Soakaways for rainwater

Designing and Constructing a  Rainwater Soakaway

What are Rainwater Soakaways


How Soakaways are constructed

Even the smallest construction projects need some method of handling rainwater. If its construction results in rainwater being deposited on a piece of ground even a small play house or greenhouse etc may require a simple ‘system’. Rainwater soakaways are a method of dissipating the rainwater accumulated on and around buildings and other hard surfaces when the rainwater doesn’t drain into a sewer or watercourse. And rainwater soakaways are, (apart from Rainwater Harvesting) the most environmentally friendly and sustainable way of handling it.

Soakaways keep the rainwater within the local water table where nature intended rather than

sluicing it out into rivers and eventually into the sea. Only very occasionally, where for


Soakaways must be protected with polythene

example the water table is uncomfortably high, is that a bad thing But in such instances the

potential rainwater soakaways mathematical design calculations (see below) will indicate that they shouldn’t be built at all. Where the calculations show that rainwater soakaways are feasible they

are also environmentally beneficial

Rainwater lands on roofs and other hard surfaces and goes down gutters and pipes into what is called a ‘surface water drainage system’. You may be unaware that you had such a ‘system’. Building plans may assume that the water drains into a rainwater soakaways which may be something as straightforward as a hole filled with rubble with a drainage pipe running into it.


Excavating for a new Soakaway

Rainwater soakaways should have been constructed in accordance with building specifications but regulations have not always been adhered to with great care. The most important variable however is the soil type. If you place a rainwater soakaway in clay soil or somewhere with a very high water table you may end up with something resembling a paddy field or create a semi aquatic water

feature. Even your children’s purpose built play house may be at risk of being cut off in flash floods.

The rainwater soakaways must be able to cope with and store the rainwater running off hard surfaces and roofs for long enough allow the water to disperse into the ground leaving the rainwater soakaway ready to handle the next deluge or to cope with an extended period of rainwater entering it continuously.

Rainwater soakaways were usually made in the form of pits packed with rubble or constructed with a lining made of perforated rings of concrete or dry masonry. Modern Rainwater soakaways are more likely to be constructed from specialised rainwater soakaway kits, built as lined pits or may be of trench construction. Domestic rubble filled rainwater soakaways usually need to be renewed after about ten years. Very recently however new proprietary rainwater soakaway systems have been arriving to suit rainwater soakaway demands of all sizes

 Rainwater Soakaways Regulations

The Building Regulations are quite prescriptive in their demands as to how the rainwater, is handled. The Regulations even specify an order of priority for rainwater handling

The first choice must always be rainwater soakaways but with a number of conditions attached.

  • The pipe depositing water into rainwater soakaways would need to be at least 5 metres from the building and at least 2½ metres from next door’s fence and should not be located where the land is unstable or where the water table rises to the height of the rainwater soakaway bottom at any time in the year
  • Rainwater soakaways are also prohibited where contamination in the run off might cause the groundwater to become polluted, near any drainage mound or field, or so close to another rainwater soakaway that the overall capacity might be affected
  • Where the rainwater soakaway is an impractical proposition a water course is advised and failing that an appropriate sewer

Preformed concrete Soakaways

In recent times there have been anecdotal complaints of water and sewerage companies and planning authorities having unreasonably insisted on rainwater soakaways where they are unsuitable and the proper option should be a sewer. Take care not to accept the regulator’s word if you disagree with it. Give it some further thought and if you still disagree take advice.

Owing to legislation which came into force recently the water companies have to take responsibility for the upkeep of private sewers so they may not be impartial judges as to whether a sewer is actually the only option. There have been allegations that the water companies have tried to fob off some applicants with an insistence that they construct an unsuitable or difficult rainwater soakaway where it is not reasonable to insist on one.

And inspectors do not always get it right. An inspector may not for example be aware that your soil is as clay in nature as you have found it actually to be and therefore so impervious to water that he may be insisting on you constructing what will turn out to be a rubble filled rainwater reservoir as opposed to a functional rainwater soakaway.

Our advice describing how to do the soil tests and calculations will give you ammunition in any dispute. If a serious dispute occurs however you might need to take advice. It is not sensible to build  rainwater soakaways that may not work.

But course some uncertainty as to what’s going on underground often arises as a by product of the construction process. If by some oversight, for example, a channel gets left between the rainwater soakaway and the nearest foul water manhole, who would know.


You can design and build your own rainwater soakaways but the smaller it is the more likely you will be able to successfully do it yourself. But the dimensions required are not optional. Working out the correct size for the rainwater soakaway is arguably the most demanding part of the design and construction process.

