Quality.

    Below-ground drainage: what good looks like

    Foul and surface water drainage, connections and testing.

    What good looks like

    Below-ground drainage: what good looks like

    Foul and surface water runs laid to the right falls and depths, bedded correctly, tested before backfill, and with access where it will be needed.

    Below-ground drainage is judged on falls, bedding, cover and access. Get those four right and the run will work for the life of the building; get one wrong and the defect is under a driveway. Gradients need to be shallow enough for solids to be carried and steep enough to be self-cleansing. Bedding and surround protect the pipe from point loads, and the depth of cover determines whether a protective slab or concrete surround is required. Access points must be positioned so every length can be rodded — at junctions, changes of direction and gradient, and at the head of each run. The stage is signed off on a test, not on appearance. Air or water test before backfill, then re-test after, because most damage happens during backfilling and compaction.

    Basis in regulation

    Before you start

    Before you start: below-ground drainage

    Checks to clear before the first trench, so the run is not dug twice.

    Confirm the invert level at the point of connection and work the falls back from it — not from the house. Check the adoption status of the sewer and whether a build-over or connection agreement is needed; that approval takes longer than the trench. Confirm soakaway sizing against a percolation test and the position against the minimum distance from any building or boundary. Check the trench support and the bedding material on site before pipes are laid, and mark up service crossings so drainage is not laid through a duct route.

    Basis in regulation

    Key requirements

    Access point location and dimensions

    Drainage access points must be located exactly as specified in the design and be readily accessible for rodding and cleaning. Inspection chambers and manholes must have sufficient internal dimensions relative to their invert depth, and covers should not obstruct boundaries or kerb lines. These dimen

    Drainage access points must be located exactly as specified in the design and be readily accessible for rodding and cleaning. Inspection chambers and manholes must have sufficient internal dimensions relative to their invert depth, and covers should not obstruct boundaries or kerb lines. These dimensions are critical for both access and safety.

    Basis in regulation

    Minimum dimensions for drainage access points

    Minimum dimensions for various types of drainage access points, including rodding points, access fittings, and inspection chambers, are specified relative to their depth to invert. These dimensions ensure adequate space for inspection, maintenance, and the passage of rodding equipment. Clear opening

    Minimum dimensions for various types of drainage access points, including rodding points, access fittings, and inspection chambers, are specified relative to their depth to invert. These dimensions ensure adequate space for inspection, maintenance, and the passage of rodding equipment. Clear openings must also meet specific size requirements.

    Basis in regulation

    Groundwater drainage pipework and discharge

    Perforated pipework used for groundwater drainage should have perforations (holes or slots) appropriate for the ground conditions to facilitate water collection. Discharge from groundwater drainage systems can be directed to a soakaway or watercourse, ideally via a catchpit, or through a catchpit in

    Perforated pipework used for groundwater drainage should have perforations (holes or slots) appropriate for the ground conditions to facilitate water collection. Discharge from groundwater drainage systems can be directed to a soakaway or watercourse, ideally via a catchpit, or through a catchpit into a foul, surface water, or combined drain.

    Basis in regulation

    Sewer connection agreements

    Connecting to a public sewer requires agreement from the local sewerage undertaker, who must be consulted on connection type and location. For private sewers, agreement from the sewer owner is essential during the design phase, and the sewerage undertaker should be notified if the private sewer subs

    Connecting to a public sewer requires agreement from the local sewerage undertaker, who must be consulted on connection type and location. For private sewers, agreement from the sewer owner is essential during the design phase, and the sewerage undertaker should be notified if the private sewer subsequently connects to a public one.

    Basis in regulation

    Septic Tank Outfall and Site Suitability

    Prior to installation, it is crucial to confirm that discharge consent can be obtained for the septic tank's outfall and that the ground conditions are suitable for the proposed system. A satisfactory percolation test result and all necessary consents and approvals must be secured before work begins

    Prior to installation, it is crucial to confirm that discharge consent can be obtained for the septic tank's outfall and that the ground conditions are suitable for the proposed system. A satisfactory percolation test result and all necessary consents and approvals must be secured before work begins, with site topography ensuring water drains away from the building.

    Basis in regulation

    Protection of pipework under finishes

    Drainage pipework laid under roads, drives, or other finished surfaces requires specific protection and appropriate compaction to prevent future settlement. Rigid pipes under 1.2m deep may need concrete encasement and movement joints, while flexible pipes under 0.9m may require concrete bridging sla

    Drainage pipework laid under roads, drives, or other finished surfaces requires specific protection and appropriate compaction to prevent future settlement. Rigid pipes under 1.2m deep may need concrete encasement and movement joints, while flexible pipes under 0.9m may require concrete bridging slabs or reinforced concrete surrounds.

    Basis in regulation

    Silt Traps and Soakaways for Treatment Plants

    When a cellular soakaway is used for the outfall from a single-unit treatment plant or septic tank, a silt trap chamber must be installed before the soakaway. This chamber is essential for minimising the discharge of suspended particles into the soakaway, thereby preventing clogging, and requires re

    When a cellular soakaway is used for the outfall from a single-unit treatment plant or septic tank, a silt trap chamber must be installed before the soakaway. This chamber is essential for minimising the discharge of suspended particles into the soakaway, thereby preventing clogging, and requires regular maintenance alongside the treatment plant.

    Basis in regulation

    Key requirements: below-ground drainage

    The drainage requirements most often picked up on inspection before backfill.

