Author: Paul Smith

Plastic Drainage Crates and Soakaways in the UK, How to Choose and Install a SuDS System

Plastic drainage crates can provide a compact way to manage roof and paved area runoff, provided the system matches the site and has been designed for the water it will receive. The key decision is whether water should infiltrate into the ground, be held and released at a controlled rate, or be stored for later use. That choice depends on soil, groundwater, catchment size, discharge permissions and long term access for maintenance.

In England, Defra’s 2025 National SuDS Standards set out expectations for the design, construction, operation and maintenance of surface water systems. The 2026 National Planning Policy Framework gives flood risk and coastal change a dedicated policy chapter and strengthens the planning context for managing runoff. Requirements and approvals still vary by project and location, so treat national policy as a baseline and check with your local planning authority and Lead Local Flood Authority.

What plastic drainage crates do

A geocellular crate is a rigid, lightweight module with a high proportion of internal void space. Connected modular soakaway crate systems form an underground chamber that temporarily holds rainwater. The crates themselves do not decide where that water goes. Their wrapping and pipework determine whether the installation infiltrates, attenuates, or stores water.

A soakaway allows collected rainwater to disperse into suitable surrounding soil. An infiltration system is the broader term for drainage that returns water to the ground. Attenuation storage systems temporarily hold runoff and release it through a flow control to an approved destination, such as a surface water sewer or watercourse. Storage retains water for use, often in a rainwater harvesting arrangement, and may need separate overflow management.

Choose the function before choosing a product. For a permeable garden site, a light duty infiltration crate kit may work. Where the ground drains slowly or infiltration is unsuitable, a lined attenuation tank system with controlled outflow may be considered. Reuse storage can reduce demand for mains water, though it needs a suitable use, pump or gravity supply and planned maintenance.

Check the site before choosing a system

A crate system is only as reliable as the information behind its design. Start with the area draining to it, including roofs, patios, driveways and any other connected surfaces. Consider the surface finish and how rainfall can reach the inlet. Then establish likely runoff volumes and the permitted discharge route. A supplier’s crate volume or a quick online calculator cannot replace a project specific drainage design where planning approval or a safe outfall is involved.

Test infiltration and groundwater conditions

Soil that absorbs water slowly may need a large infiltration structure or may rule out infiltration altogether. Clay can drain poorly, while made ground, variable strata or fractured rock can create uncertain results and pollution risks. Arrange on-site infiltration testing at the proposed location and depth. BRE Digest 365 is a widely used UK method for soakaway assessment, and the test should reflect the actual ground conditions rather than assumptions based on a nearby property.

Check seasonal groundwater, not only the level seen on one dry day. The 2025 National SuDS Standards call for at least one metre of unsaturated ground between the base of an infiltration feature and the maximum likely groundwater level. This separation helps the soil treat runoff and reduces the risk of groundwater entering the system. Local guidance can require extra investigation or more cautious design in sensitive groundwater areas.

Protect structures and locate services

Plan the position in relation to building foundations, retaining walls, roads, boundaries and buried utilities. Approved Document H gives important siting guidance for infiltration features, including separation from buildings and roads. A commonly applied minimum is five metres from a building or road, but conditions and local guidance can differ, and some ground types call for greater distances. Confirm the relevant requirement for the specific design rather than treating a rule of thumb as universal.

Obtain utility plans and arrange a safe site survey before excavation. Water, gas, electricity, telecoms and existing drainage may cross the proposed footprint, and marking any exposed services with underground warning tape during the works helps protect them from future disturbance. Keep access for excavators and future inspection in mind too. A technically suitable spot can become impractical when it conflicts with services, mature trees, foundations or safe working space.

Allow for the whole site

Check the topography, flood risk, available outfalls and space for construction. Ask where overflow will go during exceptional storms, since no crate system can store water indefinitely. A controlled discharge needs an approved destination and may need consent from the sewerage undertaker, watercourse owner or relevant authority. Sites with contamination or a risk of polluted runoff require careful consideration before allowing water to infiltrate.

Design details that make the system work

Geotextile and membrane choices

For an infiltration soakaway, a suitable permeable geotextile membrane normally wraps the crate structure and helps keep surrounding soil from migrating into its voids. Select the fabric to suit the ground and the manufacturer’s installation specification. Poor overlaps, tears or an unsuitable grade can allow fine particles inside, where they may reduce storage and obstruct flow.

An attenuation tank needs to hold water rather than release it through its sides. It is therefore usually enclosed in an impermeable geomembrane liner, protected by a non-woven protection fleece to reduce puncture risk. The membrane seams and penetrations need to be sealed with a waterproof membrane tape to the system design. Do not use permeable wrapping for a tank intended to attenuate water, or assume an impermeable liner is suitable for infiltration.

