Underground drainage systems are crucial for maintaining the structural integrity of properties and ensuring efficient water management. Whether you're dealing with blockages, installation or selecting the right materials, understanding the intricacies of these systems can save time, money and effort. Here are some frequently asked questions about underground drainage supplies.
Table of Contents
What is the Difference Between Wastewater and Surface Water?
What’s the Difference Between Plain and Socketed Drainage Pipes?
What is Underground Drainage?
The term ‘underground drainage’ describes the system of pipes, fittings and access points installed below ground level to safely collect and transport wastewater and surface water away from buildings and infrastructure. The primary function of underground drainage systems is to manage water efficiently, prevent flooding and protect structures from water damage.
Broadly speaking, two main types of water need managing. Wastewater, also known as foul water, is directed to the municipal sewer system, a septic tank or a treatment plant. Surface water, such as rainwater, is typically discharged to a soakaway or surface water sewer (sometimes known as a storm drain).
How Does it Work?
Underground drainage systems work by using gravity to move water through a network of pipes laid to carefully calculated gradients. Wastewater and surface water enter the system via gullies and soil stack pipes, before the effluent flows through the underground pipe network towards its approved discharge point.
Inspection chambers, manholes and rodding points are positioned along the system to allow easy access for maintenance. Correct pipe sizing, joint integrity and consistent gradients are essential to maintain flow, prevent blockages and ensure the system operates reliably under varying load conditions throughout its life.
Types of Underground Drainage
An underground drainage system is made up of several key components, each performing a specific function within the overall network:
- Drainage pipes: Typically PVC, used to transport foul and surface water, making up the majority of the system
- Fittings and connectors: Including bends, junctions and couplers, to change direction or join pipe runs together
- Gullies: Which collect surface water and prevent debris from entering the system – see our drain gullies guide for more
- Inspection chambers and manholes: Providing easy access for inspection, testing and maintenance needs
- Surface water solutions: Such as channels, soakaways and attenuation systems, where required
Together, these components form a complete and compliant underground drainage system suited to commercial groundworks applications.
What is the Difference Between Wastewater and Surface Water?
Wastewater
Wastewater is the water discharged from toilets, baths, showers, basins, sinks, washing machines and dishwashers. If it comes from toilets, it is also known as foul water, while from other sources it is known as grey water.
Foul water must always be directed to a main sewer, septic tank or sewage treatment plant. On the other hand, grey water can either be returned to the environment or used for applications such as flushing toilets to reduce the consumption of potable mains water.
Surface Water
Surface water, also known as rainwater or stormwater, is the result of rain or melted snow collected from roofs, roadways and driveways. Providing it isn’t run-off from a potentially contaminated surface, such as a fuel station forecourt, surface water can be returned directly to the environment.
Uncontaminated surface water can also be harvested, filtered and reused for functions such as irrigation and car washing.
The Importance of Keeping Wastewater and Surface Water Separate
When looking at what underground drainage is, it’s important to consider that today, it essentially covers two separate systems. Until 1970, most houses were fitted with combined drains that mixed all wastewater and surface water in one system. Often, this untreated mixture was simply discharged into the nearest river or watercourse. But by 1970, concerns about public health had reached such a pitch that separate drainage systems for wastewater and surface water began to be introduced.
Even now, some older properties still have a combined system. Some houses built after 1970 also have combined systems, but these direct both wastewater and surface water to a water treatment facility.
While property owners aren’t legally obliged to change older drainage systems and some areas lack the infrastructure to create separate systems, combined drains still run a higher risk of polluting the environment.
What do Underground Drainage Pipes Look Like?
While underground drainage pipes were traditionally made from vitreous clay, cast iron, pitch fibre or asbestos cement, modern systems are mostly manufactured using PVC, a hard wearing and almost maintenance-free plastic. Clay systems are still manufactured in the UK; they remain popular with some developers, particularly for commercial installation.
The pipes and fittings used in modern underground drainage systems are almost always terracotta orange in colour. This makes it easier to differentiate them from above-ground soil pipes, which are usually black, white or grey.
What are the Benefits of PVC Underground Drainage Systems?
Compared with more traditional materials, PVC pipes and fittings are more resistant to ground movement, don’t corrode and have a smoother inner wall that gives better flow efficiency than the older materials.
They are also much lighter and easier to transport, and even rigid PVC pipes have a degree of flexibility that other materials don’t. PVC pipe is less fragile but more easily cut than other materials, using a fine-toothed handsaw.
How Long do PVC Underground Drainage Systems Last?
Providing it is properly installed and regularly maintained, a PVC underground drainage system should last for fifty to a hundred years or more.
Any signs that a drain might be starting to block should be investigated promptly; these include reduced flow, gurgling sounds or foul odours.
Can a PVC Underground Drainage System be Connected to Another Pipe Material?
A new underground drainage system will often need to connect to older clay, iron or cement pipes, particularly where drains are installed in previously developed areas. One of the simplest ways to join different systems is to use a flexible connector. Made of a rubber-like material, these connectors have differing inlet and outlet diameters to match the new and old pipes and use stainless steel worm drive hose clips to create a lasting, watertight seal.
