What is a Drain

A drain is a pipe, generally installed underground, that serves the specific function of conveying wastewater and/or surface water from a property, premises, or surface to a sewer or an alternative discharge point such as a treatment system, soakaway, or watercourse. Drains form the essential first stage of any foul or surface water drainage system, operating primarily within the boundary of a single property.

Drains differ from sewers in both their scope and ownership. While a drain serves one building or premises and is typically the responsibility of the property owner, a sewer serves multiple buildings or properties and is usually maintained by the local water and sewerage company (a statutory undertaker). Understanding the distinction between the two is fundamental for correct installation, regulatory compliance, maintenance responsibilities, and infrastructure planning.

Function and Purpose

The fundamental purpose of a drain is to safely transport liquid waste away from a source to a designated disposal point without causing environmental contamination, structural damage, or public health hazards. The design of the drain must ensure:

  • Efficient removal of waste liquids from within a building or off impermeable surfaces 
  • Prevention of blockages, backflow, and leaks that could damage the structure or pollute surrounding soil or water 
  • Integration into the wider drainage network, ultimately connecting to a sewer, septic system, or drainage field 

Depending on the nature of the waste, drains are usually classified into two broad categories:

  • Foul drains: Carry wastewater from toilets, kitchens, bathrooms, and other plumbing fixtures 
  • Surface water drains: Carry rainwater runoff from roofs, drives, and other exterior surfaces 

In a separate drainage system, these are distinct pipe networks. In older or combined systems, a single pipe may carry both types of water, posing greater challenges for treatment and pollution control.

Components of a Drainage System

A drain typically includes several components that ensure the effective collection and movement of water. These include:

  • Pipework: Underground pipes made of clay, PVC, cast iron, or concrete, depending on age, location, and required capacity 
  • Gullies and traps: Collect water from downpipes, sinks, or appliances and prevent foul gases from escaping 
  • Inspection chambers (manholes): Provide access for maintenance, rodding, or CCTV survey 
  • Rodding eyes and access fittings: Allow for internal or external cleaning of pipework 
  • Vent pipes: Prevent the build-up of gases and pressure in enclosed drainage systems 
  • Connections and junctions: Interface between the property’s internal plumbing and the external drain 

In most new developments, drains are designed according to current Building Regulations and British Standards (notably BS EN 752 and Part H of the Building Regulations), ensuring consistent quality and performance across the industry.

Legal and Regulatory Framework

In the United Kingdom, the installation and management of drains are governed by various legal and technical standards. These are intended to safeguard the environment and public health and to ensure effective and equitable management of drainage systems.

Key legislation and regulations include:

  • Building Regulations 2010 (Part H): Covers the design and installation of drainage systems within and around buildings 
  • Water Industry Act 1991: Governs the adoption of private drains and sewers by statutory undertakers 
  • Environmental Protection Act 1990: Addresses pollution control and responsibilities for discharges 
  • Public Health Act 1936 (legacy legislation): Influences drainage duties and permissions 
  • Sewerage Sector Guidance (SSG): Provides adoptable standards for drainage assets to be transferred to sewerage undertakers 

According to current law, drains serving a single property remain the responsibility of the owner up to the point where they connect with a public sewer or shared lateral drain. Once outside the curtilage of the property and shared by more than one household, the pipework is considered a lateral drain and is typically maintained by the local water company.

Types of Drains

Drains can be categorised based on their function, construction, and location. Understanding the different types helps ensure correct design and installation in accordance with site-specific needs.

Based on function:

  1. Foul drains: 
    • Carry wastewater from toilets, sinks, showers, washing machines, and dishwashers 
    • Must be sealed and vented to prevent gas build-up and odour 
    • Require connection to a foul sewer, treatment plant, or septic system 
  2. Surface water drains: 
    • Convey rainwater runoff from roofs, yards, patios, and other impermeable surfaces 
    • May discharge into watercourses, soakaways, or surface water sewers 
    • Cannot legally receive foul wastewater 
  3. Combined drains: 
    • Found in older properties, particularly pre-1970s 
    • Carry both surface water and foul water in the same pipe 
    • May require separation when upgrading or redeveloping a site 

Based on construction:

  • Gravity drains: Use natural fall and pipe gradients to move wastewater without mechanical assistance 
  • Pumped drains: Use lifting stations or ejectors to move wastewater when gravity flow is not possible 
  • Lined or sleeved drains: Incorporate additional internal lining (e.g. CIPP lining) for leak prevention or repair 
  • Channel drains: Surface-mounted linear systems with grates, used to intercept water at ground level (e.g. ACO drains) 

All types of drains must be sized and sloped correctly to ensure self-cleansing velocity, avoiding stagnation and blockages.

Design Considerations

Proper drain design requires an understanding of site conditions, building layout, and regulatory requirements. Critical considerations include:

  • Gradient: The fall of the pipe should support self-cleansing; typically 1:40 for foul drains, and 1:100 for surface water 
  • Diameter: Standard domestic foul drains are usually 100 mm in diameter; larger pipes are used for higher flows 
  • Access: Adequate inspection points must be included to enable cleaning and maintenance 
  • Ventilation: Necessary to prevent gas accumulation and pressure fluctuations 
  • Protection: Drains must be protected from root ingress, frost, heavy loads, and accidental damage 
  • Backflow prevention: Where risk exists, non-return valves or anti-flood devices should be installed 

For new-build properties, a drainage layout plan is required at the design and handover stages, detailing routes, depths, and connection points.

Inspection, Maintenance, and Repair

Drains are often hidden underground, making routine inspection and maintenance essential for identifying and resolving issues before they escalate. The most common problems include blockages, structural failure, root ingress, and infiltration.

Common inspection and maintenance tasks:

  • CCTV surveys: Allow visual assessment of internal pipe condition 
  • Rodding and jetting: Used to clear obstructions or debris 
  • Smoke or dye testing: Helps locate leaks or misconnections 
  • Root cutting: Removes invasive roots without excavation 
  • Patch lining or relining: Repairs damaged sections without full replacement 

Responsibility for drain maintenance lies with the property owner unless the pipe serves multiple properties or lies outside the property boundary, in which case it may fall under the remit of the local water company.

Misconnections and Environmental Risks

One of the most common problems associated with drains is the misconnection of appliances—particularly washing machines, sinks, and dishwashers—to surface water drains. This leads to untreated sewage entering rivers and watercourses, causing pollution and legal violations.

The Environment Agency and water companies frequently run awareness campaigns to combat misconnections, and surveys of older properties often uncover illegal or accidental installations.

Property owners and contractors must ensure:

  • Foul drains only connect to foul or combined sewers 
  • Surface water drains do not receive wastewater 
  • Discharges to watercourses comply with environmental permits or binding rules 

Conclusion

A drain is far more than just a buried pipe—it is a fundamental part of the infrastructure that enables safe, hygienic, and sustainable modern living. Whether carrying wastewater from a toilet, runoff from a roof, or effluent from a treatment plant, drains play a pivotal role in the protection of public health and the environment.

Properly designed, installed, and maintained, drains offer decades of reliable service. But neglect, misconnections, or poor construction can lead to costly failures, environmental damage, and regulatory breaches. For engineers, planners, contractors, and property owners, a thorough understanding of how drains function—and how they fit into the broader sewerage network—is essential.

With climate change and urbanisation placing increasing pressure on drainage systems, the role of the humble drain is more important than ever in building resilient, adaptable, and efficient infrastructure.