Pipe connections, access and pretreatment

Connect inlets and outlets at the specified levels, using compatible underground pipe fittings and a stable pipe bedding. The inlet should direct flows into the intended part of the structure. Attenuation systems need a calibrated flow control chamber at the outlet, and every system needs a safe overflow route. Removable silt trap buckets or leaf guards can capture debris before it reaches the crates. This matters because compact underground units are difficult to clean once sediment has entered them.

Include inspection chamber access points at locations that allow the inlet, outlet and upstream silt controls to be checked. A buried crate field with no practical access may function at first and prove costly to maintain later. Keep access covers visible and reachable after landscaping and paving are complete.

Depth, loading and construction

Installation depth affects groundwater separation, cover requirements, access and the load the system can support. Crates beneath gardens may face different loads from those beneath a driveway or vehicle route. Confirm the chosen product’s structural rating, required cover, base preparation and permitted installation configuration. The design should account for soil cover, traffic loading, groundwater pressure and any constraints from nearby structures.

Excavate to the designed dimensions, prepare a level and stable base, and check levels before placing modules. Assemble and connect crates as the manufacturer specifies, then install the specified geotextile or membrane and its protective layers. Backfill with suitable material and compact in controlled stages without damaging the tank or liner. Keep machinery and sharp objects away from exposed membranes. As built locations, levels, connections and product details should be recorded for future owners and maintenance teams.

Planning, standards and long term care

Defra published the 2025 National SuDS Standards for England as non statutory standards covering surface water drainage design, construction, operation and maintenance. They set a common framework for matters such as runoff destinations, everyday rainfall, water quality, flood risk and system upkeep. The standards are relevant to designers and decision makers, though the route by which a proposal is assessed depends on the development and local planning process.

The 2026 NPPF places flood risk and coastal change within a dedicated policy chapter and reinforces the expectation that development manages surface water responsibly. Planning decisions also take account of flood risk assessments, local plan policy, LLFA advice and local SuDS guidance. A domestic installation can still need building control input or consent for its outfall, while a larger development may require a detailed drainage strategy, calculations, maintenance arrangements and evidence of site testing. Confirm the current requirements with the planning authority, LLFA and relevant sewerage or watercourse authority before committing to a design.

SuDS design should consider more than underground capacity. Where space allows, source control such as rain gardens, permeable paving, green roofs or shallow planted features can slow and treat water before it reaches a crate system. A crate tank can form one component in a broader drainage train, while relying on a single buried structure can miss opportunities for water quality treatment and visible amenity.

Maintenance begins before installation. Keep a record of the system layout, specifications, inspection locations and responsible owner. Inspect chambers, silt traps, pipework and flow controls at planned intervals and after significant storms. Remove leaves and sediment from pretreatment devices before they can wash into the crates. Investigate standing water, slow drainage, damaged covers or unusual overflow promptly. Maintenance frequency depends on site use and observed debris, so the designer should provide a practical schedule rather than a vague instruction to inspect occasionally.

Frequently Asked Questions

Are plastic drainage crates suitable for every garden?

No. They need a suitable design and an appropriate destination for the water. Infiltration depends on tested ground conditions and groundwater separation, while attenuation requires a permitted outfall and flow control. Space, structures, services and local requirements also affect suitability.

Can soakaway crates be used in clay soil?

Clay often has low infiltration capacity, which can make a soakaway ineffective or impractically large. A site infiltration test is needed before deciding. Where infiltration is unsuitable, a designer can assess an alternative such as controlled attenuation, subject to an acceptable discharge route.

Does a crate soakaway need a liner?

An infiltration system generally uses permeable geotextile around the crate structure so water can pass into the soil while limiting soil entry. An attenuation tank typically uses an impermeable membrane to retain water. The correct arrangement depends on the intended function and product specification.

How often should a crate system be maintained?

Inspect access chambers, inlet and outlet pipework, silt traps and flow controls at the intervals set by the design and after major storms. Clear accumulated debris before it enters the crates, and keep inspection records. A site with trees, heavy sediment or vehicle areas may need more frequent attention.

Do the 2025 SuDS Standards apply to a small home project?

The standards inform surface water design in England, while the approvals and evidence required depend on the scale and context of the work. Check with your local planning authority, building control and LLFA where relevant, and confirm any outfall permissions before installation.

Choose the drainage route before you buy

The right crate system follows from evidence about soil, groundwater, runoff, structures, services and the lawful destination of water. Match the wrapping and pipework to the chosen function, make inspection possible, and plan for maintenance from the outset. Before ordering crates or digging, ask a qualified drainage designer to assess the site and confirm local requirements. A little project specific advice now can prevent a costly drainage failure later.

