Key Takeaways
- When I explain “What is a soakaway drain” to folks, I get them to think of it as a temporary underground reservoir rather than a drain in the traditional sense. It doesn’t carry water away to the street – it just allows the water time to disperse into the soil naturally, and at a rate the soil type can cope with.
- Soakaways are most commonly used in house construction to manage runoff – bypassing stormwater drains. In landscaping, I use soakaway systems to deal with with poor drainage on lawns or around features I build, such as patios or driveways.
- Most garden soakaways should be 1 to 2m³, sited at least 5m from any building, and fed by a downpipe or drainage channel.
- In my experience they work best on free-draining sandy or loamy soil. On heavy clay you’ll need a bigger soakaway or a different fix entirely
- A basic DIY soakaway is a weekend job for one person with a spade, a bag of gravel and some other equipment. Anything feeding into it from a main building, or near a boundary, I always advise clients to check with local council first
- I’ve seen limestone chippings soakaways clogged from the gravel slowly dissolving, so in my experience, clean, non-alkaline gravel is always the choice for drainage.
Why Your Lawn Is Waterlogged?
I get this call a lot, usually in October or November once the ground’s had a proper soaking: “the garden’s basically a pond, what do I do?” Nine times out of ten, the problem isn’t the lawn itself it’s that the water landing on it has nowhere to go. Rain falls on your roof, patio, or driveway, runs off, and lands somewhere low in the garden. If the soil underneath is compacted, clay-heavy, or already saturated, it just sits there. That’s your waterlogged patch.
A soakaway gives that water an escape route – a pit underground where it can collect and drain away gradually, instead of sitting on your lawn killing the grass and creating muddy footprints inside when the kids are done playing. In this article, I look mainly at the use of soakaways in gardening applications for DIY landscapers.
So What Is a Soakaway, and How Does It Work?
While the term ‘soakaway’ can sound a bit intimidating to the novice, really it’s just a glorified hole in the ground, filled with something that creates space – rubble, gravel, or soakaway crates – with a permeable, non-woven geotextile fabric to slow the soil down from clogging it up over time. Excess rainwater is piped in from different areas, which fills the void, before it slowly disperses into the surrounding earth over time.
It’s worth knowing there are two places you’ll see the word “soakaway” used. In housing construction, it is used for stormwater management on new developments. That type work hard and silt up faster, since they’re taking a lot of water off a large roof or road area. The kind I’m talking about here – a garden soakaway dealing with a boggy lawn, a patio, or a single downpipe – can be done on the cheap, has a much easier life, and lasts a lot longer as a result.
A soakaway, done properly, also does a bit of good beyond your own garden: instead of that water racing into the storm drains and out to a river, it recharges the groundwater locally, the way it would if the ground weren’t built on at all. Handy to mention if a neighbour or the council ever asks what you’re digging a hole for. 😉
| Type | What it is | Best for |
|---|---|---|
| Rubble-filled pit | Broken brick/stone hardcore, wrapped in membrane | Cheapest option, small domestic jobs |
| Clean stone/gravel pit | 40-75mm clean angular stone, wrapped in membrane | Better void ratio than rubble, easier to install neatly |
| Crate soakaway | Plastic modular crates (roughly 95% void space) | Best performance for the footprint, easiest to size precisely |
Crates cost more upfront but you need far less excavation for the same water storage – I’d say it’s worth it if space is tight.
Can You Use Limestone Chippings For Soakaways?
My honest answer is no, and I never recommend it, even though I know of people who use it and get away fine for years. Here’s the problem. Rain picks up carbon dioxide from the air as it falls and becomes mildly acidic, carbonic acid. That acid reacts with calcium carbonate, which is what limestone is mostly made of, and slowly dissolves it into calcium bicarbonate, a compound that’s soluble and simply washes away with the water rather than staying put as a space filler way gravel does.
Over years, that means the stone itself is slowly eroding underground. What’s left behind is a mix of fine sediment and softened, crumbling stone, which is much better at silting up a soakaway than draining one. Our rainfall stays on the acidic side almost everywhere, so that process doesn’t stop on its own.
It’s also worth knowing that “limestone” isn’t one material. It covers a huge range of stone with very different properties. Hard, dark bluestone like you’d find around Kilkenny is technically limestone, and so is the soft, chalky limestone across southern England, and the two behave completely differently underground. That’s part of why advice on this varies so much depending on where you are. In a lot of the north of England, Scotland, and Ireland, limestone barely gets used for drainage at all because better, more stable stone is close at hand. There are exceptions of course!
I’ve come across 1970s land drains on jobs that were choked solid with degraded lime, decades of that slow dissolving finally catching up with the pipe. It’s not just my own experience either. I’ve heard there are plenty of Building Control Officers, particularly in the North West, who won’t sign off limestone as pipe bedding, and most drainage engineers I’ve worked alongside won’t recommend it for a land drain.
