line of muddy gravel that water no longer moves through. This is the commonest failure by a distance.
The wrong stone. Fine or mixed-size aggregate packs together and leaves far less void space than clean single-sized stone, so it holds much less water and moves it more slowly. Builders' ballast is not the material for this.
No fall, or fall in the wrong direction. Water in the pipe has to run somewhere, and a trench dug level or falling away from the discharge point holds water rather than moving it.
Roots. Perforated pipe is an attractive proposition for a root system looking for water, and a drain running past a mature tree or a hedge line will eventually be found. That is a maintenance question rather than a design flaw, though the route can be chosen with it in mind.
Clay, Slope and Back Courts
Two things about the ground here make french drains both more relevant and more likely to disappoint, and know which situation you are in before committing to one.
A great deal of the Glasgow area sits on heavy clay soil, often over glacial till. Clay holds water and drains very slowly, which is exactly why gardens here stay saturated for days after heavy rain and why people start looking at french drains in the first place. It is also why a soakaway frequently does not work as a discharge point: a soakaway relies on the surrounding ground absorbing water, and clay does that badly. So the discharge question is harder to answer here than in a sandy or gravelly area, and it should be settled first rather than last.
The city's topography cuts the other way and in favour. Glasgow is built across a lot of slope, and a sloping site with water moving downhill through the ground toward a building is the single situation where a french drain is unambiguously the right tool. An intercepting run across the slope above a property does exactly what it is designed to do.
Tenement back courts are the awkward middle case. They are frequently flat, enclosed by walls on several sides, sit on made ground that has been dug over repeatedly, and are commonly shared between several properties. There may be nowhere lower to discharge to at all, the ground may not take a soakaway, and any work is a shared-property decision rather than an individual one. Where a back court is persistently waterlogged, the honest answer is often that the problem is the levels and the surface rather than something a trench solves.
And in a flatted building, anything involving digging in common ground goes through the factor rather than being arranged by whichever household is tired of the mud.
When to Call Someone Out
Worth a look before digging rather than after, because the two questions that decide whether a french drain will work are where the water is actually coming from and where it can legally be discharged to, and neither is obvious from a wet lawn. Also worth calling when the problem is damp at a wall rather than water in a garden, since a trench outside is sometimes part of that answer and is frequently not. Paul leads the drain team and Kyle runs the jetting side. If the property is a flat and the ground is common, expect the factor to be involved before any work starts.
Drain Repairs in GlasgowCommon Questions
Water moves through the spaces between clean stone far more easily than through soil, so a gravel-filled trench becomes the easiest path available for water travelling through the ground. It falls to a perforated pipe at the bottom and runs along that to a discharge point. The gravel is doing its job by being empty rather than by absorbing anything.
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What Is Surface Water Drainage?
Surface water drainage takes rainwater from roofs and hard surfaces to a surface water sewer, a soakaway or a watercourse.
What Is Foul Water Drainage?
Foul water is waste from inside the property, and it goes to treatment rather than to a watercourse.
How to Install a French Drain
Establish the discharge point first, then dig a trench falling consistently toward it at roughly one in a hundred.
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