    Gradients within the design range for the pipe size and the number of appliances served. Pipes bedded and surrounded in the specified granular material, with no bricks, spoil or timber packing under the barrel. Depth of cover appropriate to the loading, with protection where cover is shallow. Access points at every junction, change of direction and change of gradient, and within the maximum rodding distance. Rocker pipes and flexible joints where the drain passes through a wall or off a structure. Air or water test carried out and recorded before backfill, and repeated afterwards. Trenches backfilled in compacted layers with no large stone against the pipe.

    Basis in regulation

    Septic Tank Dip Pipes and Soakaway Design

    Septic tanks require an inlet dip pipe to prevent disturbance of the surface scum layer and to ensure effluent is drawn from below it. Soakaway systems for septic tanks in porous subsoils must be designed to allow effluent to percolate into the ground above the winter water table, often constructed

    Septic tanks require an inlet dip pipe to prevent disturbance of the surface scum layer and to ensure effluent is drawn from below it. Soakaway systems for septic tanks in porous subsoils must be designed to allow effluent to percolate into the ground above the winter water table, often constructed as excavated and filled or lined pits.

    Basis in regulation

    Underdrainage System Construction

    Where ground conditions necessitate, underdrains are installed beneath field drains to manage high groundwater levels. These systems involve excavating deeper trenches, filling the lower section with permeable material like pea gravel, and laying a secondary pipe system at the bottom to convey exces

    Where ground conditions necessitate, underdrains are installed beneath field drains to manage high groundwater levels. These systems involve excavating deeper trenches, filling the lower section with permeable material like pea gravel, and laying a secondary pipe system at the bottom to convey excess water, often protected by a tar paper or similar barrier.

    Basis in regulation

    Drainage component material standards

    Drainage systems must be constructed using materials that comply with relevant British Standards to ensure long-term satisfactory performance. Acceptable components include vitrified clay pipes (BS 65), cast iron pipes (BS 437), and thermoplastics (BS 4660) among others, chosen for their durability

    Drainage systems must be constructed using materials that comply with relevant British Standards to ensure long-term satisfactory performance. Acceptable components include vitrified clay pipes (BS 65), cast iron pipes (BS 437), and thermoplastics (BS 4660) among others, chosen for their durability and suitability for below-ground use.

    Basis in regulation

    Information provision for drainage design

    Comprehensive drainage designs and specifications must be presented clearly and distributed to all pertinent personnel, including site supervisors and subcontractors. This documentation should detail drain layouts, invert levels, ground floor levels, junction locations, and results from percolation

    Comprehensive drainage designs and specifications must be presented clearly and distributed to all pertinent personnel, including site supervisors and subcontractors. This documentation should detail drain layouts, invert levels, ground floor levels, junction locations, and results from percolation tests for soakaways or field drains.

    Basis in regulation

    Drainage fall and connection positioning

    All drainage runs must be installed with a continuous fall away from the building to ensure efficient waste removal and prevent blockages. Additionally, the precise position of pipework and soil outlets for fittings like WCs should be confirmed on site using the actual sanitaryware or manufacturer's

    All drainage runs must be installed with a continuous fall away from the building to ensure efficient waste removal and prevent blockages. Additionally, the precise position of pipework and soil outlets for fittings like WCs should be confirmed on site using the actual sanitaryware or manufacturer's data to ensure correct connection clearances.

    Basis in regulation

    Rainwater Disposal for Garage Roofs

    Rainwater disposal systems for garage roofs must comply with Building Regulations. Roofs, or combinations of roofs, with a total area exceeding 6m² typically require a dedicated rainwater drainage system. Precautions must be taken to prevent erosion if water from a larger roof discharges onto a gara

    Rainwater disposal systems for garage roofs must comply with Building Regulations. Roofs, or combinations of roofs, with a total area exceeding 6m² typically require a dedicated rainwater drainage system. Precautions must be taken to prevent erosion if water from a larger roof discharges onto a garage roof, and rainwater should not discharge directly onto paths or drives.

    Basis in regulation

    Air Admittance Valves for Drainage Systems

    Air admittance valves (AAVs) are components used in drainage systems to allow air into the pipework, thereby preventing siphonage. They do not eliminate the need for primary ventilation of the entire drainage system. AAVs must comply with BS EN 12380 and be installed in locations that are protected

    Air admittance valves (AAVs) are components used in drainage systems to allow air into the pipework, thereby preventing siphonage. They do not eliminate the need for primary ventilation of the entire drainage system. AAVs must comply with BS EN 12380 and be installed in locations that are protected from freezing and allow free air movement.

    Basis in regulation

    Field Drain Construction Standards

    Field drains for foul water systems must be sited and constructed to ensure proper effluent dispersal while protecting the building and watercourses. They are laid with perforated pipes in trenches, bedded in specific aggregate, with sufficient undisturbed ground between trenches and away from struc

    Field drains for foul water systems must be sited and constructed to ensure proper effluent dispersal while protecting the building and watercourses. They are laid with perforated pipes in trenches, bedded in specific aggregate, with sufficient undisturbed ground between trenches and away from structures, and backfilled with a silt-preventing layer.

    Basis in regulation

    Critical dimensions at this stage

    Browse all critical dimensions
    England
    Low-rise traditional

    Original editorial guidance based on publicly available Approved Documents. Not building control approval, a warranty determination, or a substitute for the full source document.

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