Where and How to Build a French Drain in Your Garden

Key Takeaways

• A typical garden French drain is a trench 30 to 45cm wide and 45 to 60cm deep, lined with non-woven geotextile, holding a 100mm perforated pipe laid holes down inside an envelope of 20-40mm washed gravel.
• In the UK you will hear the same thing called a French drain, a land drain or a weeping drain
• French drain pipe needs a continuous fall of at least 1%, which works out at roughly 2.5cm of drop for every 2 to 2.5m of run.
• The layer order for French drains, from the bottom up, is: fabric lining, a 10cm gravel bed, perforated pipe holes down, 10cm of gravel over the pipe, fabric wrapped over the top, then backfill. Burrito style!
• Fit a cleanout (a capped access pipe you can wash down with a hose) at the highest point of the French drain, moisten and then tamp the backfill so the trench line does not sink into the lawn later.
• A single-pipe garden French drain is realistic for DIY. Very long or complex runs, anything near foundations, and anything connecting to sewers – call the professionals.

If you want to build a French drain that still works in ten years, two things matter more than everything else put together: a continuous fall, and clean gravel big enough not to pack solid. Nearly every failed drain I get asked about comes down to one of those two, and the main cause is a trench dug too flat.

This guide covers how to install a French drain at garden scale, at DIY level. The soggy lawn, the puddle at the base of the steps, the low edge of a patio. I will give you the trench sizes, the layer order, a labelled diagram, and the two features I build in that almost no online guides mention: a clean out at the top of the system, and a backfill method that stops the lawn sinking. Also, a real treat: a variation you can walk on.

What you need to build a French drain

The materials list is short and specific, but every item matters. I have seen slight variations of all three bought and the result is a trench that clogs within a couple of years.

• Non-woven geotextile. This is a permeable fabric that lets water through but filters soil out. It must be non-woven – the woven weed membrane type silts over and stops passing water. Our landscape fabric guide covers the difference in full.
• 100mm perforated pipe. For anything I want to last, I advise my clients to go with rigid PVC. It costs more than corrugated coil but is more durable and far easier to rod (clear with drain rods) if it ever blocks. Corrugated is ok on a tight budget and easier to handle bends.
• 20-40mm washed gravel. Not pea gravel, not unwashed stone. Small and dirty material packs down over time and clogs the trench. Some guides recommend gravel as small as 10mm; I would not. Never buy stone smaller than your pipe’s perforations, or it will end up inside the pipe, blocking and cutting its capacity. Check the hole size, buy bigger.

Tools

• a spade (a narrow trenching spade saves a lot of soil moving)
• wheelbarrow
• string line and line level (a small spirit level that hangs on the string)
• tape measure
• a hosepipe for the backfill stage

For a long run in clay, hiring a trencher or mini digger is money well spent. Hand-digging clay is where most of the DIY regret stories start.

French drain diagram: how the layers fit together

This is the cross-section I work to. The gravel envelope is not decoration: it protects the pipe, keeps soil out of it, and adds carrying capacity, because the stone itself conducts water alongside the pipe.

The pipe sits holes facing down. It looks wrong the first time you see it, but the perforations sit at the bottom so the pipe starts collecting water at the lowest level of the trench. That pulls the standing water in the trench down to the pipe’s invert (the inside bottom of the pipe) and keeps soil from washing straight in from above.

Before you dig: services, route and where the water will go

On every client job, I try to get hold of service plans for the property and run a CAT (a handheld scanner that detects buried cables and pipes) over the proposed run before anything. Gardens hide electric cables to sheds, water supplies and old drainage no conveyancer could discover because nobody remembers.

Next, work out the line the trench will actually take. French drains catch water by sitting uphill of the problem, so the run needs to sit on the higher side of whatever is getting wet, ready to intercept it before it reaches there. Watch for the usual trouble spots: steps that pool water at the bottom, a patio edge that sits a fraction low, or any bit of garden higher than the house itself.

Only once the route is set should you think about where that water actually goes. Pick the destination before the spade comes out, because the pipe has to fall towards it without a single flat stretch: lower ground on the plot, a soakaway, or a rain garden all work. A neighbour’s garden does not; it is rude at best and against the rules at worst, and the foul sewer is off the table entirely. Every outlet option gets a proper side-by-side, but for most gardens a soakaway is the one I use, so it is worth reading our soakaway guide.

How deep should a French drain be?

For a typical garden run, dig 45 to 60cm deep and 30 to 45cm wide. Too shallow is the bigger risk: a drain that sits above the water it is meant to intercept simply does nothing. On heavy clay, go deeper and larger, and test how fast your soil actually drains before committing to the outlet via a percolation test.

Depth is only half the geometry. The pipe needs a fall of at least 1% along its whole length, which is about 2.5cm of drop for every 2.5m of run. A 10m run therefore needs at least 10cm of fall end to end, with no flat spots or bellies.

In my opinion this is the step that decides whether the drain works at all, so check it during digging, not after. Set a string line at the finished pipe level and measure down to the trench bottom as you go. A flat or inconsistent fall is the single most common foundational failure, and it costs nothing to fix while the trench is open and a small fortune once the gravel is in.