None of this means a limestone soakaway will fail catastrophically, especially if you’re somewhere it’s genuinely the easiest stone to get hold of. But when there’s an alternative available, a clean, angular gravel or crushed stone does the same job without the slow degradation, which is what we stock for exactly this reason.
What Gravel Should You Use for a Soakaway?
If limestone is out, what goes in? Two different jobs, two different stones.
Around the inlet pipe, use 10-20mm rounded pea shingle. Rounded matters here. Round stone settles evenly along the whole length of the pipe and supports it; angular stone locks against itself, bridges, and leaves voids underneath that crack the pipe a few winters later.
Through the body of the pit, use clean washed gravel at 20mm or 40mm. The word doing the work is clean. Gravel with fines, dust or soil mixed through it silts the pit up from the inside before the surrounding ground ever gets a chance to. If you tip it out and there is a cloud, it is the wrong bag.
That is the whole specification. No fancy grading, no membrane-and-stone system to buy in a kit. We deliver 20mm washed gravel and 10mm gravel in bulk bags for exactly this job, and if you are not sure how many bags your hole takes, get in touch and we will work it out with you.
How Do I Know If I Already Have a Soakaway?
Plenty of houses have one and nobody living there knows about it. Before you dig a new one, it is worth ten minutes finding out.
Start at the downpipes. If one disappears into the ground rather than into a gully with a visible grate, it is going somewhere, and on a house built since the eighties that somewhere is usually a soakaway. Then walk the garden after heavy rain and look for the patch that dries out noticeably faster than everything around it, or a rectangle of grass that browns off first in a dry summer. Both are the pit underneath giving itself away.
If the house came with plans, the drainage layout will be marked on them. Failing that, your local authority building control may hold them, and for anything built recently the surface-water strategy had to be approved before the house went up.
Finding an old one changes the job. Clearing a silted pit is a far smaller day than digging a new one from scratch.
Do I Need a Soakaway?
Not every damp lawn needs one. Before you start digging, it’s worth ruling out the cheaper fixes:
- Check your gutters and downpipes. Half the “drainage problems” I get called out for turn out to be a downpipe with no proper outlet, dumping water straight onto the lawn. Worth ruling out first since it’s the quickest check, but if the guttering itself is actually faulty, that’s a job for whoever fitted it, not a landscaper. I’ll flag it, but I won’t pretend to fix it.
- Check the grading. If the ground slopes toward the house, or toward a low point with nowhere for the water to go, sometimes a section just needs regrading rather than a soakaway. There’s more to this than eyeballing a slope though, I’ve seen plenty of “flat” gardens that actually have a hidden fall once you test them properly, and settlement around old trenches or removed tree roots can create a new low spot years after a garden was graded correctly.
- Rule out a blocked existing drain. If there was a soakaway or land drain there before and it’s stopped working, in my experience it’s almost always silted up rather than genuinely failed, usually because the pipe wasn’t laid with enough fall to keep the water moving fast enough to carry silt through it.
If none of that’s the issue and water is consistently pooling after rain, a soakaway is usually the right call.
Sizing It Properly: How Big and How Deep?
A soakaway’s size needs to match two things: how much water it’s going to receive (your roof or paved area, and local rainfall) and how fast your soil can actually absorb it. Get the sizing wrong and you’ll either dig a hole that’s bigger than it needs to be, or one that overflows the first time it rains properly.
The way I find out how fast a soil drains is a simple on-site percolation test: dig a small test hole, fill it with water, and time how long it takes to drop. As a rough starting point before you test: I generally find a typical UK semi with an average roof area needs somewhere in the region of 1m³ of void space on decent free-draining soil, but I never dig based on that number alone. Test first.
Before you size anything though, it’s worth knowing what you’re actually dealing with underground. If you’re not sure whether you’ve got sandy, loamy, clay, or chalky soil, our guide to identifying your soil type will help you work it out, since it changes everything about how you approach the next step.
On depth specifically: there is no standard number, and anyone who gives you one has not tested your ground. A soakaway pit is sized by volume, not by depth, so what you are aiming for is a void, and how you get there is up to the ground. Most garden pits land at 1 to 2 cubic metres of void in total. On slow ground I would rather go wider and shallower than deep, because the top of the soil profile almost always drains better than the bottom of it.
Will It Work on Clay Soil?
Short answer: it can, but not built the same way as on free-draining soil, and it’s the question I get asked more than any other on this topic.
Clay drains slowly, sometimes barely at all once it’s saturated. Dig a test hole in heavy clay and you’ll often find water still sitting in the bottom a full day later rather than soaking away. A standard-sized soakaway on ground like that will just fill up and stay full, which defeats the point.
That doesn’t mean give up on the idea. It usually means going wider and shallower rather than deeper, since clay tends to drain a little better nearer the surface, and it often means combining the soakaway with a French drain to spread the load rather than asking one pit to do everything.
