Underground Drainage: Explained

Posted: 12th January 2023 | Author: Jamie Brown

Back to news

---

img: Underground drainage fittings layout

What is Underground Drainage?

Domestic underground drainage, or foul drainage, refers to the network of pipes, manholes, and other structures that are used to collect and transport sewage and surface water away from homes and other buildings in the United Kingdom. This system is a crucial part of the infrastructure that keeps our homes and streets clean and hygienic, and it is the responsibility of homeowners and property owners to ensure that their underground drainage is properly maintained.

Surface water drainage is a crucial aspect of modern construction, as it helps to prevent water damage to buildings and other structures. The system is designed to collect water from hard surfaces such as roofs, driveways, paths, and patios and transport it away from the building.

Roof drainage is the above-ground component of the surface water drainage system. It consists of gutters and downpipes that collect water from the roof and channel it into the underground drainage system. The gutters are typically made of metal, plastic, or rubber, and are installed along the edge of the roof to catch and channel water from the roof into downpipes. Downpipes are vertical pipes that are connected to the gutters and carry the water from the roof to the underground drainage system.

It's important to note that the surface water drainage system is separate from the foul water drainage system which carries waste and sewage. A separate system of pipes and manholes are responsible for this.

Overall, the surface water drainage system is a complex network of pipes and components that work together to prevent water damage and keep buildings and surrounding areas safe and dry.

 

How Does Domestic Underground Drainage Work?

img: builder installing drainage

In a typical domestic underground drainage system, rainwater and other surface water is collected by gutters and downpipes on the roof of a building and directed into a network of underground pipes. These pipes, which are usually made of plastic or clay, transport the water to a manhole, which is a large, above-ground structure that allows access to the underground drainage system. From the manhole, the water is carried through a series of larger pipes to a treatment plant or other disposal site, where it is treated and released into the environment. All the flow is gravity based so pipes require a small fall to assist in this process. 

The standard UK domestic underground drainage size is 110mm (4”) and in new builds and extensions is most commonly plastic uPVC in terracotta colour rather than the legacy clay.

 

Installing Underground Drainage

Domestic underground drainage refers to the system of pipes and fittings that is installed beneath the ground in order to collect and dispose of rainwater and waste water from a house or building. This type of drainage system is necessary in order to prevent water from accumulating around the foundation of a house or building, which can cause structural damage and create a breeding ground for pests and diseases.

There are several steps involved in installing an underground drainage system for a domestic property in the UK:

1. Planning and design: The first step in installing an underground drainage system is to plan and design the layout of the system. This will involve considering the size and layout of the property, the location of the existing drainage system, and any local regulations that must be followed.
2. Excavation and preparation: The next step is to excavate the area where the drainage system will be installed. This will typically involve digging trenches around the perimeter of the property to a depth of around 60cm.
3. Laying the pipes: Once the trenches have been dug, the pipes for the drainage system can be laid. These pipes are usually made of plastic or concrete, and are connected using specialized fittings and then covered with 10mm pea shingle to cover and protect the pipes in the drainage system, as well as to provide a stable and permeable surface for water to flow over.
4. Backfilling and compaction: Once the pipes have been laid, the trenches should be backfilled with soil and compacted to ensure that the pipes are secure and properly supported.
5. Testing and final connections: Finally, the drainage system should be tested to ensure that it is functioning properly. This may involve flushing the pipes with water or using specialized testing equipment. Once the system has been tested and is deemed to be in good working order, it can be connected to the house or building and put into use.

It is important to note that installing an underground drainage system can be a complex and time-consuming process, and it is generally recommended that it be carried out by a professional contractor with experience in this type of work.

 

The fall required for Underground Drainage

img: Diagram showing the fall of underground drainage pipe

Domestic underground drainage falls refer to the slope or gradient of the pipes in an underground drainage system. The term "fall" refers to the difference in elevation between the inlet and outlet of the pipes, and it is important to ensure that the fall is sufficient in order to allow the water to flow through the system properly.

There are several factors to consider when designing the falls for an underground drainage system:

• Pipe size: The size of the pipes in the system will affect the required fall. Larger pipes will require a steeper fall in order to maintain an adequate flow rate.
• Length of the pipe run: The longer the distance that the water has to travel through the pipes, the steeper the fall will need to be in order to maintain a sufficient flow rate.
• Type of soil: The type of soil that the pipes are laid in can also affect the required fall. Soils with a high clay content will require a steeper fall, as they are more prone to clogging and can obstruct the flow of water.
• Local regulations: In the UK, there are regulations governing the minimum fall required for underground drainage systems. These regulations will vary depending on the location and type of property, so it is important to check with the local authorities to determine the required fall for a specific project. Gradients from 1 in 40 to 1 in 110 will normally give adequate flow velocities. If the gradient is steeper than 1 in 40, the liquid may run faster than the solids in the sloping foul water pipe thus leaving the solids stranded, which could then block the pipe. A gradient of 1 in 80 is suitable for commencing calculations for pipe schemes. If the gradient is less than 1 in 110, then the pipe could still block if the solids slow down and become stranded.

