If your project needs a soakaway, for a new roof, an extension, or a conversion off mains drainage, the ground decides whether it will work. On clay and other slow-draining ground, a soakaway built without a test can fail in its first wet season. The test that tells you in advance is a small cost next to digging it up again.
Key Takeaways
- A soakaway is a designed structure, not a hole filled with rubble. It has to be sized from how fast your ground actually takes water away, measured on site.
- There are two different tests, for two different jobs. A surface water soakaway is usually tested and sized to BRE Digest 365. A drainage field for effluent from a septic tank or treatment plant uses the percolation test in Approved Document H. They are not interchangeable.
- On clay and other low-permeability ground, the test often shows a soakaway will not work at a practical size. The answer then is a different route, such as discharge to a watercourse or a sewer, often with storage and a controlled outflow, not a bigger hole.
- Rainwater should not be run into a septic tank, a treatment plant, or its drainage field. Keep the two systems separate.
- A soakaway shown “for costing only” on a drawing is a placeholder, not a design. Do not build it until it has been tested and designed.
A common way it goes wrong
This is a pattern, not a single case. A drawing shows a soakaway with a note such as “soakaway for costing only, design and test to follow”. The note is there because nobody has tested the ground yet. The groundworker prices a pit, the test is treated as someone else’s job, and the pit is dug to the size on the drawing. Sometimes the roof water and the outlet from a treatment plant are both piped into it to save a trench.
On free-draining sand or gravel, that may get away with it. On clay, the first long spell of wet weather fills the pit faster than the ground can take the water away. The pit overflows, the ground around it stays waterlogged, and water can back up the pipes. The fix is a dig-up, a new design, and a delay, all of which a test at the start would have avoided.
Where surface water is allowed to go
Requirement H3 of the Building Regulations covers rainwater from roofs and paved areas, and it sets an order of priority for where that water goes. First, an adequate soakaway or other infiltration system. Where that is not reasonably practicable, a watercourse. Only where that is not reasonably practicable either, a sewer. Approved Document H is the government’s guidance on how to meet it.
The important words are “adequate” and “reasonably practicable”. Whether a soakaway would be adequate has to be shown, not assumed, and an infiltration test is how you show it. If the test shows the ground cannot take the water at a useful rate, that is the reason to move down the list, not a reason to build the soakaway anyway.
Moving down the list brings its own permissions. A new outfall to a watercourse needs the agreement of whoever owns the land it crosses. On an ordinary watercourse, a new outfall or other work in the channel may also need consent from Kirklees Council as the Lead Local Flood Authority. Work on or near a main river is a matter for the Environment Agency. Some small watercourses around Huddersfield run in culverts under gardens and roads, so finding the watercourse can be the first job. A new connection to a public sewer has to be applied for through the sewerage company, which around Huddersfield is Yorkshire Water, and rainwater should not go into a sewer that carries only foul water. On new houses, a planning condition may also set how much water can leave the site and how fast.
The surface water test: BRE Digest 365
BRE Digest 365, Soakaway design, is the design method most often used for surface water soakaways, and the one Building Control commonly asks to see. A trial pit is dug on the line of the proposed soakaway, to about the depth it will be built, filled with water, and allowed to drain. This is done three times, and the time the water takes to fall between two set levels is recorded each time. From those times, the soil infiltration rate is worked out.
That infiltration rate, the area of roof and paving draining to the soakaway, and a design rainstorm are then used to size the soakaway so that it can hold the water from the storm while the ground takes it away, and empty again in a reasonable time before the next one. On sand and gravel, the numbers usually work at a sensible size. On clay, the water may barely drop during the test, and the soakaway needed becomes impractically large, or the test shows infiltration is not a realistic option at all. The base of the soakaway also has to stay above the highest winter groundwater level, which a test done in a dry summer can miss.
When the test shows that a soakaway will not work, it has done its job. It has told you, before anything has been built, that you need a different design. That usually means a connection to a watercourse or sewer, often with underground storage that lets the water out slowly.
The effluent test: Approved Document H percolation test
If a property has a septic tank or a package treatment plant, the effluent usually goes to a drainage field: perforated pipes laid in gravel-filled trenches that spread it slowly through the ground. The test for a drainage field is a different one, set out in Approved Document H. A trial hole is dug first to check the groundwater level. Small test holes are then dug below the level of the proposed trenches, soaked, refilled, and timed as the water drains. The result is a percolation value, called Vp, which shows whether the ground is suitable for a drainage field at all and, if it is, how large the field needs to be.
So there are two tests with two different results: a BRE 365 infiltration rate for rainwater, and a Vp for effluent. If a drawing or a report mixes them up, ask which test was done and what it was done for.
In England, discharging septic tank or treatment plant effluent to the ground must either meet the Environment Agency’s general binding rules for small sewage discharges or be covered by an environmental permit. Those rules expect the discharge to go through a properly designed drainage field, not a soakaway pit, and a new private system may not meet them where a public foul sewer is close by. Check this before the system is designed.
Clay, and ground around Huddersfield
Clay is a fine-grained soil that water moves through very slowly. In a test on heavy clay, the water level can barely move over hours. The same is often true of the weathered mudstones and glacial till found in many parts of the district around Huddersfield. Sandstone can drain well, but it can also be patchy, with water running along cracks in the rock rather than soaking evenly into it, and it can come out again lower down the hill: at a spring line, in a neighbour’s garden, or through a cellar wall.
Parts of the district also have a legacy of old, shallow coal workings. Check the coal mining report for the address before a soakaway is designed. Where shallow workings are recorded, putting water into the ground needs particular care and may not be acceptable.
Slopes and made ground matter too. On a hillside, a soakaway uphill of a house can send water towards its foundations or into its cellar, and one near a retaining wall adds water pressure behind a wall that was never designed for it. Filled ground can settle when it is soaked. Approved Document H gives guidance on siting, including keeping soakaways away from buildings and out of unstable ground. The rest is engineering judgement for the particular site, and it belongs in the design, not on the day of digging.
On slow-draining ground, the honest answer is often that a soakaway will not work. Accept that early and the project moves on by a different route. The alternative is to build it, watch it fail, and then have the conversation next to a waterlogged pit.
Keep rainwater and effluent apart
A drainage field is sized for the effluent from the house, not for rainwater from the roof. Sending rainwater into a septic tank, a treatment plant, or its drainage field overloads it, can flush partly treated effluent through, and on slow-draining ground makes a failure far more likely. Rainwater and effluent need their own systems, each tested, designed, and sited for its own job.
What “for costing only” means
Drawings sometimes carry notes such as “soakaway for costing only, design to follow” or “indicative drainage layout, subject to ground investigation”. These notes mean the designer did not yet have the site information needed to design it. They are not permission to build.
Before a soakaway is built, you need a result from the right test, a design based on that result, and, where the work falls under the Building Regulations, confirmation that Building Control is satisfied with it. If any of these is missing, do not build it yet.
When to get an engineer involved
Drainage on conversions, extensions, and self-builds often falls between packages, with the designer, the groundworker, and the treatment plant supplier each assuming someone else has it covered. Get the drainage designed before the groundworks start, not after. If a soakaway has already been built and is slow to empty or overflows in wet weather, have it checked before it fails completely, and do not connect anything more to it in the meantime.
I design soakaways, drainage fields, and the alternatives when a soakaway will not work, from test results you provide. The tests themselves, BRE 365 or percolation, are carried out by a separate specialist.
This post is general information about how soakaways and drainage fields are tested and designed. It is not a design for your site, which depends on your own test results, ground, and layout.
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