The short version: a French drain is a gravel-filled trench with a perforated pipe inside it that moves water away from where you don't want it to somewhere you do. Get the slope wrong by even half an inch over ten feet and the whole thing becomes an expensive gravel-filled ditch that does nothing. Most DIY failures come down to that one detail, not the digging.
What a French drain does
It's not magic and it's not complicated. You dig a trench, lay a perforated pipe along the bottom, surround it with gravel, wrap the whole lot in landscape fabric so soil doesn't clog it, and give it a gentle downward slope so gravity pulls the water somewhere useful. That's it. Water that would otherwise sit on your lawn or seep into your foundation finds the path of least resistance, which is the gravel, and travels along the pipe to an exit point.
The name is misleading. It's not French. It's named after Henry French, a Massachusetts judge who wrote about field drainage in 1859. Worth knowing if you want to sound clever at a barbecue, useless for the actual job.
When you need one (and when you don't)
You need a French drain if you've got standing water that doesn't clear within 24 to 48 hours after rain, a damp patch that kills grass in the same spot every year, or water pooling against your house foundation. You don't need one if the whole garden is just heavy clay and slow to drain everywhere, because a trench in one spot won't fix a soil problem across the whole plot. In that case you're better off improving the soil structure first with compost and grit over a season or two, or regrading the whole area.
Say you've got a garden that slopes gently toward the back fence, and every winter a three-metre patch near the shed turns into a bog that doesn't drain for a week after rain. That's a textbook French drain job. A whole garden that squelches everywhere you step is a different, bigger problem, and a single trench won't solve it.
What you'll need
- A trench, typically 18 to 24 inches deep and 6 to 12 inches wide
- 4-inch perforated PVC pipe (flexible corrugated pipe is easier for DIY, rigid is more durable long term)
- Landscape fabric, enough to line the trench with overlap
- Washed gravel, 3/4 inch, clean and angular (not rounded pea gravel, it compacts and moves)
- A level, string line and stakes to check your slope as you go
- A spade, mattock or a mini digger if the run is long or the ground is stony
- An exit point: a dry well, a soakaway, a storm drain connection, or daylight at a lower point in the garden
Step by step
1. Plan the route and the exit
Walk the garden after rain and watch where water collects and where it would naturally flow if it had somewhere to go. The drain needs to run from the wet spot to somewhere lower, ideally a spot at least 10 feet from your house foundation if you're draining away from the building. If there's no natural low point, you'll need a dry well or a soakaway pit filled with gravel at the end of the run.
2. Dig the trench with a slope
This is the step people rush and it's the one that decides whether the whole thing works. You need a fall of at least 1 percent, which is 1 inch of drop for every 8 to 10 feet of trench. For a 20-foot run that's a 2 to 2.5 inch drop from start to finish. Check it with a string line and a level as you dig, not after. Eyeballing a slope is how you end up with a trench full of still water that smells by August.
3. Line the trench
Lay landscape fabric down the length of the trench with enough overlap on both sides to wrap over the gravel later. This stops soil migrating into the gravel and clogging the whole system within a couple of years, which is the second most common reason these drains fail.
4. Add a base layer of gravel
Put down 2 to 3 inches of washed gravel before the pipe goes in. The pipe shouldn't sit directly on bare soil.
5. Lay the pipe
Perforated pipe goes in holes facing down, not up. This surprises people every time, but the water needs to enter from below and sides where it's pooling under the gravel, and holes facing up just let debris fall in. Connect lengths with couplers if you're using rigid pipe.
6. Fill with more gravel
Cover the pipe with gravel up to within 2 to 4 inches of the surface, depending on whether you want the trench visible as a gravel strip or hidden under turf.
7. Wrap and close
Fold the landscape fabric over the top of the gravel to seal it in, then finish with topsoil and turf, or leave it as a visible gravel channel if that suits the garden design.
