What Is a French Drain? How it Works and When You Need One

Key Takeaways

• A French drain, which many suppliers and landscapers also call a land drain, is a gravel-filled trench with a 100mm perforated pipe at the bottom that collects excess water and carries it away using gravity.
• The pipe needs a fall of at least 1%, which works out at a 2.5cm drop for every 2.5m of run, and the pipe perforations always face down.
• Use 20-40mm clean washed gravel and a non-woven geotextile lining. Do not use fine pea gravel, limestone or woven weed fabric these “shortcuts” will silt a drain up within a few years.
• The water has to end up somewhere legal, usually a soakaway. Never discharge onto a neighbour’s land or the sewer, and avoid the street drains, unless you have council planning.
• A French drain will not fix damp inside a house, a high water table or a garden with no fall. Those problems need a different solution – often a drainage professional.
• If a French drain is to run near the house, keep it at least 1m out from the wall and never dig deeper than the foundations.

I have dug up more failed French drains than I can count, and almost every one died the same way: either the pipe was laid flat so the water never moved, or there was no fabric and the whole trench had silted into a solid, useless sausage of muddy gravel. The thing is; neither mistake is expensive to avoid, but both are expensive to fix.

I once got called to a backyard that was a total mess – constant pooling from poor clay drainage, a rotting fence, and a huge tree stump right where the client wanted a new garden bed. With no room for a digger, we spent three grueling days on that stump: two digging a 15m trench (60cm deep, 30cm wide), prying out over 200kg of roots and stuck clay after the saturated ground made it heavy as lead. I installed triple-wall perforated HDPE pipe wrapped in copper mesh as an extra root deterrent, gravel, and drainage fabric for the French drain, switching to solid pipe near the fence to discharge via a pop-up emitter into a mini soakaway, down gradient. Then I raised the grade with fresh topsoil over the line (using HDPE for frost protection instead of burying PVC deeper), and finally got to plant a beautiful mixed border of hardy perennials, grasses, and shrubs that now thrive in the improved conditions, leaving the client thrilled with a dry, usable yard after every rain.

This page is the overview: what a French drain actually is, whether your garden needs one, and the parts of the job people get wrong.

What Is a French Drain?

A French drain is a trench, typically 30-45cm wide and 45-60cm deep for garden work, lined with non-woven geotextile (a permeable fabric that lets water through but holds soil back), part-filled with clean gravel, with a perforated pipe laid near the bottom. Water soaks down through the gravel, enters the pipe through its holes, and gravity carries it along the pipe to an outlet somewhere it can do no harm.

In the UK you will hear the same thing called a land drain, and builders’ merchants sell those big rolls of black pipe as perforated land drain pipe. There are small technical distinctions people argue about, and I cover them in the comparison section further down, but for a garden job the two names describe the same trench. I’ve even seen older versions that are just a gravel-filled trench with no pipe, and some people still build them that way. From my knowledge, there are limited situations where that works, but for anything you want to last, the pipe is in.

Once finished, the drain is invisible. You can turf straight over it, or top it with decorative chippings so it reads as a deliberate gravel feature rather than some stealth plumbing job. That second option is one I like, because a French drain that surfaces as a neat stone seam can double as a design feature, especially where it feeds into a dry stream or creek bed.

Do I Need a French Drain?

You probably need one if water is pooling somewhere and hanging around for hours after the rain has stopped. The classic spots are the base of a set of steps, the edge of a low patio, a soggy strip of lawn that never firms up, and any ground that sits higher than the house, because everything that lands on it is heading your way.

A quick test: find or dig a wide, shallow hollow in the problem area, soak it with a hose until water stands on the surface, and keep an eye on it. If it is still standing hours later, the ground is not absorbing much, and that means during a storm most of the rain water will run off across the surface. That is what a French drain is built for: catching water on the move and rerouting it.

Be certain as to why you don’t go with the cheaper option first, though. If the real problem is that the ground slopes toward the wrong place, regrading the lawn or cutting a shallow diverting channel can fix it for the cost of a weekend’s labour, no pipe required. I would rather lose a gravel sale than have you trench a garden that just needed reshaping.

