Underground drainage must be correctly installed to manage wastewater, prevent leaks and allow maintenance access. In this guide, we will cover the regulations, British Standards, pipework requirements, installation methods and inspection provisions relevant to underground drainage projects.
What are the Regulations for Underground Drainage in the UK?
Underground drainage is governed by building regulations, supporting guidance, British Standards and, where applicable, planning and environmental requirements. The precise rules depend on the location, building type and proposed drainage system.
Building Regulations Part H
Part H of Schedule 1 to the Building Regulations 2010 covers drainage and waste disposal in England. Its six requirements address:
- Foul water drainage
- Wastewater treatment systems and cesspools
- Rainwater drainage
- Building over sewers
- Separate drainage systems
- Solid waste storage
For underground drainage, Part H requires an adequate foul water system that carries wastewater to an appropriate outfall while minimising the risks of leaks, blockages, flooding and foul air entering the building.
Approved Document H Explained
Approved Document H provides practical guidance on meeting Part H in common building situations. It covers pipe sizes, gradients, materials, bedding, access points, testing, foul drainage and surface water disposal.
Following the document is not the only way to achieve compliance. Alternative designs may be accepted if they satisfy the underlying requirements of the Building Regulations. Any departure should be agreed with the relevant building control body and supported by suitable calculations or technical evidence.
When do Building Regulations Apply in Underground Drainage?
Building Regulations generally apply when constructing or extending a building, installing or extending controlled drainage services or making alterations that affect an existing system’s compliance. This can include installing new foul or rainwater drainage, rerouting drains as part of an extension or changing connections to a sewer.
Minor repairs may not require a separate building control application, but the completed work must still meet applicable standards. Contractors should confirm the position with the building control body before starting where the scope is unclear.
British Standards for Underground Drainage
British Standards provide detailed specifications and performance requirements for drainage products and systems. The relevant standard will depend on the pipe material, application and wider system design.
BS EN 1401
BS EN 1401-1:2019+A1:2023 specifies requirements for solid-wall PVC-U pipes, fittings and joints used in non-pressure underground drainage and sewerage.
It covers dimensions, stiffness, joint performance, marking, test methods and application areas. Products marked with application code U are intended for burial outside the building structure. Code UD covers products buried both within and outside the structure.
BS EN 752
BS EN 752:2017 establishes objectives and functional requirements for drain and sewer systems outside buildings. It applies from the point where wastewater leaves a building, roof drainage system or paved area to its eventual treatment or discharge point.
The standard covers planning, design, installation, operation, maintenance and rehabilitation. It is particularly relevant to larger or more complex drainage networks where a broader system-level approach is required.
Underground Drainage Requirements
Every underground drainage system should be designed around its expected flow, proposed use, ground conditions and connection point. These requirements are implemented to protect the surrounding environment.
Pipe Sizing
The required pipe diameter depends on the flow rate, gradient and type of wastewater being conveyed. These factors determine the capacity needed to carry waste efficiently without blockages or overloading.
A foul drain should have an internal diameter of at least 75mm. A drain carrying discharge from a WC or trade effluent should have an internal diameter of at least 100mm.
In standard UK product ranges, 110mm underground drainage pipes are commonly used for foul drainage serving individual buildings. 160mm underground drainage pipes provide greater capacity and may suit larger flows or sewer applications. Read the Drainfast underground drain pipe size guide for a more detailed comparison.
Approved Materials
Several materials are suitable for below-ground gravity drainage, subject to the correct product standard and application. These include:
- PVC-U
- Polypropylene
- Structured-wall thermoplastics
- Vitrified clay
- Concrete
- Grey iron
- Ductile iron
The material must be appropriate for the ground conditions, loading, depth of cover and substance being conveyed.
Colour Requirements
Colour is commonly used to help distinguish systems, but markings, product certification and application codes provide more reliable confirmation of suitability.
Brown or orange-brown PVC-U is widely used for non-pressure underground foul and surface water drainage. Other systems, including structured-wall surface water pipes, may be black. Contractors should follow the project specification and check that each product is certified for its intended application rather than selecting it by colour alone.
Underground Installation Requirements
Correct installation protects the pipework from movement, deformation and leakage. For further practical guidance, read Drainfast’s guide to installing underground drainage.
Minimum Pipe Falls
Pipes should be laid to an even gradient, with changes in gradient positioned at suitable access points. Approved Document H recommends the following minimum gradients for foul drains:
- 75mm pipes carrying a peak flow below one litre per second: 1:40
- 100mm pipes carrying a peak flow below one litre per second: 1:40
- 75mm pipes carrying more than one litre per second: 1:80
- 100mm pipes carrying more than one litre per second and at least one WC: 1:80
- 150mm pipes carrying more than one litre per second and at least five WCs: 1:150
These are recommended minimum gradients, not substitutes for hydraulic calculations. Larger and more complex systems should be designed around their anticipated flows and applicable standards.
Trench Width
The width must provide enough working space to prepare the bed, position and joint the pipe and compact sidefill without damaging the system.
Pipe diameter, trench depth, ground conditions and the proposed support system will all affect the required width. Manufacturer guidance and the project design should be followed. Excavations must also be planned and supported in accordance with HSE excavation safety guidance.
Bedding and Backfill
Bedding and backfill should be selected based on the pipe’s material, diameter, strength and depth of cover. Flexible plastic pipes depend on correctly placed bedding and sidefill for structural support, while rigid pipes may require a different bedding arrangement.
