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How to Install a French Drain
The digging is the easy part. Four decisions made before the spade comes out determine whether this drain is still working in fifteen years or silted solid in five, and three of them are invisible once the trench is backfilled. Get them right and the rest is labour.
The Short Answer
Establish the discharge point first, then dig a trench falling consistently toward it at roughly one in a hundred. Line it with a permeable geotextile membrane, bed perforated pipe on a layer of clean single-sized stone with the perforations downward, fill with more of the same stone, fold the membrane over the top, and cap it. The four things that decide its life are the fall, the membrane, the stone size and where the water goes. Everything else is digging.
Decide These Four Things First
Before any digging, because none of them can be fixed afterwards without opening the whole trench again.
- Where the water discharges to. A surface water drain, a soakaway sized for the ground, or a watercourse where permitted. Never the foul sewer. If there is no legal discharge point, there is no french drain, and that is the answer rather than a problem to work around.
- Where the water is coming from and where it is going. A french drain intercepts water moving through the ground, so it goes across the flow, uphill of whatever you are protecting. A trench dug in the wettest looking spot without knowing the direction of travel frequently misses it entirely.
- The fall. Around one in a hundred, meaning roughly ten millimetres of drop per metre of length, consistently toward the discharge. Not steeper, which drains too fast for the stone to feed it, and never level or reversed.
- The depth. It follows from the fall and the discharge point rather than from a standard figure. Work backwards from the depth at the discharge end and the length of the run, and check the numbers before digging rather than discovering halfway that the far end needs to be a metre and a half down.
Materials That Matter
Three, and substituting cheaper versions of any of them is the usual reason these fail early.
Clean, single-sized stone, typically twenty to forty millimetre. Single-sized is the important word: uniform pieces leave large voids between them, and those voids are the storage and the pathway. Mixed aggregate or builders' ballast packs together, the fines fill the gaps, and you get a trench with a fraction of the capacity that drains far more slowly. This is not a place to use what is left over from something else.
A permeable geotextile membrane, wrapped around the stone and folded over the top. It lets water through and keeps fine soil out. Without it, silt migrates into the voids over a few years and the drain becomes a line of muddy gravel that looks identical from above while no longer moving water. This is the single commonest cause of premature failure and the cheapest thing on the list.
Perforated pipe, usually a hundred millimetre, laid with the perforations facing DOWNWARD. That surprises people who assume holes should face up to catch water. Water fills the trench from the bottom, so downward-facing perforations let it in at the lowest point and let the pipe drain fully rather than holding a standing puddle along its length.
Digging and Building It
With the four decisions settled and the materials right, the build is straightforward.
- Check for services before digging anything. Gas, water, electricity and telecoms all run through gardens and back courts, frequently shallower than they should be and rarely where anyone remembers. This is not optional and it is the one step on this page where getting it wrong is dangerous rather than expensive.
- Mark the run and dig the trench, typically three hundred millimetres wide, working to your calculated depth at each end so the fall is consistent. Use a string line and a level rather than eyeballing it; a fall that looks right over three metres frequently is not.
- Line the trench with the membrane, leaving plenty of overlap at the top on both sides so it can be folded over later.
- Put down fifty millimetres or so of stone as a bed, then lay the pipe on it with perforations downward, checking the fall along its length as you go.
- Fill with stone to within about a hundred and fifty millimetres of the surface, keeping it clean as you shovel it in.
- Fold the membrane over the top so the stone is fully wrapped, then cap: either more stone left visible, or topsoil and turf, or a permeable surface. Do not cap with anything that seals the top, since that stops water getting in.
- Connect the discharge end properly rather than letting the pipe simply end in the ground, and make sure that connection is accessible for future rodding.
The Mistakes That Kill Them
Five, in the order we find them.
No membrane. The drain works for a few years, silts up invisibly, and stops. By far the most common.
Wrong stone. Ballast or mixed aggregate instead of clean single-sized, so the voids are already partly filled on day one.
Perforations facing up. The pipe holds standing water along its length and only fills once the trench is nearly full, which is the opposite of how it should behave.
No proper discharge. A trench ending in the ground with nowhere to go fills up and stays full, and is then a linear pond rather than a drain.
Fall that is wrong, absent or reversed. This is the one that is hardest to detect afterwards and impossible to fix without redigging.
Every one of those is invisible once the trench is backfilled, which is why a french drain that has stopped working usually cannot be diagnosed without opening it, and why the four decisions at the start are worth the time.
Doing It Yourself or Not
An honest read, because this is one of the more DIY-able drainage jobs and also one where the failure is slow and invisible.
In favour of doing it yourself: the technique is simple, the materials are ordinary, and the main input is labour. A short run in an accessible garden with an obvious discharge point is a realistic weekend.
Against: the decisions that determine whether it works are the ones needing judgement rather than effort, and the failure mode is silent. A drain built without a membrane behaves perfectly for three years, so you get no feedback that anything was wrong until long after you would connect it to the build.
The parts worth getting help with regardless are establishing where the water is actually coming from, confirming a legal discharge point, and locating services before digging. Those three are where the genuine risk sits, and none of them is about how well you can dig a trench.
Digging in Clay, and What Is Already Down There
Two local realities change this job here, and both are about the ground rather than the technique.
The clay makes the digging heavy and the discharge question hard. Heavy clay is slow and unpleasant to excavate by hand, and it holds water rather than releasing it, which is why gardens here stay wet and why people want a french drain in the first place. It also means a soakaway is frequently not a viable discharge point, because a soakaway depends on the surrounding ground absorbing water and clay does that badly. So in this area the discharge question needs settling first far more often than in a free-draining sandy area where a soakaway is a safe default.
The other is what is already buried. Back courts and gardens in this city have been dug over repeatedly across a century and more, and what comes out of a trench is regularly not soil: old foundations, demolition rubble from earlier buildings, ash and clinker layers, and services laid at whatever depth was convenient at the time. Gas, water and electricity runs in older property are frequently shallower than current practice would allow, and their positions are often not recorded anywhere.
That makes checking for services before digging important here rather than a formality, and it makes hand digging the sensible approach on anything close to a building or a boundary. It also means a trench takes longer than the length suggests, because a good proportion of the time goes on getting rubble out rather than soil.
And in a flatted building, a back court is usually common ground. Digging a trench in it is a factor decision rather than an individual one, however reasonable the drainage argument.
When to Call Someone Out
Worth a look before digging rather than after, because the decisions that determine whether this works are made before the spade comes out and cannot be corrected once the trench is backfilled. Specifically worth calling to establish where the water is coming from, whether there is a legal discharge point, and what services are in the ground on the intended run. Paul leads the drain team, and where a discharge connection into an existing drain is needed, that is the part most worth having done properly since a bad connection is a defect that shows up on a survey later. In a flatted building expect the factor to be involved before any work in common ground.
Drain Repairs in GlasgowCommon Questions
Enough to be worth lifting and turning while the trench is open, which is a short job compared with what it costs later. A pipe laid holes-up still works after a fashion once the trench is saturated, but it holds standing water along its length between downpours and responds much more slowly, and correcting it after backfilling means digging the whole run out again.
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Read Next
Full guide
What Is a French Drain?
A trench, a perforated pipe at the bottom, a permeable membrane lining it, and clean stone filling the rest.
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.
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