And there are jobs where a French drain is the wrong tool entirely:

• Damp inside the house, a wet basement or a suspected high water table. That is a job for a damp specialist or drainage guy, not a landscaping trench. A garden drain treats the garden’s water, and I don’t pretend otherwise.
• Roof water. Do not run downpipes into a French drain. Gutters and downpipes are a guttering trade issue and belong in their own dedicated drain run or soakaway.
• A garden with genuinely no fall and nowhere for the water to go. Water will not flow uphill for anyone. See below for what to check before deciding.

If the drain will run near the house, keep it at least 1m out from the wall and never dig deeper than the foundation footings (the widened concrete base the walls sit on). Anything closer or deeper than that stops being landscaping and starts being structural work, which is professional territory.

How Do You Install a French Drain?

Here is the short version of the build. The full trench-by-trench detail, with a labelled cross-section diagram, will be in an installation guide (to come).

• Check for buried services before any digging. I check the service plans and run a cable avoidance tool (a handheld scanner that detects buried cables and pipes) over it. Ten minutes of scanning against a cut power main is not a trade I need to explain to anyone.
• Dig the trench 30-45cm wide and 45-60cm deep, setting the fall as you dig, not after. String line and a level, minimum 1% fall, which is 2.5cm of drop for every 2.5m of run. A flat pipe is the single most common reason drains fail.
• Line the trench with non-woven geotextile, leaving enough spare fabric each side to fold over the top later.
• Lay a 10cm bed of clean gravel, then the 100mm perforated pipe with the holes facing down. Holes down stops soil dropping in from above and lets the water table drain right down to the bottom of the pipe rather than only to hole height.
• Cover with another 10cm of gravel, fold the fabric over the top so the stone is completely wrapped (the trade calls this the burrito wrap), and back fill.
• Before the trench is completely full, soak the back fill and tamp it down firmly. Skipping this is why so many lawns grow a sunken stripe along the drain line a year later. Settle it now, on purpose, rather than letting it settle on its own.
• Build in an access point at the highest point of the run, even just a capped upstand. It lets you flush the whole pipe downhill with a hose years later instead of digging blind.

Is it a DIY job? For a garden run, yes, if your back can take the digging. Hand-digging 10m of trench through heavy clay is the part people end up regretting, and hiring a mini trencher for a day changes the job completely. Long runs, anything near foundations, and anything connecting to existing drainage – I would personally call in a professional.

What Gravel, Pipe and Membrane Does a French Drain Need?

Three materials, and a wrong choice on any of them shortens the drain’s life. Here are the factors that matters most.

Gravel: 20-40mm, clean and washed. The one rule I want you to remember at the point of purchase is that every stone must be bigger than the holes in your pipe, otherwise the stone either blocks the perforations or ends up inside the pipe. That rules out pea gravel and unwashed stone, because the fines (the dust and grit clinging to unwashed aggregate) pack down into exactly the sort of solid plug you built the drain to avoid. Rounded gravel drains slightly faster; crushed angular stone locks together and holds the pipe steadier. I use crushed on runs where the pipe needs to stay put, but either works.

Pipe: 100mm perforated land drain pipe. Rigid perforated PVC is more durable and easier to rod clean; flexible corrugated pipe is cheaper and easier to snake around obstacles. In my experience, the corrugated coil works for a straightforward lawn run, but wont save you anything, anywhere you need to clean or drive over the drain in future.

Membrane: non-woven geotextile, and specifically not the woven weed-suppressant fabric sold for planting beds. Woven fabric is built to block growth, and its tight weave blinds over with silt. Non-woven fabric is built to filter, passing water while holding the soil back. They look similar on the roll and behave completely differently underground. If you want the full comparison, my landscape fabric guide covers both types in depth.

Where Should a French Drain Drain To?

A French drain moves water but it does not make it disappear. Before you start any trench, you need to know where the pipe ends, and the pipe must fall all the way there. The main options, roughly in order of how often I would use them:

• A soakaway (a buried pit of large aggregate or specialised plastic crates that stores a surge of water and lets it soak slowly into the ground). This is the standard partner for a French drain in a garden,. I cover them properly in my soakaway guides.
• Daylighting to lower ground: simply running the pipe out to a bank, ditch or low corner of your own land where the water can disperse harmlessly.
• A landscaped outlet, such as a dry stream bed, that takes the discharge and makes a functional feature of it. I cover this as well.
• A surface water drain, only with planning permission. Connecting to any public system needs checking with local council or the water company first, and making the connection is professional work. Never, under any circumstances, run into the sewer.

