A waterlogged lawn, overflowing downpipe or puddle beside a patio is rarely solved by adding more gravel to the surface. The water needs somewhere suitable to go. Soakaway crates for gardens provide underground storage that lets clean rainwater disperse gradually into surrounding soil, helping to manage runoff without taking up valuable garden space.

For a domestic garden, a crate system can be a practical answer to roof water from extensions, garages, sheds, conservatories and outbuildings. However, it is not simply a case of digging a hole and dropping in a few plastic units. The soil, available space, roof area and drainage route all determine whether a soakaway will perform properly.

What soakaway crates do

A soakaway crate is a high-void plastic attenuation unit. When several crates are clipped together, they create a large underground volume that temporarily holds rainwater. Water enters through underground pipework, fills the open structure and then passes through a permeable geotextile membrane into suitable ground around the excavation.

This approach is more efficient than a traditional rubble-filled soakaway. Rubble has significant gaps, but it also occupies much of the hole. Crates usually offer a void ratio of around 95%, so more of the excavated volume is available for water storage. That can reduce the footprint needed in a compact garden, although the excavation still needs enough room for the crate assembly, bedding, surround and safe working access.

The system is normally connected to rainwater downpipes using underground drainage pipe and fittings. A roddable access point, silt trap or suitable gully arrangement can make maintenance easier by reducing the amount of leaves and debris reaching the crate void.

Start with the ground, not the crate size

The biggest question is whether the ground can accept the water. Clay-heavy soil, made ground, shallow bedrock and areas with a high water table may drain too slowly for a conventional soakaway. Installing extra crates will not solve poor infiltration. It may simply create a chamber that stays full after prolonged rain.

A percolation test is the sensible starting point for anything beyond a very small garden project. For new-build and formal drainage design, testing and calculation are commonly based on BRE 365 guidance. A groundworker or drainage designer can excavate a trial pit, fill it in a controlled way and measure how quickly the water level falls. This produces an infiltration rate that can be used to size the system properly.

If the ground test shows slow drainage, alternatives may include a surface water connection where permitted, attenuation with a controlled discharge, a rain garden, or reducing runoff through water butts and permeable landscaping. Local drainage requirements and any planning conditions should be checked before work starts, particularly on extensions and larger developments.

Ground conditions can also vary across one property. A garden may appear free-draining at the surface but have compacted subsoil below, while a sloping plot may direct water towards a neighbour’s land. The proposed location should be assessed in context, rather than relying on how quickly a puddle disappears from the lawn.

Sizing soakaway crates for gardens

Correct sizing balances two figures: how much water the roof can produce during a design rainfall event, and how quickly the soil can disperse it. The roof catchment is generally measured as its plan area – the footprint viewed from above – rather than the actual slope length of roof tiles.

As a simple illustration, a 50 m² garage roof receiving 20 mm of rainfall produces approximately 1 m³ of water. In reality, the required crate volume depends on the rainfall criteria used in the design and the tested infiltration rate, not just one rainfall depth. A larger roof, a slow-draining soil or a location exposed to intense storms will require more storage.

Crate dimensions and void volume matter. A crate may have an external volume close to a cubic metre but a slightly lower usable water volume once its structure is allowed for. Always use the manufacturer’s declared storage capacity when calculating the number of units. Allow for the chosen crate’s loading classification too. A crate suitable beneath a lawn may not be suitable beneath a driveway, where vehicle loading, cover depth and installation detail are more demanding.

For domestic roof drainage, it is usually better to retain a sensible margin than to choose the smallest theoretical system. It is also worth considering future changes. An extension, replacement garage or additional hard landscaping can increase the area draining to the soakaway later on.

Keep the soakaway clear of structures

A soakaway should be positioned where infiltrating water will not undermine foundations, retaining walls or neighbouring land. A commonly used rule of thumb is to keep it at least 5 metres from a building and around 2.5 metres from a boundary, but site conditions and local authority requirements can call for greater separation.

Avoid low points close to the house, ground where water naturally collects, and areas above utility services. Before excavation, identify buried cables, water pipes, gas services and existing drainage. If the garden has a septic tank, private treatment plant or other below-ground installation, keep drainage systems clearly separated and plan the route carefully.

Choosing crates, membrane and pipework

The crate is one part of the system. The geotextile membrane is equally important in a conventional soakaway because it allows water to pass into the soil while restricting fine particles that could clog the crate over time. The membrane should fully wrap the crate assembly with sufficient overlaps, and any tears should be repaired before backfilling.

Do not confuse a permeable geotextile with an impermeable geomembrane. A non-woven geotextile is generally used where water is intended to infiltrate into surrounding ground. An impermeable liner is used for attenuation systems where water is deliberately retained and discharged through a controlled outlet. Using the wrong material can prevent a soakaway from working as intended.

Use compatible underground drainage pipework for the inlet run. For many domestic applications, 110mm underground drainage pipe is suitable for carrying roof water, subject to flow requirements and the rest of the drainage layout. Fittings should provide a continuous fall from the downpipe connection to the crate. Sharp changes in direction, inadequate falls and unsealed joints are common causes of poor flow and soil ingress.

A leaf guard at gutter level and a trapped gully or filter point upstream can reduce maintenance. No filter arrangement is completely maintenance-free, especially below trees. Periodic clearing of gutters and gullies protects the drainage system as well as the soakaway.

Installation points that make the difference

Excavate to the required plan size and depth, allowing for the specified granular bedding and side surround. The base should be level, stable and free from sharp stones. Where ground is soft or uneven, follow the crate manufacturer’s installation guidance rather than trying to correct it with loose, uncompacted material.

Lay the geotextile into the excavation with enough excess to fold over the completed crate structure. Assemble the crates on the prepared base, ensuring connectors are fully engaged. Fit inlet pipework at the intended connection point and check the fall before the system is enclosed. Where access or inspection provision is specified, install it at this stage rather than assuming the crate can be reached once buried.

Wrap the membrane carefully around the sides and top, keeping overlaps generous. Backfill in even layers with the specified clean angular stone or granular material where required by the system design, compacting with care. Heavy compaction equipment directly above lightweight crate systems can cause damage, so cover depths and loading instructions must be followed.

Before reinstating turf, paving or decorative gravel, test the route from the downpipe to confirm water enters the crate freely. It is far easier to rectify a poorly aligned pipe or blocked fitting before the garden is finished.

When a crate soakaway is not the right answer

Soakaway crates are designed for relatively clean surface water. They should not receive foul water from toilets, sinks, washing machines or kitchens. Water contaminated by oil, chemicals or significant sediment also needs a different treatment approach before any discharge is considered.

They are not automatically suitable for every paved area either. A driveway may collect silt, tyre debris and fuel residue, while a large patio can generate runoff faster than a modest garden soakaway can handle. In these cases, consider the catchment, treatment requirements and loading class as a complete drainage design.

For a straightforward garden installation, buying compatible crates, geotextile, 110mm pipe, bends and access components together avoids delays once excavation begins. Speedy Plastics supplies the connected drainage materials needed to build a practical rainwater route, with competitive VAT-inclusive pricing and fast UK delivery. Measure the roof, test the ground and plan the pipe run before ordering – those three checks will do more for long-term performance than choosing the largest crate on the page.