How to build a French drain step by step

1. Mark the route and set your fall. Stakes at each end, string line between them, line level on the string. Work out your total drop first: run length divided by 100 gives the minimum fall in metres, so 12m of trench needs at least 12cm of drop.
2. Dig the trench, checking the fall as you go. 30 to 45cm wide, 45 to 60cm deep, measuring down from the string line every metre or so. Smooth the trench bottom; the pipe should sit evenly, not bridge across humps – especially the perforated stuff.
3. Line the trench with non-woven geotextile. Run the fabric down one side, across the base and up the other, leaving enough spare on both sides to fold over the top later, like wrapping a parcel.
4. Lay a 10cm deep gravel bed. Spread the clean aggregate evenly along the base and rake it to follow your fall.
5. Lay the perforated pipe holes down. Use proper connectors at any joins rather than improvising, and check the fall along the laid pipe one more time before covering anything.
6. Fit a cleanout at the highest point of the run. A cleanout is a capped pipe teed off the run and brought up to just below the surface. Most online guides that mention access at all put it at the outlet end. I put it at the top of the system, because it lets you flush the whole run downhill with a hose years from now, with gravity doing the work.
7. Cover with 10cm of gravel and wrap the fabric over. Another 10cm of the same stone over the pipe, then fold the spare fabric over the top so the whole envelope is wrapped, with little or no gap between the fabric edges. This is the ‘burrito’ wrap that keeps soil out of the gravel, long term.
8. Backfill, moisten and tamp. Backfill with soil in stages. Before the trench is full, soak the backfilled soil thoroughly with the hose and tamp it down firmly, then finish filling. This forces the settlement to happen now, on your schedule. Skip it and the trench settles by itself, and that infamous sunken stripe appears on your lawn about a year later. Finish with turf over the top, or decorative stone if you would rather track where the drain runs.

Pro Tip: For planting the ground immediately alongside a working French drain ends up noticeably drier than the rest of the garden. Choose plants for that strip accordingly, or water more often!

The gravel-path variation: a French drain you can walk on

Here is a variation you will rarely find anywhere, and it suits an aggregates-minded garden perfectly. Instead of backfilling with soil, fill the trench with drain rock right to the surface and set timber headers (roughly 50 x 150mm boards) along both edges, standing about 25mm proud of ground level to keep soil off the stone.

Surface water now enters the drain directly through the top, the trench intercepts runoff coming down a slope before it reaches the garden, and what you actually see is a neat, fast-draining gravel path. One trench, two jobs.

Should you build a French drain yourself or hire a professional?

A single-pipe run with a simple outlet is realistic DIY if you can dig and hold a fall, and this guide gives you everything the job needs. That includes a run feeding a modest soakaway you dig yourself. It really depends on your level of strength and available time as to the length of run and size of soakaway you could tackle.

The following I would hand to a professional without hesitation:

• long or complex multi-run systems;
• anything against or near building foundations (that is damp-proofing and structural territory, not landscaping); and,
• any connection to a public storm sewer, which needs permissions on top of expertise.

The other real reason to hire is the type of soil: if your garden is heavy clay and the run is long, the digging alone breaks people. A pro with a mini excavator will finish in a day what takes a bruising week by hand.

Next Steps

If your outlet is going to be a soakaway, dig that first and run the percolation test before you order materials, because a trench with nowhere to send its water is just a buried pond. To work out gravel quantities for your trench, the gravel calculator will do the arithmetic for you, and we carry the right the 20mm and 40mm washed gravel for the envelope. Get the fall right, wrap the burrito properly, and this is one of those jobs you only ever do once. Enjoy the dry lawn.

If you would rather talk the gravel for your French drain through with a person, ring Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

If you have built a French drain yourself, how was the dig? Did you use perforated pipe, and how has it performed since? Tell us in the comments.

FAQs

Do you need a pipe in a French drain?

A gravel-only trench can move light surface water in free-draining soil, and for a very short interception run it sometimes earns its keep. It has less capacity and ages worse than a piped drain, so for anything you actually care about, the pipe is worth its modest cost.

Which way do the holes go on a French drain pipe?

The holes on a French drain pipe always face down. The perforations at the bottom let the pipe collect water from the lowest point of the trench, which keeps the water level in the trench down at the pipe’s invert and stops soil dropping straight into the holes from above.

Do French drains need an outlet?

Yes. The pipe must fall continuously to somewhere the water can safely go, and a drain with nowhere to send its water just becomes a buried pond. If your garden has no natural low point, a soakaway is usually the answer, provided your soil percolates; if it does not, a French drain may be the wrong tool for the site altogether.

Can you cover a French drain with grass?

Yes. Backfill properly, tamp as described above, and lay turf straight over the top. The drain disappears completely. The alternative finish is a band of loose decorative stone, which looks deliberate and makes the run easy to find again (but you shouldn’t need to if you build it right, first time!).

How long does a French drain last?

Built without filter fabric, or with the wrong fabric, there is a clogging window of 2 to 15 years. Built with proper non-woven geotextile, clean 20 to 40mm stone and a working outlet, expect decades.