I always work from the ‘love thy neighbour’ rule in gardening and construction: so, the water must never end up on their land. It is inconsiderate, it is frequently unlawful, and neighbour-flooding disputes are some of the ugliest threads on every gardening forum I’ve read. If you are on the receiving end of someone else’s drain, document it, talk to them first, and check your options with the council before escalating.

What if there is genuinely nowhere for the water to go? Then a French drain is the wrong answer. Test whether the ground will accept a soakaway first, and if it will not, look at surface solutions or regrading instead.

French Drain vs Soakaway, Land Drain and Channel Drain

The easiest way to understand is transport versus disposal. A French drain transports water from where it is causing trouble, usually to a soakaway that disposes of it at the far end.

French drain versus land drain is mostly naming. Both describe perforated pipe underground moving water by gravity. Where people draw a distinction, it is usually that a land drain may be laid without the full gravel-and-fabric envelope, while a French drain always has the gravel trench. In Britain, suppliers use the terms interchangeably, and the pipe on the shelf is labelled land drain either way.

A channel drain (the long, narrow grated drain set flush into paving, sometimes called an ACO drain after the best-known brand) handles surface water on hard landscaping: driveways, patios, in front of garage doors. If the water you are fighting runs across paving, you want a channel drain. If it is soaking through soil, you want a French drain.

Why Do French Drains Fail?

There are three main causes account for nearly every dead garden drain I have repaired:

1. the pipe was laid without enough gradient;
2. the trench had no fabric so soil migrated straight into the gravel; or,
3. the gravel was too small and/or dirty and packed itself solid, blocking the holes.

All three are choices are made on day one. As a working rule of thumb, I would give an unlined drain somewhere between 2-15 years depending on the soil type, while a properly wrapped one built with washed 20-40mm aggregate can outlast the backyard fence.

I will flag a common counterpoint, because it is a fair one: the fabric is not always the whole story. Tree roots hunting for water and a blocked or badly chosen outlet can kill well-built drains too, which is why the outlet planning above matters as much as the trench itself.

Maintenance

Maintenance is pretty straight forward. Once a year, run a hose into the access point at the top of the run and watch how freely it discharges at the outlet. Sluggish flow means silt is building, and a good flush now is vastly cheaper than excavation later. If an old drain has already failed, flushing sometimes buys a season or two, but a silted, fabric-less trench usually means digging it out and rebuilding properly. It’s a judgement call!

FAQs

How deep should a French drain be?

45-60cm covers most garden runs. The depth that matters is relative: the pipe has to sit low enough to intercept the water you are chasing, so a drain protecting a planting bed can sit shallower than one cutting off water flowing downhill toward a patio. Too shallow and the water simply travels underneath it.

How much does a French drain cost in the UK?

The honest answer is that it depends almost entirely on the digging: length, depth, whether a machine can reach the trench line, and what the ground is made of. Clay roughly doubles the labour of the same trench in loam. Materials are the cheap part; excavation is the cost.

Can I connect my downpipes to a French drain?

No, and this one catches people out because it looks efficient. A downpipe in heavy rain delivers a concentrated surge that overwhelms a gravel trench, backs the water up and flushes debris into the system. Roof water belongs in its own sealed drain run to a storm drain, a soakaway or a water butt.

Can a French drain just end underground?

It should not. Water reaching a dead end pools there, saturates the surrounding soil and slowly silts the pipe from the far end forward. Every drain needs a real outlet: a soakaway, lower ground, or a permitted connection. A dead-ended drain is just a very long puddle you cannot see.

Why is it called a French drain?

Nothing to do with France. The name comes from Henry Flagg French, a Massachusetts judge and farmer who popularised the technique in his 1859 book Farm Drainage. Farmers were digging versions of it long before he wrote it down; he just did the marketing.

Next Steps

If you are ready to plan the job, work out your trench volume and stone quantity with the gravel calculator, and read the soakaway guide before you decide where the water ends up, because that decision shapes everything upstream of it. The step-by-step installation guide is the next read once the route and outlet are settled.

Have you ever had a French drain on a property? Did it shift the water, and how long has it been in? Let us know in the comments below.