It is important to ensure that the falls in an underground drainage system are properly designed and installed, as a lack of sufficient fall can cause problems such as slow drainage, blockages, and flooding. It is generally recommended that this type of work be carried out by a professional contractor with experience in drainage design and installation.

 

Why is pea shingle used around underground drainage?

img: Compaction of materials above minimum of 150mm shingle

Shingle is a type of small, angular stone that is often used in the installation of underground drainage systems. It is typically used to cover and protect the pipes in the drainage system, as well as to provide a stable and permeable surface for water to flow over.

There are several benefits to using shingle in the installation of underground drainage systems:

• Protection: Shingle helps to protect the pipes in the drainage system from damage caused by soil movement, water flow, and other factors. It also helps to prevent the pipes from becoming blocked by debris.
• Permeability: Shingle is a porous material that allows water to pass through it, which helps to prevent standing water and excess moisture in the soil.
• Stability: Shingle provides a stable and supportive surface for the pipes in the drainage system, which helps to prevent the pipes from shifting or collapsing.
• Drainage: Shingle helps to facilitate the drainage of water away from the pipes and into the surrounding soil, which helps to prevent the build-up of excess water in the system.

Overall, the use of shingle in the installation of underground drainage systems helps to protect and support the pipes, as well as to facilitate the proper drainage of water away from the property.

 

Main Components for Underground Drainage

A uPVC (unplasticized polyvinyl chloride) underground drainage system is a type of drainage system that is commonly used in residential and commercial properties. The main components of a PVC underground drainage system include:

• Pipes: uPVC pipes are typically used for underground drainage systems because they are strong, durable, and resistant to corrosion. These pipes come in a range of sizes and are used to transport sewage and surface water from the building to the treatment plant or other disposal site. The most common sizes are 3m Pipes and 6m Pipes; these can come as plain ended lengths or with an in-built socket coupling.
• Fittings: Fittings are used to connect the pipes in the underground drainage system. These fittings may include elbows, tees, couplers, and other types of connectors.
• Manholes: Manholes are large, above-ground structures that allow access to the underground drainage system. They are typically located at regular intervals along the route of the pipes and are used to inspect and maintain the system.
• Drainage pits: Drainage pits, also known as catch basins, are structures that are used to collect and filter surface water before it is transported through the underground drainage system. These pits may include grates or other features to prevent debris from entering the system.
• Stormwater and sewage treatment plants: The final destination for the water in an underground drainage system is typically a stormwater or sewage treatment plant. These plants are used to clean and treat the water before it is released back into the environment.

 

Common Problems with Domestic Underground Drainage

Despite their importance, underground drainage systems are not immune to problems. Some common issues that can occur include:

• Blockages: Debris, grease, and other substances can build up in the pipes and cause blockages, leading to slow drainage and potentially causing sewage to back up into the home or garden.
• Collapsed or damaged pipes: Pipes can become damaged or collapsed due to age, ground movement, or other factors, leading to leaks and blockages.
• Tree roots: Tree roots can grow into underground pipes, causing blockages and damage.
• Sewer flooding: In some cases, the underground drainage system may become overwhelmed during heavy rain or other extreme weather events, leading to sewage flooding.

 

Maintaining Domestic Underground Drainage

It is important for homeowners and property owners to regularly maintain their underground drainage systems to prevent problems and ensure that they are functioning properly. Some simple steps you can take to maintain your system include:

• Regularly cleaning gutters and downpipes to prevent debris from entering the system
• Avoid flushing non-biodegradable items down the toilet
• Use strainers in sinks and showers to prevent hair and other debris from entering the system
• Avoid pouring grease or oil down the drain
• If you are experiencing issues with your underground drainage system, it is important to contact a qualified professional to diagnose and fix the problem. 
• Ignoring problems with your drainage can lead to serious damage and costly repairs, so it is always best to address any issues as soon as possible.

Do I need planning permission to make changes to Underground Drainage at a property?

You may not need to apply for planning permission for repairs, maintenance, and minor changes to underground drainage at your home in the UK, but it's always best to check with your local council. Permitted development rights may apply for larger changes, but it's important to confirm this with the local planning authority. Before making any changes, it's important to clarify ownership and responsibility for the drainage system, as it may be shared with neighbours or owned by a sewerage company. Failing to comply with relevant standards or legislation could result in legal and financial consequences. It is recommended to seek advice from your local planning authority.

You may need permission from your local council to make changes to the underground drainage at your home in the UK. This is typically done through the building control department and is known as a building regulations application. They will need to approve the plans for the changes to ensure that they comply with local building codes and regulations. Additionally, if you live in a conservation area or have a listed building, you may need to apply for planning permission as well.

 

The Author: Jamie Brown

From my early working life as a labourer, then becoming a builder and a property developer, I have been buying and using building materials for over 25 years. I set about to start a builders merchant which can offer high-quality products and significant savings along with speedy delivery to keep your sites moving. Still having a foot in the construction industry on my own property, I always make sure the products we sell are up to our high standards which is important for us to keep a great image for the company and superior reputation to supply quality!

View Author Profile

Back to news