Surface Water Drainage: Gullies, Soakaways, and Building Regulations Part H

Surface Water Drainage: Gullies, Soakaways, and Building Regulations Part H

Surface water — rainwater falling on roofs, driveways, patios, and gardens — must be managed. If it is not collected and directed away from the building, it causes flooding, damp, structural damage, and contributes to sewer overloading. Building Regulations Part H (Drainage and Waste Disposal) sets the requirements for surface water drainage in England and Wales, and the approach has changed significantly over the last two decades as sustainable urban drainage systems (SuDS) have become the preferred solution.

This guide covers the principles of surface water drainage, the types of gully and channel drain used at ground level, soakaway design, connection to sewers, and the regulatory position under Part H and planning requirements.

The Hierarchy of Surface Water Disposal

Approved Document H1 (2002, updated 2015) establishes a hierarchy for disposing of surface water from new developments:

1. Infiltration: Soak into the ground via a soakaway, filter trench, or permeable pavement. This is the preferred option where ground conditions allow.
2. Watercourse: Discharge to a ditch, stream, or other watercourse. Requires landowner consent and may require consent from the Environment Agency or Lead Local Flood Authority.
3. Sewer: Connection to a public sewer is the last resort — surface water to a combined sewer increases flooding risk and is often restricted by the water company.

In practice, for domestic extensions, conversions, and new builds, the choice is usually between a soakaway and a watercourse, with sewer connection permitted only where neither alternative is practicable. The water company may refuse permission to connect surface water to a combined sewer, and a drainage impact assessment may be required for any connection to a public sewer.

Gullies and Channel Drains

Road Gullies and Yard Gullies

A surface water gully collects water from a paved or surfaced area and directs it to underground drainage. Gullies are available in:

• Trapped (P-trap or bottle trap) gullies: Incorporate a water seal that prevents sewer gases (if connected to a combined system) from entering the atmosphere above. Required where the gully connects to a combined sewer. Not needed where draining to a surface water sewer or soakaway, but still commonly fitted as standard.
• Untrapped gullies: Simple access chamber — used where trapping is not required and regular rodding access is needed.
• Back-inlet gullies: Allow waste pipe connections at the back of the gully body above the water seal. Used where a drainpipe or waste pipe is brought into the gully — common for domestic downpipes and appliance wastes.

Materials: polypropylene and PVC gullies are standard for new domestic work. Ductile iron and clay gullies are used in commercial and highway applications. Grid covers are either plastic (domestic), galvanised steel, or cast iron (commercial/highway).

Standard domestic gully sizes: 110 mm (4") body, with 110 mm outlet for standard soil/surface water drainage. The body depth (from grid to invert) determines the available sediment capacity — deeper gullies collect more silt before blockage.

Channel Drains (Linear Drains)

Channel drains (also known as slot drains, ACO drains, or linear drains) collect water along their entire length rather than at a point. They are used:

• Across driveway entrances to prevent water flowing from the driveway to the pavement/highway (a legal requirement where a new or altered driveway drains toward the highway)
• Along the edges of patios and paved areas
• In shower wet rooms (linear shower drain)
• In commercial kitchens and industrial flooring

Channel drains connect to underground drainage at one or both ends of the channel run via 110 mm or 160 mm outlets. Load class (A15 for pedestrian, B125 for light vehicle, C250 for residential parking, D400 for highway) must be matched to the surface use.

Soakaways

A soakaway is an underground structure that receives surface water and allows it to percolate into the surrounding ground. It is the preferred disposal method where ground conditions are suitable.