The bed should support the pipe evenly and leave suitable recesses beneath sockets. Granular sidefill should be placed and compacted carefully around the pipe before backfilling continues. Where cover is shallower than recommended, additional protection such as a reinforced concrete cover slab may be required.
Pipe Jointing
Joints must be appropriate for the pipe material and remain watertight under working and testing conditions. Flexible joints are generally used to accommodate minor movement and reduce the effects of differential settlement.
Before assembly, installers should check that sockets, spigots and sealing rings are clean and undamaged. Pipes should be correctly aligned and joined using the manufacturer’s approved lubricant and installation method. Nothing within a fitting or joint should project into the flow path and create an obstruction.
Inspection Chamber and Access Requirements
Drainage systems need sufficient access so that blockages can be located and cleared. The layout, depth and pipe diameter determine the type and spacing of access points required.
When are Inspection Chambers Required?
Access should be provided at or near the head of a drain run, at changes in direction or gradient and where the pipe size changes. It is also normally needed at junctions unless every connected run can be cleared from another suitable access point.
The correct combination of rodding eyes, access fittings, inspection chamber bases and risers and manholes helps keep the system maintainable throughout its service life.
Chamber Spacing
For drains up to 300mm, the guidance permits up to 45m between shallow inspection chambers. Up to 90m may be allowed between deep inspection chambers or manholes, rising to 200m for some man-entry drains and sewers. Shorter limits apply where access fittings or rodding eyes are used.
Chamber Depth
For drains up to 150mm in diameter and 1.2m deep, a shallow inspection chamber must measure at least 450mm by 450mm, or 450mm in diameter.
Where an inspection chamber is deeper than 1.2m, access should be restricted to deter entry, with the guidance specifying a maximum opening of 350mm.
Deeper systems may require a properly sized manhole rather than a standard inspection chamber. Product-specific maximum depths must also be observed.
Manhole Covers and Load Classes
Manhole covers and frames should be selected for the loads expected at their installation location. BS EN 124 divides covers and gratings into six principal classes:
- A15 for pedestrian and cyclist-only areas
- B125 for footways, pedestrian areas and comparable car parks
- C250 for kerbside channels
- D400 for carriageways and areas used by cars and lorries
- E600 for locations with heavy wheel loads, including loading areas and docks
- F900 for exceptionally heavy loads, including aircraft pavements
Learn more in the Drainfast guide to manhole cover shapes, sizes and load classes.
Surface Water vs Foul Water Regulations
Foul water carries wastewater from toilets, washing facilities and food preparation. Surface water is predominantly rainwater collected from roofs, paved areas and other external surfaces. Drainfast supplies a wide range of surface water drainage products for commercial projects.
Separate Drainage Systems
Where the public sewer network has separate foul and surface water sewers, the building’s drainage must also keep these flows separate. This prevents rainwater from unnecessarily entering and overloading the foul sewer.
Surface water should not be discharged into a foul-only system. A connection to a combined sewer may be possible where other discharge options are not practicable, but the sewerage undertaker must confirm that the system can accept the additional flow.
Soakaway Requirements
Surface water should discharge to a soakaway or another infiltration system where practicable and ground conditions allow. A suitable infiltration assessment or percolation test is required to establish whether the ground can accept the anticipated flow.
Soakaway crates temporarily store surface water runoff, allowing it to gradually infiltrate through a permeable membrane into the surrounding soil. Crate strength, storage volume, ground conditions and vehicle loading must be considered during specification.
Sustainable Drainage (SuDS)
SuDS manage rainwater closer to where it falls by controlling runoff rates, reducing flood risk and, where possible, improving water quality and amenity. Examples include rainwater harvesting, permeable paving, swales, infiltration systems and attenuation storage.
The 2025 National Standards for SuDS are intended for new infrastructure and development in England. They prioritise non-potable reuse, followed by infiltration, discharge to an above-ground water body, a surface water sewer and finally a combined sewer.
Appropriate planning applications should demonstrate how the standards have been addressed. Local planning policies, Lead Local Flood Authority requirements and site-specific approvals must also be considered.
Building Control Inspections
Building control inspections help confirm that drainage work matches the approved design and complies with applicable requirements. Inspection arrangements should be agreed with the building control body before work starts, as the required stages will depend on the project and approval route.
Inspectors may check pipe sizes, gradients, bedding, connections, access points and separation between foul and surface water systems. Drain runs should remain available for inspection where requested rather than being concealed prematurely.
Following installation, gravity drains and private sewers should be tested for watertightness using an appropriate air or water test. Any defects should be corrected before the system is put into service or the work is signed off.
Common Underground Drainage Compliance Mistakes
Common design and installation errors include:
- Using above-ground pipework underground: Products intended only for internal soil or waste systems may not have the strength or application classification required for burial
- Selecting the wrong fittings: Tight bends, unsuitable adaptors and incorrectly positioned junctions can restrict flow or make the system difficult to clear
- Insufficient falls: A gradient that is too shallow can allow solids to settle, while excessive falls may cause liquids to move ahead of solid waste
- Poor bedding and sidefill: Uneven support, unsuitable aggregate or inadequate compaction can cause movement, deformation and joint failure
- Incorrect joint assembly: Dirty seals, damaged sockets, poor alignment or unsuitable lubricant can compromise watertightness
Contact the Experts at Drainfast
Drainfast supplies underground drainage pipes, fittings, inspection chambers, manhole covers and surface water drainage products for commercial construction and groundworks projects.
With over 20 years of industry experience, our technical team can help you identify suitable products for the project specification.
Shop underground drainage supplies or contact Drainfast for practical product and specification support.