Positioning
Position matters as much as size. These are the rules I stick to on every job:
- At least 5m from any building, which is the Approved Document H minimum rather than my preference (Scotland works to its own building standards). Read “any building” literally, including the neighbour’s garage.
- Not tight against the boundary, because the 5m applies to the neighbour’s garage or extension the same as it does to your own house, and because water pushed to the fence line becomes their problem.
- Away from trees, since roots will happily find and clog a soakaway over a few years
- On the lowest practical point of the area you’re draining, so water reaches it by gravity rather than fighting uphill
DIY Installation, in Brief
A straightforward garden soakaway is a weekend job for one person with a decent spade. In short, the job is:
- Dig the pit at your chosen, tested spot
- Line it with a permeable geotextile membrane so soil doesn’t work its way in and clog it
- Backfill with clean stone: smaller, rounder stone near the inlet pipe, larger clean stone through the body of the pit
- Connect and bed the inlet pipe
- Fold the membrane over the top and cover
There’s a fair bit of technique in steps 2-4 that’s worth getting right the first time: how the membrane’s wrapped, how the pipe’s bedded in, what size stone goes where. The membrane wrap, the pipe bedding and what size stone goes where are all in how to build a soakaway, step by step, in both the crate and the gravel versions.
Common Problems & Fixes
These are the problems I see most often on site:
- Blocked by silt or roots: usually a sign the membrane wasn’t up to the job, or wasn’t sealed properly at the joins. I’ve learned not to cut corners on geotextile quality.
- High water table: if the ground around the pit is already saturated below a certain depth, no soakaway will disperse water into it. I’d look at a different solution entirely rather than just digging a bigger pit.
- Overflowing: almost always an undersizing problem in my experience. Back to the percolation test.
Silt and root ingress nearly always trace back to the membrane and how well it was sealed at the joins and around the pipe, and the full build guide covers the wrap in detail.
Can you unblock a soakaway rather than replace it? Often, yes, and it is worth trying first. Start at the top of the system, because the fault is frequently upstream of the pit rather than in it: a blocked gully, a silt trap nobody has ever emptied, or a flat spot in the feed run holding silt. If the pipe run is clear and the pit itself is full, the pit has silted and it has to be opened up. This is the moment you find out whether whoever built it left an inspection point. If they did, it is a look-and-clear job. If they did not, it is a dig, which is precisely why I put one in on every soakaway I build.
Maintenance
A soakaway isn’t fit-and-forget. On any job of real size, such as handling roofing runoff, it’s worth building in inspection points (even just a marked access spot) so you can check and clear it years down the line rather than digging blind. Membrane-wrapped pits can silt up over 10-15 years. You’ll know this sign when water starts taking noticeably longer to disappear after rain, or pooling starting to return.
Regs, Costs & DIY vs. Pro
You generally won’t need planning permission for a standard domestic soakaway, but if it’s taking water from a new extension or a large area of new hard paving, it can fall under Building Regulations, so it’s worth a quick check with your local authority if you’re not sure. Which set of regs applies depends on where you are:
- England and Wales: Building Regulations Part H (Approved Document H, Section H3 covers rainwater drainage). See the official GOV.UK document.
- Scotland: Section 3 (Environment) of the Scottish Building Standards Technical Handbooks
- Northern Ireland: Part N (Drainage) of the Building Regulations (Northern Ireland)
On cost: a DIY gravel soakaway is mostly the cost of your stone and membrane, plus a weekend of digging. In my experience, it’s worth bringing in a professional for anything larger than a single downpipe, anything near a boundary or structure where getting it wrong is expensive to fix, or if you’ve hit the kind of clay soil problems above and want it sized properly first time.
If you would rather talk the stone for your soakaway through with a person, ring Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.
Have you ever had a soakaway put in, or inherited one with the house? Is it doing its job? Tell us in the comments.
FAQs
Do soakaways work in clay soil?
Soakaways do work in clay, though not built the same way as on free-draining ground. Wider and shallower beats deeper, and pairing the pit with a French drain stops one hole being asked to do everything.
Crate or rubble, which should I use?
I’d use rubble/gravel for cheaper, easier DIY garden jobs. Crates hold more water per m³ and can bear weight, which is why I reach for them on driveways and larger construction jobs.
How deep should a soakaway be?
Soakaway depth follows the percolation test rather than a standard figure, because the void you need depends on how fast the soil takes water and how much roof or paving feeds it. Most garden pits come out at 1 to 2 cubic metres of void in total, and on slow ground I would rather go wider than deeper.
Can I cover a soakaway with grass?
Yes, once it’s backfilled and the membrane’s folded over, you can turf or seed straight over the top. I’d always keep the exact location marked somewhere so you (or whoever has the garden next) know where it is if it ever needs digging up.