How do I install a French drain in a garden?

Where a French drain does and does not help, the fall and materials it needs, and the outfall question that has to be answered before you dig.

Difficulty: advanced
Time: 10 hr
Read: 4 min
Safety: warning

Short answer

A French drain is a stone-filled trench with a perforated pipe that intercepts water moving through the ground and carries it somewhere else. It only works if there is somewhere legitimate for the water to go and a continuous fall to get it there. Answer the outfall question before you buy anything.

Most garden waterlogging in the UK is not solved by a drain. Compacted clay with nowhere for water to go stays wet whatever you install, because a drain moves water it cannot make it disappear. The drain is the right answer when water is arriving from higher ground or from a specific source.

Safety

Check for buried services before digging — water, gas, electricity and drainage runs are often shallow in gardens, and a cable locator or a service plan is worth the cost. Never dig alongside a house foundation or a boundary wall without advice, as you can undermine it. Surface water must discharge to a soakaway, a watercourse or a surface water drain — connecting it to a foul sewer is an offence, and discharging onto a neighbour's land is a civil liability.

What you'll need

Spade or hired mini-digger
Perforated land drain pipe
Clean angular stone, 20mm
Permeable geotextile membrane
Line and level
Barrow

Cross-section of a garden French drain

Step by step

Find where the water comes from.Water running off a slope, off a driveway or from a neighbouring property is intercepted well by a drain. Water simply sitting on flat compacted clay is not — that needs soil improvement or a different approach.
Establish the outfall first.The water must end somewhere: a soakaway sized for the catchment, a ditch, or a surface water drain you are permitted to use. Without a legal outfall, do not start.
Set the line across the flow.The drain runs across the slope, intercepting water moving downhill, not straight down it. It should sit above the area you want to protect.
Dig with a continuous fall.Around 300mm wide, and falling at least 1 in 80 towards the outfall. Use a line and a level; a drain that dips anywhere along its length silts up at that point.
Line the trench with membrane.Permeable geotextile against the trench walls, with enough spare to fold over the top of the stone. This is what stops soil silting the stone up over a few years.
Bed the pipe on stone.50 to 100mm of clean angular stone in the bottom, then the perforated pipe laid on it with the holes downwards so water enters from below rather than sitting in the pipe.
Backfill with clean stone.Clean angular 20mm stone, not builders' ballast or sand. Fines block the voids and the drain stops working.
Fold the membrane over and finish the top.Either bring the stone to the surface so water enters along the whole run, or fold the membrane over and put 100mm of topsoil and turf on top if it only needs to work underground.
Add rodding access where you can.An inspection chamber at a bend or at the outfall makes clearing possible later. Buried pipe with no access is a problem the day it blocks.
Check it after the first heavy rain.Water should be discharging at the outfall within an hour or two of prolonged rain. Nothing arriving means either no fall or the wrong diagnosis.

Tips

Perforations go downwards. It looks wrong and it is right — water rises into the pipe from the surrounding stone.
Clean angular stone only. Anything with fines in it will silt up.
A drain with no legal outfall is just a very expensive stone-filled hole.

Common mistakes

Digging a drain into flat waterlogged clay with nowhere to discharge — The trench fills with water and stays full, because there is no gradient and no outfall. The answer for that situation is raised beds, soil improvement or planting for wet ground.
Omitting the geotextile membrane — Silt migrates into the stone from the surrounding soil and the drain becomes solid within a few years. The membrane is the cheapest part and the one that determines its lifespan.

If it doesn't work

Nothing discharges after rain

Cause: No fall, or water is not reaching the drain — Fix: Check levels along the run. Re-examine whether the problem is inflow or standing water.

Drain worked for three years then stopped

Cause: Silted up, usually from missing or torn membrane — Fix: Rod or jet the pipe if there is access; otherwise it needs excavating and relaying.

Water now pooling at the outfall

Cause: Outfall undersized or into ground that cannot absorb it — Fix: Enlarge the soakaway or find a different discharge point. A soakaway in clay often will not work.

Ground still wet either side of the trench

Cause: Clay soil moves water very slowly sideways — Fix: Closer spaced runs, or accept that soil improvement and planting choice will do more.

Questions people ask

Do the pipe holes go up or down?

Down. Groundwater sits in the stone around the pipe and rises into it through the perforations. Holes upwards means water must fill the trench above pipe level before it can get in.

Will a French drain fix a waterlogged clay lawn?

Usually not on its own. If water is arriving from higher ground, yes. If it is simply sitting on impermeable clay, the drain has nothing to intercept and no fall to work with.

Where can the water discharge to?

A properly sized soakaway well away from buildings, a ditch or watercourse you have rights to use, or a surface water drain. Never a foul sewer, and never straight onto a neighbour's land.