For most of our readers the question of a DIY rainwater soakaways is most likely to arise when you build something in a garden. Perhaps an extension, some outhouse, or some other feature which is prone to depositing water on the land.

The first things you need to consider are as follows

  1. You must establish whether your site is a Groundwater Source Protection Zone, If it is you will not be permitted to pollute it with sewage effluent. If all that’s entering the soakaway is rainwater this is not an issue
  2. You must dig a Trial Site Assessment Hole to find whether the bedrock or water table ever comes within one metre of the rainwater soakaways pipe bottom. Sites in the UK commonly  fail this test
  3. If your project survives the pipe bottom test you must then conduct a percolation test to ensure that the rainwater will dissipate into the soil at an acceptable rate. Clay soils usually fail this test but you may still be able to consider an above ground mound rainwater soakaway instead or consult our articles on Rainwater Harvesting for an alternative use for your rainwater

These are design hurdles that have to be overcome before you can proceed any further. The feasibility and the actual dimensions of soakaways for rainwater must be settled at the outset otherwise you may find yourself with an unsuitable construction which could be expensive and time consuming to put right.

Even supposedly professional builders sometimes fail to carry out the necessary rainwater soakaways capacity tests with (what should have been) predictable consequences. The exact arrangements for rainwater handling should best be determined before the building is itself constructed. The rainwater soakaway is part of the project as a whole.

Size and Volume

Building Regulation Approved Document H specifies that a rainwater soakaways serving 25 square metres can only be used for rainwater landing on it at rate of no more than ten millimetres every five minutes. Readers are referred to BRE Digest 365 and BS EN 752-4 for relevant amounts for larger areas.

Go to  for rainfall data for your area. However we can tell you in advance that 10mm in five minutes is a serious deluge and would be exceptional in any part of the UK. It would amount to a freak flash flood. It is not so rare however for much larger amounts of rainwater to fall over a period of hours. And it is this fact which makes the rainwater soakaways dimension calculations demanding.

But when you consider rainwater soakaways you will also need to make your own judgment as to whether more than 10mm of accumulated rainwater from somewhere else, having suddenly been released following prolonged heavy rain, is likely to fall in any period of five minutes onto the area which the soakaway serves. If the answer is yes the rainwater soakaway will, regardless of whether it is regulation compliant, overload and flood, possibly causing significant damage.


percolation test for Soakaways

Soil Capacity Percolation Tests

Soil capacity is critical to the functioning of rainwater soakaways. If the soil cannot absorb the rainwater at the required rate then the rainwater soakaways will not be regulation compliant and will be liable to flood. The soil capacity needs to be tested by means of a percolation test which can be carried out as follows

  1. A 300 square millimetre hole should be dug to a depth of 300 millimetres lower than the depth of the proposed rainwater soakaway pipe. If a very deep drain is required specialist advice should be sought
  2. Fill the hole with water to a depth of about 300 millimetres and leave it overnight. The next day top it back up to the same level and measure the time (in seconds) for the water to drop from ¾ full to ¼, i.e. fall by 150 millimetres. Then divide the time by 150 millimetres

For example 75 minutes (4,500 seconds) divided by 150 millimetres is 30. The rainwater will take 30 seconds to subside by one millimetre. That number (30) is known as the Vp

  1. Ideally the test should be tried at least three times with two experimental holes and then the average taken. If however the two holes give hugely different results investigate further to see what’s causing it

Obviously only do the test when the ground is in a normal condition and not when it’s frozen, when it’s raining, when the ground is already saturated with rainwater or when it’s completely parched

Filtration Rate and Rainwater Storage Volume

Filtration Rate and Rainwater Storage volume then needs to be worked out to see if throughout the duration of a storm, the rainwater soakaway can contain the difference between the inflow and the outflow volume. Inflow volume is derived from the depth of the rainfall and the drainage area which the soakaway serves.

Building Regulation Approved Document H is a key document which you need to consult. Go to This document provides significant further technical detail which may be of assistance depending on circumstances.

You now need to make an important decision as to how much rainfall (and therefore rainwater) the soakaway will need to cope with. You also need to know the amount of rainfall (in millimetres) that is likely to fall on it is short bursts. This is partly a matter of judgment, and partly a matter of how cautious you want to be. Rainfall figures and other interesting facts are available on

Next you need to do the demanding mathematical calculations described below.