Choosing an underground drainage system involves more than selecting a pipe that fits the trench. The pipe material, stiffness, fittings, fall, bedding and destination of the water all affect whether the installation will work reliably and meet the relevant requirements. This guide explains the practical choices for UK projects, from a domestic foul drain to a surface water run beneath a driveway.

Building regulations and sewer ownership can vary by nation and by site. Approved Document H applies to England. Wales, Scotland and Northern Ireland have their own regulatory arrangements, so confirm the rules with the relevant building standards authority and water or sewerage company before work begins.

Choosing a pipe system

PVC-U solid wall pipe

PVC-U, or unplasticised polyvinyl chloride, is the familiar orange or brown solid wall plastic drainage pipe used for many domestic underground gravity systems. It is light to handle, resistant to corrosion and available with compatible bends, junctions and access fittings for foul drainage and surface water runs where the product is specified for the intended application and installed to its instructions.

For solid wall PVC-U systems, check whether the product conforms to BS EN 1401-1, which sets requirements for pipes, fittings and joints for non-pressure underground drainage and sewerage. The standard covers buried systems outside building structures. Verify the current edition and any amendments through the manufacturer or standards catalogue, since standard references can be revised.

Structured wall and twinwall pipe

Structured wall pipe has a profiled exterior and a smooth bore. Twinwall drainage pipe is a common type, often supplied in larger diameters for surface water collection, attenuation, culverts or other designed drainage uses. Its corrugated outer wall can provide stiffness with less material than a solid wall of similar diameter, while the inner surface supports flow.

Many structured wall systems are specified to the BS EN 13476 series, which covers non-pressure underground drainage and sewerage systems made from PVC-U, polypropylene or polyethylene. Check the specific part and pipe type stated by the manufacturer. A structured wall pipe is not automatically interchangeable with solid wall drainage. Confirm that fittings, seals, stiffness class, diameter and intended application all match as a system.

Separate the water by purpose

Foul water carries wastewater from toilets, sinks, baths, showers and appliances. Surface water comes from roofs, yards and paved areas. The systems may use similar plastic materials, yet they have different design destinations. Foul water normally goes to a foul sewer or an approved private treatment system such as a septic tank or sewage treatment plant. Surface water should be managed through an appropriate surface water sewer, soakaway crate system or other lawful arrangement, depending on local conditions and approvals.

Keep foul and surface water flows separate unless the relevant sewerage company or approved design specifically allows a combined system. A wrong connection can cause pollution, surcharge or flooding. Before choosing fittings, establish where each run discharges and obtain any necessary permission for a connection.

Read the specification before buying

A familiar colour or nominal diameter does not prove a pipe is suitable. Check the product marking, technical data sheet and installation guide. Confirm the standard, material, outside diameter, ring stiffness class where stated, joint type, seal specification and intended use. Check that bends, branches, access chambers and connectors are made for the same system or are explicitly approved as compatible.

For PVC-U solid wall pipe, BS EN 1401-1 is the key reference for non-pressure underground drainage and sewerage. Structured wall products may use the BS EN 13476 series. Other standards can apply to pressure systems, internal above-ground soil and waste pipework, or particular applications, so do not transfer one standard reference to every plastic pipe on site.

The standard is only one part of the decision. Installation depth, ground conditions, traffic loading and manufacturer guidance all matter. A product selected for a garden run may not be appropriate beneath a frequently trafficked area without a suitable design and a driveway-rated cover and frame. When a drain will be adopted by a sewerage company, check its current design and construction requirements as well as the product literature.

Installation details that make the difference

Set the line and gradient

Plan the route before digging. Use a consistent fall toward the outlet, avoid unnecessary changes of direction and check levels throughout the run. Approved Document H includes guidance on minimum gradients for common foul drainage arrangements, with the appropriate value depending on pipe size and flow. For example, a 100 mm drain serving a WC is commonly designed around a gradient of 1 in 80 under the relevant guidance, while lower flows and other pipe sizes can require different falls. Treat this as a design check, not a universal figure. Confirm the precise situation against the current guidance and the drainage layout.

A slope that is too shallow can leave solids behind. An excessively steep run can allow liquid to move ahead of solids in some configurations. Accurate levels, appropriate pipe sizing and a well planned layout are more dependable than relying on a rough visual estimate.

Prepare bedding and side support

Flexible plastic pipes rely on the surrounding ground and granular bedding for support. Excavate a trench wide enough to install and compact the specified bedding and sidefill. Remove sharp stones and soft spots, form an even bed, then place and compact suitable material around the pipe haunches and sides in line with the design and manufacturer instructions. A non-woven geotextile membrane can help separate bedding material from surrounding ground where filtration is needed. Backfill in layers and avoid displacing or crushing the pipe.

