Happy Drains Blog

A Quick Guide to Understanding Underground Drains

Underground drains require a plethora of right tools as well as a hefty amount of skills and knowledge. We’ve put together a quick guide to understanding what underground drains and how they work.

Building Control

The Building Control Department will become your main point of contact as you need to ensure any proposed underground drainage projects are practical, possible and avoid breaking any laws. You’ll need to contact them before you start any work on your underground drainage system.

Drain Types

Drain systems in the UK hugely vary, but generally speaking, there are two types of drainage systems:

  • Foul water, which comprises of sewage and grey waste water that has come from toilets, sinks, baths and kitchen appliances.
  • Surface water, which is made up of only rainwater.

Some older properties have both the surface water and foul water running into the same pipe. This is not a cause for worry, though you will need a trapped gully to make sure no foul air can escape. Newer properties are likely to have separate foul and surface drainage systems, which is considered standard and best practice.


Only brown pipes should be used for underground drainage systems. You are likely to need plain end drain pipes, drain couplers, standard bends, rest bends, pipe lubricant, gullies and small inspection chambers. You may also require further fittings, including P traps, hoppers and rainwater and waste pipe adaptors.


You will need to plan and have a clear idea of where all the equipment will need to go, how it will connect to one another and whether it obeys any building regulations. It is advised to create a diagram that can be referred to throughout the underground drainage system project. You’ll also need to ensure nothing interferes with the foundations of the property.

Preparing Trenches

You should ensure all building regulations are adhered to when digging trenches. For example, the tops of pipes must be at least 300m deeper than ground level. Once the trenches have been created, you should place pea shingles or other granular materials to form a bed, which will help to provide stability to the drainage runs.

Installing underground drains is a lengthy process, and you’ll have to keep an eye on our blog for the next instalment. If you’re after some help or advice regarding your underground drainage system, we can help. For more information, feel free to get in touch by calling us on 0800 849 8099 or via our contact page.