Suitability Testing

Before designing a soakaway, the infiltration rate of the soil must be established. The standard method is the BRE Digest 365 percolation test:

1. Excavate a test pit approximately 300 mm × 300 mm to the proposed soakaway depth
2. Fill with water and allow to drain fully (pre-saturation)
3. Fill again and record the time for the water level to fall from 75% full to 25% full
4. Calculate the infiltration rate: f = A ÷ (t × P), where A is the plan area of the pit (m²), t is the time in seconds, and P is the wetted perimeter (m)
5. If f is greater than 1 × 10⁻⁶ m/s, infiltration is viable. Clay-dominated soils often fail this test; sandy or gravelly soils pass easily

Soakaways must not be sited within 5 m of a building or boundary, and should be at least 10 m from any watercourse. In areas prone to groundwater contamination sensitivity (source protection zones), Environment Agency consent may be required.

Soakaway Design

For small domestic soakaways (typically serving a roof area up to 100 m²), BRE Digest 365 provides a chart-based design method. The key variable is the contributing drained area multiplied by the design rainfall intensity for the location (UK rainfall data from the Flood Studies Report or more recent UKCP18 data).

Construction options:

• Rubble-fill soakaway: An excavated pit backfilled with clean angular stone or brick rubble, covered with geotextile membrane to prevent silt ingress. Simple and low cost, but limited inspection and maintenance access. Suitable for small domestic applications.
• Ring soakaway: Precast concrete soakaway rings (600 mm or 900 mm internal diameter) with perforations, seated on a granular base. The rings act as a temporary storage chamber while water percolates through the perforated walls. Inspection can be carried out through a surface access cover. Better for larger applications or where future maintenance is anticipated.
• Geocellular (plastic crate) soakaway: Modular HDPE crate units with high void ratio (up to 95% storage volume) arranged in a block, wrapped in geotextile. Much greater storage volume per cubic metre of excavation than rubble fill. Used where there is limited space or where attenuation of peak flow is required before infiltration. Brands include Wavin Aquacell, Graf Platin, Polypipe stormcell.

Soakaway Maintenance

Soakaways fail primarily through silt ingress blocking the voids or perforations. Preventive measures:

• Fit a silt trap or settling chamber upstream of the soakaway inlet
• Ensure all gullies feeding the soakaway have traps that are cleaned annually
• For geocellular and ring soakaways, inspect the inlet chamber annually and remove accumulated silt

Part H Regulatory Requirements

Approved Document H covers surface water drainage under H3. Key requirements for domestic work:

• Surface water must not be drained to a foul sewer without Local Authority and water company consent (and these are rarely given for new connections)
• Where a new driveway or hardstanding exceeds 5 m² in a front garden, planning permission may be required unless a permeable surface or drainage to a soakaway is provided (GPDO amendment)
• New roof drainage should connect to surface water sewer, watercourse, or soakaway — not the foul sewer
• Rainwater harvesting (collecting rainwater from roofs for garden irrigation or WC flushing) satisfies Part H where it is the primary disposal method for the contributing area

Rainwater Pipes and Guttering

For completeness: surface water begins at the roof. Standard domestic gutters are 112 mm half-round or square (OG) section, draining to 68 mm round or square downpipes. For larger roofs or high-rainfall areas, 150 mm gutters with 110 mm downpipes provide greater capacity. All downpipes should connect to a trapped gully at ground level (preventing rodents from accessing the underground drain via the open downpipe) or terminate via an offset to a back-inlet gully.

PVC guttering and downpipes are the standard for domestic new-build and replacement. Cast iron remains the material for conservation areas and heritage properties where the local authority requires it. Aluminium guttering is an intermediate option — lighter than cast iron, more durable than PVC, and acceptable to some conservation officers.

Summary

Surface water management is required by law (Part H) and is increasingly scrutinised by local authorities and water companies as urban flooding becomes a greater concern. The preferred solution — infiltration via soakaway — is viable in most gardens with appropriate ground conditions; a BRE Digest 365 percolation test establishes suitability before design. Where ground conditions are poor, connection to a dedicated surface water sewer or watercourse is the next option. Sewer connection, particularly to a combined sewer, requires water company consent and is increasingly restricted. Fitting gullies and channel drains at ground level, keeping them trapped and maintained, and routing the collected water to an appropriate disposal point is the complete solution for most domestic applications.

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