How to Build a Soakaway That Won’t Block Up | DIY Guide

Key Takeaways

• A soakaway is a buried reservoir – either crates or gravel wrapped in geotextile fabric – that stores stormwater and allows it to soakaway into the ground over time. No fabric and the soakaway clogs early.
• Site it at least 5m from any building and 2.5m from a boundary, on ground that actually drains, and above the water table. If a test hole fills with water on its own, stop – that ground is telling you no.
• Bed and surround the feed pipe in 10-20mm rounded pea shingle, not angular stone – rounded shingle settles evenly and won’t leave voids under load.
• For a rubble or gravel soakaway, layer the fill by size: smaller, rounder stone around the inlet pipe, larger stone through the body of the pit.
• Backfill over the top in 150-200mm layers, compacting each one, and crown the surface slightly proud – it will settle over the first few months.
• Build in an inspection point for a soakaway on day one, as it turns a future blockage from an excavation job into a look-and-hose-down job.

The soakaways I get called out to are almost never badly sized. They are badly built – no membrane, fines washed into the gravel, a feed pipe laid flat with no fall, and often, nobody can remember exactly where it is buried. The digging is the easy half of learning how to build a soakaway. The details underground are what decide whether it works for thirty years or three.

Dig up a failed one and it is nearly always the same picture. A gravel pit from the eighties or nineties, decent depth, clean enough stone, and no membrane anywhere. The voids between the stones are packed solid with fine silt, so the thing looks less like drainage and more like a buried flowerbed. That pit did not fail as such. It slowly filled, over twenty-odd years, with the soil it was sitting in – probably just missing layer of geotextile fabric that would have cost nothing at the time.

This guide covers the two ways I build garden soakaways: modern plastic crates, and the traditional gravel pit. Both work. Which one suits your job comes down to the ground you are digging in and how much water you are shifting.

What Is a Soakaway and How Does It Work?

A soakaway is an underground void that takes a sudden volume of rainwater – off a roof, a patio, a boggy lawn – stores it, and releases it slowly into the surrounding soil. The old way was a hole filled with rubble, half-bricks and broken flagstones. The modern way is reinforced plastic crates (engineers call them modular cells) that create as much empty storage space as possible without collapsing under the ground above them.

That storage-and-slow-release behaviour has a trade name – attenuation – and it is why regulators love soakaways. New developments are now required to send surface water to SUDS installations (sustainable drainage systems) rather than piping clean rainwater miles away, and some water companies will even knock money off your bill if your property disposes of all its surface water this way. Worth a phone call to yours; the rebates are real but patchy by region.

One rule before anything else: a soakaway is for rainwater only. Never connect it anywhere near the sewerage system, and never build one that sheds water towards a neighbour’s garden – inconsiderate, and in many cases illegal.

Where Can You Put a Soakaway?

The location decides more than the build quality does. Four requirements, and I treat all four as fixed:

• At least 5m from any building. Closer than that and you risk softening the ground under foundations. If the ground slopes back towards the house, go further than 5m, not less.
• At least 2.5m from a boundary. This one catches people out far more often than the 5m rule, because a fence line feels like the edge of your responsibility and it is not.
• In ground that drains. The water has to have somewhere to go. Sand and gravel subsoils are ideal; heavy clay is the difficult case, and it changes the whole approach.
• Above the water table. A soakaway sitting in groundwater is just a buried tank of water – it has nowhere to empty to.

Planning, Services and Regulations

Those two distances are not my preference, they are the law. Approved Document H of the Building Regulations sets a rainwater soakaway at a minimum of 5m from any building and 2.5m from a boundary (Scotland works to its own building standards rather than Approved Document H, so check locally if you are north of the border). My recommendation is to treat both numbers as a floor rather than a target, and to read “any building” literally – the neighbour’s garage, their extension, the outbuilding the other side of the fence all count. It is entirely possible to clear 5m from your own house and still be far too close to theirs. Where the garden gives you the room, I would rather put the extra metre in now than have a conversation about a damp boundary later.

Before you put a spade anywhere, check for buried services – electric, gas, water, comms, existing drains. If two drain chamber covers are visible in a garden, assume the pipe runs dead straight between them, because it nearly always does. When in doubt, get the site plans from your local authority. And if the soakaway will sit anywhere near an existing sewer run, ask your water company about a build-over agreement before committing – a conversation now is cheaper than a survey later.

On planning: a garden soakaway for rainwater does not normally need planning permission, but new drainage work does fall under the Building Regulations (Part H covers drainage – check the current detail with your local building control rather than taking a blog’s word for it, including mine). If you are on a new build, the surface-water strategy should already be marked on the plans – work with it, not around it.

Test the Ground Before You Dig the Hole

The quickest way to waste a weekend is to build a soakaway in ground that cannot absorb water. So test first. Dig a hole where the soakaway will go, fill it with water to saturate the ground, then fill it again and watch. Water still standing after several hours means the ground sheds water rather than soaking it up. And if the hole fills with water on its own, you have hit a high water table – stop there.

Pro tip: the British Geological Survey publishes free borehole records online, so you can often get a read on your local geology and water table before digging anything.

For anything feeding a large roof or paved area – or anything building control will look at – there is a formal version of this test called BRE 365, which gives you a measured percolation rate and a calculated soakaway size. For a modest garden soakaway, the bucket-and-patience version above tells you the main thing you need to know: yes, no, or clay.

Tools and Materials

How to Build a Soakaway with Crates: Step by Step

TLDR: dig, level a gravel base, wrap crates in membrane, connect the pipe on a proper fall, surround with shingle, backfill in compacted layers, and leave yourself a way to inspect it.