What’s the Difference Between Plain and Socketed Drainage Pipes?
Plain drainage pipes maintain the same internal and external diameter along their entire length and require separate couplings to join them.
Socketed drainage pipes incorporate a wider (female) opening at one end with an internal diameter equal to the external diameter of the other (male) end. This allows lengths of socketed drainage pipe to be slotted together without couplings.
Plain-ended drainage pipes tend to be more popular nowadays for domestic drainage, where a choice of couplings will typically be used to create drainage systems that serve multiple outlets. Longer stretches of uninterrupted drain, however, may be faster and easier to install if socketed pipe is used.
How is a Drainage Pipe Measured?
In the UK, 110mm is the most common PVC drainage pipe size used for domestic installations, while 160mm is used for a main flow pipe that serves several dwellings. Larger pipe sizes – 200mm, 250mm, 315mm and beyond – are also used for higher-capacity and heavier-duty applications.
These measurements refer to the outside diameter of the pipe. On a plain end pipe, the measurement will remain constant along the entire length of the pipe, whereas on a socketed pipe, it will be the external diameter of the wider (female) end only.
How do I Unblock an Underground Drain Pipe?
To unblock an underground drain pipe, start by determining where the water is backing up to, as the blockage will be downstream from that point. If the blockage is not far down the drain, a plunger can increase the air pressure inside the pipe and often force the blockage to break up. Create a tight seal around the drain opening before working the plunger vigorously until the blockage is cleared.
If plunging doesn't work, use a drain snake. Insert the snake into the pipe through the nearest access point and feed it until you encounter resistance, which signals the blockage. Rotate the snake to break up the blockage and push it through the pipe, continuing until the snake moves freely.
Once the blockage is broken up, flush the pipe with water using a garden hose. Insert the hose into the drain and run water at high pressure to clear any remaining debris. For stubborn blockages, a high-pressure water jet can be very effective in blasting away blockages and cleaning the pipe walls.
If the blockage persists, check with a pipe inspection camera to identify the exact location and nature of the blockage and confirm that it has been fully cleared.
You can also use a commercial drain cleaner or an enzyme-based cleaner to break down organic material, following the manufacturer's instructions carefully. Always wear protective gloves and eyewear when dealing with chemicals or sewage.
If you cannot clear the blockage yourself or if it is severe, call a professional plumber. Regular maintenance, such as using enzyme-based cleaners monthly, can also help prevent future blockages.
Which Pipe Material is Best for Underground Drainage?
When discussing what underground drainage is, pipe material is an important consideration. The choice of pipe material used depends on various factors, including durability, cost and application. Plastic pipes are most commonly used in new underground drainage systems. PVC (Polyvinyl Chloride) is popular for its corrosion resistance, light weight and ease of installation. HDPE (High-Density Polyethylene) is flexible, resistant to corrosion and suitable for various soils.
However, non-plastic pipes also have their advantages. Clay is traditional and highly durable, though heavier and more difficult to install. Concrete is suitable for heavy-duty applications due to its strength.
How Should Underground Piping Systems be Installed?
Proper drainage installation is vital for the effectiveness and longevity of an underground drainage system. This process begins with thorough planning and design, ensuring proper gradients and appropriate pipe sizes.
Excavation follows, where trenches are dug to the required depth and width. A bedding layer of gravel or sand is then laid to support the pipes.
During pipe laying, it is crucial to position the pipes correctly, ensuring that the joints are secure and watertight. Finally, backfilling involves covering the pipes with selected backfill material, which is compacted in layers to prevent settlement.
What are the Key Components of a Well-Designed Underground Drainage System?
A well-designed underground drainage system includes several essential components to ensure efficient and effective water management.
- Pipes, made from suitable materials and sized appropriately for the application, form the backbone of the system.
- Manholes serve as access points for maintenance and inspection, while gullies collect surface water.
- Channels and gratings facilitate surface water drainage, and inspection chambers, which are smaller access points compared to manholes, provide additional accessibility.
- Soakaways are structures designed to manage surface water runoff, completing the comprehensive drainage system.
What is the Correct Depth to Install an Underground Drainage Pipe?
The correct depth varies depending on the pipe's purpose and location. Domestic drainage pipes are typically installed at a depth of 600mm to 900mm to prevent damage from freezing and surface loads. Main sewers are often deeper, ranging from 1.2m to 1.5m. Surface water drainage pipes require a minimum depth of 450mm, though local regulations may vary.
What Drainage Pipe Sizes are Used in the UK?
Different underground drainage pipe sizes serve various purposes. A 110mm pipe is common for domestic waste pipes, while a 160mm pipe is used for larger domestic systems or small commercial properties. Pipes measuring 225mm and above are used in commercial, industrial and main sewer lines.
What Size Pipe is Right for Domestic Drainage Systems?
For most domestic systems, a 110mm diameter pipe is standard. This size efficiently handles the typical waste output from residential properties and conforms to UK building regulations.
Should an Underground Drainage Pipe be Flat or Sloped?
An underground drainage pipe should always be sloped. A gradient of 1 in 40 (2.5%) is standard for wastewater, ensuring gravity assists in efficient water flow and preventing stagnation and blockages.