8. Test it
Run a hose at the highest point of the trench for ten minutes and watch the exit point. If water's coming out the far end, you've got flow. If it's pooling or backing up, check your slope before you fill anything back in.
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Cost: what you're looking at
In the UK, a professional landscaper installing a basic French drain typically charges somewhere between £50 and £100 per linear metre, so a 10-metre run can land anywhere from £500 to £1,500 depending on access, soil type and whether they need to break up patio or hardstanding to get to it. In the US, expect $30 to $75 per linear foot professionally installed, so a 20-foot drain often costs $600 to $1,500.
DIY, materials for a 10-metre trench run to roughly £150 to £350 in the UK or $200 to $450 in the US, covering pipe, gravel, fabric and a hire charge if you need a mini digger for a long or deep run. The labour you're saving is real money, but it's also a full weekend of digging, and if your soil is heavy clay or full of roots, closer to two.
The bit most guides skip
Here's what doesn't get said enough: a French drain only moves water from one place to another. It doesn't make water disappear. If your exit point is a neighbour's fence line, a shared boundary, or a spot that floods their garden instead of yours, you've just relocated the problem and possibly started an argument. Check where the water is going to end up before you dig, not after, and if the exit is a soakaway, make sure the soil there can absorb water at a reasonable rate. Clay soil at the exit point means your drain works perfectly and then just creates a new bog ten feet further along.
The other uncomfortable truth is that a French drain is a long-term maintenance item, not a install-and-forget fix. Gravel systems silt up over years, especially if the fabric gets a small tear or the slope was marginal to begin with. Plan to flush the pipe with a hose or a drain rod every couple of years, and expect to dig one up and refresh the gravel after ten to fifteen years if it's working hard.
Common mistakes that cause failures
- Not enough slope, or a slope that's inconsistent along the run, creating low spots where water sits and stagnates
- Using pea gravel instead of angular gravel, which compacts over time and loses drainage capacity
- Skipping the fabric, which lets fine soil particles clog the gravel within two or three wet seasons
- Draining toward the house instead of away from it, which can make foundation damp worse, not better
- No clear exit point, so the water has nowhere to go and the trench becomes a hidden reservoir
- Digging too shallow, so the pipe sits above the water table level it's meant to be intercepting
DIY or hire someone
If the run is short, straight, and on open ground with no underground pipes, cables or tree roots to handle, this is a doable weekend project for anyone reasonably fit and patient about checking the slope. If you're dealing with a run near the house foundation, a long distance, rocky or heavy clay soil, or you need to tie into an existing drainage system, a professional with the right equipment will save you a redo in eighteen months. Call your local utility line service before you dig regardless, because hitting a cable or a gas line with a mattock is a bad afternoon.
For more on the practical, no-nonsense side of running a home and a business at the same time, you can browse the rest of the blog at lilachbullock.com, where most of what I write about is work, not gardens, but the same rule applies everywhere: check the slope, don't eyeball it, and fix the actual problem instead of the symptom.
Frequently asked questions
How deep does a French drain need to be?
Most residential French drains are 18 to 24 inches deep, which is enough to get below the frost line in colder climates and below where surface water typically pools, without being so deep it's a major excavation job.
Can I install a French drain myself without a digger?
Yes, for a trench under about 20 to 30 feet in reasonably soft soil, a spade and a mattock are enough, though it's physically hard work over two days. For longer runs or stony, compacted ground, a mini digger saves significant time and strain.
Will a French drain stop water coming into my basement or house?
A sloped French drain positioned away from the foundation can reduce surface water reaching the house, but if the problem is already inside the basement walls, that's a different job involving interior drainage or a sump pump, and it's worth getting a structural or drainage specialist to assess it rather than guessing.
How long does a French drain last?
A well-built French drain with real fabric, angular gravel and correct slope typically lasts 10 to 15 years before it needs gravel refreshed or the pipe checked for silt buildup. Poor slope or skipped fabric can cut that down to two or three years.
