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A Simple Guide to Underground Drainage

Drain Depot - suppliers of drainage materials, believes when it comes to underground drainage not only do you need the right tools but also a good amount of skills and knowledge. Even though installing underground drainage isn’t the most complicated task in the world, if you get it wrong not only could you cause a huge amount of damage but also find yourself in trouble with your local Building Control Department! This is why Drain Depot has created our own guide explaining how underground drainage works – perfect for beginners or anyone who needs to refresh their memory.  Don’t forget that if you have any further questions to get in touch with our team of friendly experts who will be happy to help.

Tip 1 – Get to know the Building Control Department

The Building Control Department will be your main point of contact when it comes to ensuring that your underground drainage project is feasible and, more importantly, doesn’t break any laws.  If you are planning on installing or adapting underground drainage then you must contact them before you start any work. However, if you are just repairing a broken element of your underground drainage then you may not need their permission – it’s always best to double check though!  For more information visit their website or give them a call on 020 7091 6860.

Tip 2 – Learn your Drain Types

Drainage systems here in the UK can sometimes be confusing as they vary wildly depending on when they were installed.  However, generally there are two types of drainage systems: - Foul water – this comprises of sewage and grey waste water from WC’s, sinks, baths, kitchen sinks, dishwashers & washing machines. - Surface water – this is rainwater only; any other type of sewage or grey water needs to be directed to foul water pipes. Depending on the age of your property, you may find that both surface water and waste water runs into the foul water pipes.  This is fine; however you will need a trapped gully to make sure that no foul air escapes.  In newer properties you will have separate foul and surface drainage systems and this is considered standard & best practise.

Tip 3 – Know your Equipment

First things first: if you are installing underground drainage you must make sure that all the pipes you use are brown in colour, used specifically for underground drainage otherwise the Building Control Department will require you to remove them and start again. Drain Depot provides a comprehensive range of brown drainage pipe & fittings, and to make your life easier we have created a list of some of the most common items when it comes to underground drainage projects:

Plain End Drain Pipes – When purchasing pipes for underground drainage always check if they come chamfered on both ends for ease of installation. Pipes are supplied in either 3mt or 6mt lengths and conform to BSEN1401.





Drain couplers are used to join two pipes or plain ended fittings together, make sure the rubber seals are intact and are seated correctly, use a lubricant to ease installation and avoid unnecessary leaks. Certified to BS EN1401.





  • Standard Bends – Used to change direction they can be supplied in double or single socket. Double socket bends have two blown ends with rubber integral seals to accept a plain ended drain pipe and single socket bends have one blown socket with seal and one plain end. Bends come in 4 differing degrees 90, 45, 30, 22, 15 & 11. Certified to BS EN1401.





  • Rest Bends – These are used to connect soil stacks and ground floor sanitary appliances to the underground drains. It allows anything coming from the toilet pan to pass through to the drains. The bottom of the bend should never be any closer than 450 mm to the drain floor connection just above the soil stack. The difference between the WC pan outlet and bottom of the rest bend should also be less than 1.5 metres to ensure that waste can flow smoothly.




  • Pipe Lubricant – It is of the utmost importance that you never use anything other than dedicated pipe lubricant when fitting underground pipes as this can lead to leaks and unnecessary damage.






  • Gullies – It’s important that the gully you install can withstand traffic that will be passing over it, whether it’s vehicles or pedestrians. You may also want to purchase rising pieces if you need greater depth as well as sealed lids if you do want the gully to accept rainwater from the open top grating.





  • Small Inspection Chambers – The maximum depth for small inspection chambers set out by building regulators is 600mm, however risers can be cut to size if you require them to be shallower. Inspection chambers are used to access drainage runs and specific points usually when a change of direction occurs. Access allows blockages to be removed or the system rodded.

Depending on the underground drainage system you are installing you may also require further fittings including P traps, hoppers and rainwater & waste pipe adaptors.  Don’t forget to also have your safety equipment to hand including safety boots, goggles, a hi-visibility jacket and safety barrier mesh to secure the area you are working in.


Tip 4 – Plan, Plan, Plan

Before you even pick up your shovel it’s important you have a clear plan of where everything will need to go, how it will connect and whether it adheres to building regulations.  For this reason it’s often a good idea to create a diagram that you can refer to throughout your project.  Don’t forget that you need to make sure your project doesn’t interfere with the foundations of your property – for more information ask your local council for blueprints of your home.

Tip 5 – Prepare your Trenches

When digging your trenches make sure that they adhere to building regulations, which means that the top of the pipes must be at least 300m deeper than ground level.   After you have dug your trenches place pea shingles or a similar granular material to form a bed and provide stability for your drainage runs.

Tip 6 – Laying Pipes with Care

Laying underground drainage correctly takes time especially as you have to consider the gradients for each pipe to ensure that both waste and surface water flows properly.  This is possibly the trickiest part of any underground drainage project, which is why if you haven’t done it before you should take your time and potentially even ask a professional for help.

Tip 7 – Connecting pipes internally to your home

As mentioned, there are a number of rules and guidelines associated with laying drainage pipes; however these don’t just apply for when you are laying them underground!  When you are passing a pipe through a building wall you need to have a lintel with at least 50mm clearance around the pipes on each side of the wall.  You should also cover the hole with a rigid sheet so that soil or vermin don’t end up in your home!

Tip 8 – Pipe Maintenance

As we all know, PVC pipes are prone to wear and tear over the years, meaning that every now and again you may be required to perform some maintenance. This is why you need to make sure that any manholes, chambers or gullies you install can be easily accessed so as you are able to rod the pipework, clear any blockages or gain access for a repair.

Tip 9 – Check everything’s above board

As previously mentioned the Building Control Department will be your main point of contact during your underground drainage project; however did you know that you have to have them inspect your work after you have completed it?  As long as you have adhered to all the building regulations they will provide you with a certificate of approval and you can congratulate yourself on all your hard work!

Tip 10 – Further Information

If you have ever undertaken a drainage project before you will know that they range from relatively simple to extremely complex, which is why it’s always beneficial to know where you can find help should you need it. Below is a list of handy videos you can use during your next underground drainage project, however if you have any further questions please just let us know in the comments section below! Drain Depot YouTube Videos Circular Inspection Chamber with Circular Cover – https://youtu.be/meBJ5WLaIEo Square Top Bottle Gully – https://youtu.be/dZc24H1jtvs Underground Drainage T Junction - https://youtu.be/1bO6bxxUNts Underground Drainage Standard Coupling - https://www.youtube.com/watch?v=sfw4s-qNh7c Drainage Waste Pipe Adaptor - https://www.youtube.com/watch?v=oMXCfIhTDgA Underground Drainage Rest Bend - https://www.youtube.com/watch?v=NdGF8Ll5vaQ Underground Drainage bend - https://www.youtube.com/watch?v=QeR2ng2EQlo Manhole Base Inc. 2 Risers & Round Cover - https://www.youtube.com/watch?v=vxP9OddFI9I

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