Why are Drains Normally Installed Underground?
Installing drains underground hides them from view, maintaining the property's appearance, while protecting them from physical damage and weather conditions.
Underground installation also allows for natural gravity flow from all floors of a property except a basement, reducing the need for mechanical pumping.
What are the Signs of a Blocked Underground Drain?
Signs that you might have a blocked underground drain can include several key indicators:
- Water draining slowly from sinks, showers or bathtubs is a common symptom, as is hearing unusual gurgling noises from your drains.
- Persistent foul smells near drains or outside may also suggest a blockage.
- Water backing up or overflowing from gullies or inspection chambers is a clear sign of trouble.
- Unexplained damp or waterlogged areas in your garden or around your property can further indicate a blocked drain.
Can Tree Roots Damage Underground Drain Pipes?
Yes, tree roots can penetrate and damage underground drain pipes. They seek out moisture and can enter through small cracks or joints, causing blockages and structural damage over time. Regular maintenance and root barriers can help prevent this issue.
How Often Should Underground Drains be Inspected?
It's recommended to have underground drains inspected every 2 to 3 years to ensure they are functioning correctly and to catch any potential issues early. However, if you notice any signs of blockages or drainage problems, an immediate inspection is advised.
What are Soakaways and When are they Used?
Soakaways are underground structures designed to manage surface water runoff by allowing it to slowly soak into the ground. They are commonly used in areas with poor drainage to prevent flooding and standing water. Soakaways are essential for properties without access to a suitable surface water sewer.
What Regulations Govern Underground Drainage Systems in the UK?
Underground drainage systems in the UK are governed by several regulations and standards, including:
- Building Regulations (Part H): Covering drainage and waste disposal.
- Water Industry Act 1991: Relating to public sewers and water authorities.
- BS EN 752: The European standard for drainage systems outside buildings.
- Local Authority Requirements: Additional requirements may be imposed by local councils.
Can I Connect Rainwater and Foul Water Drains?
No, never. Rainwater (surface water) and foul water (wastewater from toilets, sinks, etc) should be kept separate. Mixing these can lead to environmental pollution and is against UK building regulations. Separate systems ensure efficient treatment and disposal of both types of water.
What is the Lifespan of Underground Drainage Pipes?
The lifespan of underground drainage pipes varies depending on the material used. PVC, HDPE and clay pipes typically last between 50 and 100 years, while concrete pipes have a slightly longer lifespan, ranging from 70 to 100 years. Regular maintenance can help extend the lifespan of these pipes, ensuring their continued effectiveness and durability.
What is the Role of a Gully in a Drainage System?
A gully is a component of a drainage system designed to collect and transport surface water. It typically has a grate on top to catch debris while allowing water to enter the drain. Gullies are commonly used around buildings, driveways and roads to prevent flooding.
How do I Prevent Blockages in Underground Drains?
Preventing blockages in underground drains involves a combination of regular maintenance and good practices.
To avoid issues, refrain from disposing of fats, oils or grease (FOG) down the drain, as these can cause significant build-ups. Installing drain guards in sinks and showers can help catch hair and debris, preventing clogs.
It is also important to use safe drain cleaning products periodically to keep drains clear. Regular inspection and maintenance of both drains and gutters are crucial for early detection of potential problems.
Managing tree roots by keeping trees and shrubs well away from drain pipes can prevent root intrusion and subsequent blockages.
What Happens if Underground Drains are not Properly Maintained?
Neglecting underground drain maintenance can lead to a range of serious issues. Blockages may cause water to back up into your home, leading to potential water damage. It can erode foundations and other structures, resulting in significant structural damage.
Stagnant water from poorly maintained drains can attract pests and harbour harmful bacteria, creating health hazards.
Failing to maintain underground drains also increases the risk of severe flooding during heavy rainfall, as the system may fail to effectively manage the water flow.
How to Choose Underground Drainage for Your Project
Choosing the right underground drainage system starts with understanding the needs of your project. Main considerations will include whether the system is handling foul water or surface water, the required pipe sizes and flow capacity of the system, ground conditions at the installation location and compliance with Building Regulations.
Material choice is also important, with PVC by far the most common choice thanks to its durability, workability and ease of installation. Access requirements, maintenance needs and integration with existing infrastructure should be planned from the start of the project. Accurate specifications from the outset and a reliable material take off service can help avoid delays, over-ordering and on-site issues, allowing your project to run more smoothly throughout. Drainfast’s experienced engineers are here to ensure your underground drainage needs are met.
Underground Drainage Systems With Drainfast
We hope this guide covered everything you needed to know about what underground drainage is. Drainfast supplies a large range of underground drainage products, backed by over 20 years of industry experience. With a large stockholding, fast and reliable delivery and expert technical support, Drainfast helps you specify exactly what you need for your installation.
Whether you need guidance on pipe selection or a reliable supply to your site, our team is on hand to support you throughout your project. Browse our underground drainage products or speak to us for practical, expert advice.
Written by
Bob Stone
Technical Sales
Heading up our Technical Estimating Department, Bob is our in-house quantity surveyor.