The accuracy you need at this stage depends largely on the location of the rainwater soakaway and its size. At one extreme if the rainwater soakaway is small and owing to its location a small occasional and temporary accumulation of surface water it unlikely to do any harm you can be more relaxed. However if it’s a large rainwater soakaway with significant volumes of water draining into it which could cause a serious flood and damage to something nearby you need either to be very cautious or arrive at very accurate calculations using reliable data. Either way you should not proceed on rainwater soakaway construction without regard to any calculations at all.

We hope to assist you in building your rainwater soakaway entirely as a DIY Project but if you are in any doubt or if your rainwater volume projections appear uncertain, we recommend that you seek professional advice, if only to ensure that your calculations are right. You will already have done the measuring yourself so a professional will not take long to examine your figures and give you the remaining answers.

You can also contact the Planning Department at your local council and they may send someone around to examine what you are proposing to do. Have your calculations and the figures you have obtained ready to show the inspector. It may well be obvious to an experienced the inspector that your proposal is suitable and he will advise you so. If not he will tell you what more you need to do.

Calculate the Filtration Rate and Rainwater Storage Volume

Filtration Rate

You next need to calculate the soil filtration rate (f) which is related to the number (Vp) obtained from the earlier test.

The equation to work out filtration rate (f) is-

f= 10-3 2Vp

If you are able to do this calculation keep the answer you arrive at and apply it to the example below.

Volume of outflow volume (O) is arrived at with the equation :

O = as50 x f x D

as50 is the side area of the storage volume when 50% full at its effective depth and (D) is the length of a storm measured in minutes (say 5 minutes).

For Example
Drained area = 25m2

Water incoming = 25m2 area x 10millmetres rainfall = 0.25m3 water volume available to dissipate.

Volume of outflow is O = as50 x f x D

Therefore O = 2x 0.0002 x 5 so O must = 0.002m3

Capacity required is 0.25m3 – 0.002m3 = 0.248m3.

So a traditional rainwater soakaway filled with rubble and measuring 1m3 beneath the inlet pipe and with, perhaps, 0.2m3 storage capacity, is insufficient, The soakaways volume would have to be increased to a capacity 1.24m3. If you find your tests appear to suggest the rainwater soakaway is impracticable you may consider digging deeper because the soil may be significantly different at different levels.

We appreciate that these calculations are difficult but if you are unable to do them take your test results to an appropriate professional and ask for advice. Alternatively contact us and we may be able to assist. Even if you can’t calculate your own figures however your test will give you an idea whether it’s worth carrying on. If your water stands for days without the level falling, your soil is clearly too impervious to support the functioning of a rainwater soakaway in acceptable dimensions.

All being well however this exercise will give you the cubic capacity required for the rainwater soakaway and you are ready to build it.

Building the Rainwater Soakaway

The rainwater soakaways must not encroach 5 metres of the building it serves or within 2 ½  metres of next doors fence. And whatever the calculations say, most inspectors will require the rainwater soakaway to be at least 1 metre cubed. The stone or other infill material must completely surround the pipe and come to at least 100mm above it with an impervious surface layer placed on top. This might be a tarpaulin or a concrete slab. Then finish a layer of topsoil or turf.

Ready made rainwater soakaways in kits form are available often comprising circular sections of plastic or concrete which can be joined together and completed with a cover. If it’s a small or rainwater soakaway or otherwise cautiously designed e building inspector is likely to be happy with what is in effect a hole in the ground filled with rubble.

Crate and vaulted drainage systems and products such as Stormcell and Stormblock are available which perform all the underground functions of a rainwater soakaways better than simply laying in rubble or other granular material. These rainwater soakaway systems store much more rainwater in them and allow the water to seep into the ground more slowly. Take care however to ensure that you are purchasing the correct product. Some systems store rainwater in the expectation that you will use most or all of it yourself in the form of Harvested Rainwater.

In general however for a larger rainwater soakaways project it may be best to use one of these systems. They last much longer than pits of rubble and alternative materials which decompose. The proprietary rainwater soakaway kits do however affect the mathematical calculations so it’s advisable to refer back to the manufacturers specifications before buying them.

Earlier we referred to forming rainwater soakaways mounds as an alternative where there is a high water table or a clay type soil. The complexities associated with the dimension calculations if the mound is a principle component of the rainwater soakaway are however too involved to go into here. Some of them are even the subject of theoretical computer modelling. Nevertheless if you are in a position to do so including a rainwater soakaway mound into the rainwater soakaway design is likely to give some additional insurance as to its effectiveness.

If you have any specific enquiries as to the design, construction, suitability or anything else related to rainwater soakaways we at KCS would be pleased to hear from you. Contact us and we’ll see if we can answer your enquiry by return