1. Dig the hole with working room. Size the excavation for the crates plus a gap of roughly 150mm all round for the shingle surround, plus the base layer, plus at least 500mm of cover over the top. Tight holes result in struggling and cursing; the extra digging is always worth it.
2. Prepare the base. Clear the bottom of stones, roots and flints, then spread and level around 100mm of gravel. The base is not doing anything clever – its job is to get the crates sitting firm, level and secure. Check the level in both directions; if the base leans, the whole soakaway leans with it.
3. Line the hole with geotextile membrane. Lay the membrane in a wide expanse across the excavation, the way you would drape a pond liner, so there is plenty spare to fold over the top later.
4. Assemble and place the crates. Clip or cable-tie the crates together – the ties are only there to hold everything in place until the side fill goes in, they are not structural – and lower the assembly onto the base.
5. Connect the feed pipe. The pipe run from the gully or downpipe to the soakaway needs a continuous fall of at least 1:100 – 1cm of drop per metre of pipe – and I check it with a level and string line during the dig, not after, because a flat spot in the run is the classic cause of a drain that silts up. Cut a neat cross in the membrane at the entry point, push the pipe about 150mm into the crate, then fold the membrane back around the pipe and tape or cable-tie it tight. That collar matters: it is the one place silt loves to sneak in.
6. Fit a silt trap on the feed run. If the water is coming off a roof or paving, an inline silt trap upstream of the soakaway catches the mud before it reaches your expensive void. Emptying a bucket every few months is a much better job than digging up a silted soakaway.
7. Wrap the parcel. Fold the membrane around the whole assembly – all four sides and the top – so no soil can migrate into the void. If you have read my French drain guide you know I wrap those like a burrito; same principle here, just a bigger parcel.
8. Add the side fill. Fill the gap around the crates with pea shingle or clean gravel. This braces the structure against ground movement and helps water move from the surrounding soil into the void.
9. Backfill in layers and crown it. Cover the top with 100mm or so of gravel, then backfill with the excavated soil in 150-200mm layers, compacting each layer before the next goes on. Finish slightly proud of the surrounding ground – groundwater washing through will settle the fill over the first few months, and a gentle crown now beats a sunken trench line in the lawn later. Moisten and tamp the final layers to force that settlement early.
10. Build in your inspection point. I do this on every crate soakaway I put under a driveway: an access point at the inlet end, or at minimum a marked, findable spot, so in ten years the system can be checked and jetted rather than dug up blind. Nobody remembers where a soakaway is buried. Make the soakaway remember for them.

Pro tip – pipes under paving: the feed pipe wants a minimum of 300mm of cover under patios and paths. Under a driveway, plastic pipe shallower than 600mm should be encased in concrete – or swap to clayware pipe, which deforms far less under load. Crates themselves can go under a driveway if there is genuinely nowhere else, but follow the manufacturer’s load-bearing installation spec to the letter; this is the one part of the job where their paperwork outranks my opinion.

How to Build a Soakaway with Rubble or Gravel

TLDR: same hole, same membrane wrap, same pipe rules – but the void comes from the gaps between stones, so the stone choice and the layering do the engineering.

The traditional soakaway pit predates crates by a century and still works, provided you build it deliberately rather than tipping in whatever the last job left behind. The catch is efficiency: a stone-filled pit is mostly stone, so you need a noticeably bigger hole than a crate system to store the same water. For a garden-scale build, I would treat roughly 1 cubic metre of excavation as the working minimum, and dig generously beyond it if the ground is slow.

1. Dig and test. Same siting rules, same percolation test, same 5m rule. No shortcuts because the materials are cheaper.
2. Line the pit with membrane. Non-woven geotextile across the base and up all the sides, with enough spare to close over the top. Skip this and the pit begins filling with fines from day one – the classic reason old rubble soakaways go solid.
3. Bed the pipe in rounded shingle. This is the detail that separates a built soakaway from a filled hole. The feed pipe gets bedded on and surrounded by 10-20mm rounded pea shingle – rounded specifically, because round stones settle evenly around a pipe and support it along its whole length. Angular stone bridges: the sharp edges lock against each other, leave voids under the pipe, and the pipe eventually flexes or cracks under load at exactly those points.
4. Layer the fill by size. Smaller, rounder stone goes near the inlet pipe; larger stone – 40mm gravel, clean broken brick or crushed stone – makes up the body of the pit. The fill must be clean and free of fines: soil, sand and dust in the stone will do the silting for you, from the inside, before the ground even gets a chance. Reclaimed site rubble is fine if it is genuinely clean and graded; a barrowload of anything is not.
5. Close the parcel and backfill. Fold the membrane over the top of the stone, then backfill in 150-200mm compacted layers and crown the surface slightly, exactly as with the crate build.

Which should you choose? The short version: crates buy you far more storage per hole dug and take structural loads predictably; a gravel pit is cheaper, simpler, and entirely adequate for a modest garden soakaway taking a lawn or a small patio. The full comparison comes down to void ratio, cost, and whether the ground above will be driven on.

Common Soakaway Mistakes (the Ones I’ve Actually Seen)

Next Steps

If the ground drains and the spade is sharp, a weekend and a methodical wrap will get you a soakaway that outlasts the fence. If the problem is surface water travelling across the garden rather than sitting in one spot, have a read of my French drain build guide – the two systems work beautifully together, with the drain collecting and the soakaway disposing.

We carry the 10-20mm pea shingle for the pipe surround and the 20mm and 40mm washed gravel for the fill – clean, graded, and delivered. If you are unsure on quantities for your hole, get in contact and we will do the sums with you.

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.

If you have dug a soakaway yourself, how did it go? How deep did you get before things got interesting, and has it cured the problem? Love to hear about it in the comments.