The required material and cover depend on pipe diameter, depth, soil and loading. Standards such as BS EN 1610 address construction and testing of drains and sewers, while design guidance and product instructions help determine the appropriate installation detail. Avoid assuming one bedding recipe suits every trench.

Make joints carefully

Cut pipe square, remove burrs and clean the spigot and socket. Use a specified jointing lubricant on elastomeric seals, and insert the pipe to the manufacturer’s insertion mark or specified allowance. Keep seal faces clean and correctly seated. A joint that is twisted, contaminated or forced together can leak or create a snag inside the bore. Use compatible fittings, and check the completed run with a drain air test kit for line, level and secure connections before covering it.

Provide access for inspection and clearing

Access points let a drainage professional inspect and clear the system. Provide suitable rodding eye access points at changes of direction, junctions and other points required by the layout and applicable guidance. Inspection chambers or manholes should be positioned so the route can be maintained without dismantling pipework. Keep lids accessible after landscaping or paving, and select covers suited to their location and expected loading.

Protect drains beneath traffic areas

A driveway changes the load imposed on a buried pipe. The pipe’s stiffness, depth of cover, bedding and compacted backfill act together, so a stronger pipe alone does not guarantee a safe installation. Follow the manufacturer’s cover and loading guidance, and obtain a project specific design where the route is shallow, heavily trafficked or close to foundations. Use an appropriate heavy duty cover and frame over chambers in trafficked locations. Do not drive plant or vehicles over an unfinished trench.

Approved Document H and permissions

Approved Document H gives practical guidance for meeting Part H of the Building Regulations in England. It addresses foul water drainage, rainwater drainage, building over or near drains, wastewater treatment and related matters. Following its guidance can help demonstrate compliance, though a building project may use a different solution where it meets the regulation’s functional requirements and is accepted by building control.

Check with building control before work if a new drain, alteration or extension affects the drainage arrangements. A connection to a public sewer may require the sewerage company’s approval. Building over or near a public sewer can also trigger consent requirements and constraints. Requirements differ across the UK, so use the correct national regulations and contact the relevant authority for the site.

A private drain is generally the drain serving a property before it connects to a public sewer. In England and Wales, qualifying shared sewers and lateral drains were transferred to sewerage companies in 2011, with the exact responsibility depending on location and network details. Owners generally remain responsible for private drains within their property boundary that serve only their property. A property can still have a public sewer running through it, so a boundary line alone may not settle every case. Ask the local sewerage company to confirm ownership and responsibilities before repairing, diverting or building near a drain.

When to call a drainage professional

Bring in a drainage designer or qualified contractor where levels are constrained, the route passes beneath a road or heavily used driveway, the ground is unstable, flooding has occurred, or the work involves a connection to a public sewer. Professional advice is also sensible for shared drainage, a septic tank or package treatment system, or any proposal close to foundations or a public sewer. Contact building control or the sewerage company early when permissions may be needed. Early checks can prevent expensive excavation and rework.

Frequently Asked Questions

Is PVC-U suitable for underground foul drainage

It is commonly used for non-pressure underground foul drainage pipe when the pipe and fittings are specified for that purpose, comply with the relevant product standard and are installed to the manufacturer’s instructions. Confirm the application and system compatibility before purchase.

What does BS EN 1401 cover

BS EN 1401-1 specifies requirements for solid wall PVC-U pipes, fittings and joints used in non-pressure underground drainage and sewerage. Check the current edition and product documentation when specifying a system.

Can foul water and rainwater share one plastic pipe

They should be designed as separate systems unless the approved arrangement and sewerage company requirements allow a combined system. Establish the correct destination for each flow before connecting any pipework.

Who repairs a drain on my property

Owners usually maintain private drains serving only their property, while qualifying shared sewers and lateral drains in England and Wales are generally the responsibility of the sewerage company following the 2011 transfer. Network history and site layout can affect the answer, so ask the company to confirm ownership.

Do I need approval before connecting to a public sewer

A connection or work near a public sewer may require approval from the sewerage company, and building work may also need building control involvement. Confirm permissions before starting, and check the regulations that apply in your part of the UK.

Choose carefully, install confidently

A dependable plastic drainage system begins with the right pipe for the job, a clear understanding of where the water must go and a specification that matches the installation conditions. Verify product standards, keep foul and surface water arrangements clear, set the fall accurately, support the pipe properly with appropriate underground drainage pipe and fittings and provide access for future maintenance. Beneath a driveway or near a public sewer, seek project specific advice before work starts.

Before ordering materials or opening a trench, check the current guidance with building control, the relevant sewerage company and the pipe manufacturer. A short planning conversation now can protect the system for years and help keep the project compliant.