What is a Back Inlet Gully

A back inlet gully is a specific type of ground-level drainage fitting that functions similarly to a standard surface gully but includes a concealed connection point below ground level. Unlike conventional gullies, where connecting pipework typically discharges into an open grating or hopper at the surface, a back inlet gully allows wastewater or rainwater to be directed into it through a sealed inlet connection beneath the grating, ensuring a more enclosed and controlled flow path.

These fittings are widely used in both domestic and commercial drainage systems, particularly where aesthetic concerns, safety, or environmental conditions demand that waste connections be hidden or protected from external exposure. Back inlet gullies are also essential in layouts where surface run-off and below-ground pipework must be combined into a single fitting without compromising system integrity or allowing surface water to mix with foul discharges inappropriately.

Design and Construction

At first glance, a back inlet gully appears nearly identical to a traditional yard gully, with a cast iron or plastic grating flush with the surrounding ground or paving. However, the critical distinction lies beneath the surface: instead of wastewater entering the gully by falling directly onto the grating, the pipe connection is made through an inlet located on the rear or side of the gully trap body, below the grating and above the trap seal.

The internal structure typically consists of the following components:

  • Grating or cover: Located at ground level, usually removable for maintenance and access. This protects the gully from debris and surface ingress. 
  • Gully body: The main chamber, which may be circular or square in cross-section, constructed from uPVC, polypropylene, or clayware. 
  • Back (or side) inlet: A sealed pipe socket incorporated into the body of the gully to allow horizontal connection of waste or rainwater pipework. 
  • Trap: Integral water seal that prevents foul odours and gases from escaping the drainage system. 
  • Outlet: Typically vertical, leading into the underground drainage run connected to the main system. 

The design of a back inlet gully provides versatility, as it can accept waste from pipework such as kitchen or bathroom sinks, downpipes from roofs, or even combined rainwater and greywater sources—depending on the type of drainage system in place (foul, storm, or combined).

Applications in Modern Drainage Systems

Back inlet gullies are especially useful in situations where exposed discharge from pipework is either undesirable or impractical. Their enclosed configuration allows for cleaner aesthetics and reduced risk of blockage, contamination, or accidental damage. Common applications include:

  • Garden or patio drainage: Where downpipes from roofs discharge below ground to avoid surface run-off and puddling. 
  • Wastewater connections: For sinks or appliances in outbuildings, garages, or utility areas with concealed pipework. 
  • Driveways and pathways: Where surface water must be collected efficiently without visible pipe entry. 
  • Retrofitting: In older properties where modern waste appliances must be integrated into existing underground drainage systems with minimal disruption. 

A significant advantage of the back inlet design is that it reduces the likelihood of splashing, freezing, or blockage from external sources, as all discharges are introduced directly into the body of the gully. This makes it particularly beneficial in cold climates or high-footfall areas.

Integration with Underground Drainage

Back inlet gullies are typically connected to the underground drainage system using push-fit or solvent-weld connections (in plastic systems) or through traditional collar and spigot joints (in clay systems). The back inlet allows for horizontal pipework to approach the gully without entering from above, which is critical in situations where vertical clearance is limited or where the system must be kept flush with the ground level.

The gully outlet is usually directed downward into a foul or combined drain. However, it is crucial that the system be properly designed to ensure compliance with Building Regulations and local authority requirements. The following factors must be considered:

  1. Drainage type: Whether the gully connects to a foul-only system, storm-only system, or a combined system will dictate the type of waste it can legally and safely receive. 
  2. Trap depth: Most back inlet gullies are trapped to prevent sewer gases from entering the property. The trap depth (commonly 50mm or 75mm) must comply with relevant standards. 
  3. Access and rodding: Modern gullies are often designed with accessible points to allow for rodding or jetting in the event of blockages. 
  4. Maintenance access: The grating should be removable, and the internal components of the gully should be accessible for inspection and cleaning. 

Proper bedding and alignment of the gully unit are essential during installation to prevent settlement, misalignment, or cracking under load, especially in areas subject to vehicle traffic or heavy usage.

Benefits of Back Inlet Gullies

The use of a back inlet gully provides several functional and practical advantages, both during initial construction and over the long term. These include:

  • Improved aesthetics: Pipework is concealed, which is visually cleaner, particularly in landscaped or paved areas. 
  • Reduced exposure: Waste discharges do not spill onto surface gratings, reducing splash, smell, and hygiene issues. 
  • Enhanced safety: Lower risk of trip hazards or surface contamination in pedestrian or vehicular zones. 
  • Simplified layout: Allows for horizontal connections to enter directly into the drainage system without requiring fall onto an open gully pot. 
  • Ease of maintenance: Many models include features for easy access to the gully trap and outlet for inspection and cleaning. 

For drainage engineers, these benefits make back inlet gullies a popular choice when designing compact or integrated systems in constrained locations.

Regulatory Considerations

In the United Kingdom, the installation of back inlet gullies must comply with several regulatory frameworks, most notably:

  • Building Regulations Part H: Governs drainage and waste disposal systems in new and existing buildings. It includes specifications for access, trap seals, and discharge arrangements. 
  • BS EN 752: Specifies standards for drainage systems outside buildings, including gully design, trap requirements, and access for maintenance. 
  • BS EN 1253: Covers gullies for buildings, particularly for internal or surface use in conjunction with waste appliances and systems. 

Local water authorities may also impose their own requirements regarding the connection of gullies to foul or storm systems, particularly in combined drainage areas. Where uncertainty exists, consultation with building control or a qualified drainage engineer is recommended.

It is essential that back inlet gullies are installed according to manufacturer guidance, using appropriate materials, bedding techniques, and jointing methods. Improper installation can lead to blockages, water backing up into the property, or structural failure of the unit under load.

Maintenance and Operational Guidance

Although back inlet gullies are relatively low-maintenance, routine inspection is recommended to ensure long-term functionality. Over time, leaves, silt, and debris can accumulate in the gully body or trap, leading to flow restriction or odour issues.

Regular maintenance may involve:

  • Visual inspection: Checking for damage to the grating, surface debris, or signs of water pooling. 
  • Trap cleaning: Removing accumulated solids from the trap to maintain water seal and prevent blockages. 
  • Pipe flushing: Rodding or jetting the connecting pipework to ensure free flow. 
  • Seal checks: Inspecting joints and fittings for leaks or deterioration, especially in plastic systems. 

In areas with high leaf fall or sediment load, seasonal checks may be necessary, especially in autumn or after heavy rainfall.

Conclusion

Back inlet gullies provide an elegant and efficient solution for integrating below-ground pipework into ground-level drainage infrastructure without exposing waste discharges to the open environment. Their discreet design, combined with reliable hydraulic performance and ease of maintenance, makes them a preferred choice in residential, commercial, and public realm developments.

Whether installed in patios, driveways, or garden settings, these gullies support hygienic and compliant waste management while enhancing the aesthetic quality and safety of external environments. For plumbing and drainage professionals, understanding the correct specification, installation, and upkeep of back inlet gullies is vital for delivering robust and regulation-compliant drainage systems that meet the demands of modern construction and environmental stewardship.