Attenuation Crates: Void Ratio, Load Ratings, Membrane Wrap & Bulk Supply for UK SuDS

Quick Specs for Attenuation Tanks: What an Attenuation Crate Installation Solution Must Prove Before UK SuDS Approval

Attenuation crates are a system of geocellular modules used to store surface water underground for slow release. They sit inside the surface water drainage system for UK SuDS and must be specified as storage, flow control and maintenance assets rather than loose crate packs.

What Searchers Call This Same Product A searcher may type any number of terms in for a similar product: stormwater attenuation crate, water attenuation crate, stormwater attenuation system, stormwater management crate or water management crate. While it’s customer specific in phrasing, the underlying need for clarification is the same. Whether the wording is stormwater attenuation or water management, the specifications need to clarify design options regarding attenuation and infiltration, stormwater control methods and the regulation, governing authority or municipal requirement that impacts discharge.

That difference is critical for the 2025 national SuDS standards for England cover new infrastructure, the surface water management systems they serve, and, in doing so, define SuDS as being structured as a hierarchy, a long-term maintenance liability, and as a management train. While a crate schedule can assist it can’t be a proxy for design proof.

Terminology in supplier catalogues is not always consistent. Some buyers ask for types of crates, cellular crates, a stack of modules, an underground storage system, storm water during heavy rainfall, water during heavy rainfall conditions, collected storm water or ground storm water. A practical specification still has to suit the level of surface water, the ground water table, water infiltration limits, infiltration of storm water, covering depth, excess runoff, retention duty and any flood-prone layout. Installing attenuation should be treated as a sustainability and environmental decision because the right package can retain water, promote controlled discharge, improve downstream risk and be beneficial for long-term site management.

For standard traceability, the ANSI Webstore listing for BS EN 17152-1:2019 storm water boxes is a useful prompt: ask which product standard, test certificate or factory document applies to the proposed crate family rather than relying on a catalogue strength label alone.

Download and integrate this checklist with AQUA RainWater’s UK attenuation tank design guide. For sizing based on product specifics, cross-reference the crate evidence against the AQUA geocellular tank range and geocellular stormwater modules.

Attenuation Crates vs Soakaway Crates: Which Job Is the Crate Actually Doing?

An attenuation crate and a soakaway crate may consist of identical modular units, but the functions they perform are different. A soakaway crate is porous and allows water to soak into the ground. A sealed attenuation crate system is used to hold water and discharge to a sewer, watercourse or other approved location at a specified rate.

This is often where much of the mistake begins. In the UK, large search demand is on “soakaway crates”, but SuDS hierarchy and site constraints could render easy infiltration non-viable. According to the 2025 national standard, infiltration has clear limits: ground conditions must be appropriate, infiltration rates adequate and the bottom of any infiltration must not be within 1m of the max likely groundwater.

On a driveway application or car park use case, the wrong order creates risk because a soakaway package and a sealed attenuation package solve different drainage problems. AQUA RainWater provides geocellular tank and stormwater module evidence, including 95% void space and 1000 mm x 500 mm x 500 mm module data, so buyers can confirm against product facts before a design team commits to one route.

The 95 Percent 30-Point Storage Test: Void Ratio, Excavation Volume and Crate Count

A great initial sanity-check on an attenuation crate quote, this 30-Point Storage Test works as follows; look whether the design is near 95% net void space, then ask whether the nominated load class is realistic for a 30 T/m2 baseline or higher project.

AQUA RainWater’s geocellular tank module is sold in dimensions of 1000mm x 500mm x 500mm, is roughly 95% void, and 95% storage capacity at 0.238m3. That isn’t a substitute for hydraulic design, but it allows the buyer to check crate-count errors before an expensive RFQ.

In this table, the formula is intentionally basic: target storage capacity divided by net module capacity, upscaled to the nearest whole number. It doesn’t make allowances for project differences, freeboard, silt capacity, or unusable space for connections and designers’ safety margins. This formula is primarily intended as a check during purchasing rather than a drainage design tool.

For an RFQ use case, the risk is a crate-count gap because a nominal 10 m3 tank and a real 10 m3 usable storage target are not always the same thing. AQUA RainWater provides the 0.238 m3 net module reference and a supplier-reported, roughly 95% void figure, which gives buyers a practical evidence check before asking for a quote or drawing review.

Load Ratings by Surface Use: Landscape, Driveway, Car Park and HGV Routes

The load rating is probably the most commonly observed where a crate quote appears exact but it needs careful review. ADS Pipe UK state that, “The geocellular structural assessment must consider the short term traffic loading and the long term loading due to cover material and lateral earth loads and comply with the manufacturer’s cover depth limitations and installation conditions.”

AQUA RainWater’s geocellular tank product line is defined as “Standard”, “Heavy” and “Super” for vertical load capacity of 30, 45 and 60 T/m2, respectively.

Their stormwater module product pages distinguish LD, SD, HD and XD duty families. Cite those as your product evidence then have the project engineer review your design cover, pavement build up and loading scenario.

On a domestic driveway, commercial car park or HGV service-yard application, the problem is not the product label alone; the structural risk appears because cover depth, pavement build-up and traffic frequency change the load case. AQUA RainWater provides product class data, while the engineer confirms against 30 T/m2, 45 T/m2 or 60 T/m2 evidence and the site-specific use case.

Geotextile or Geomembrane: The Membrane Wrap Decision That Changes the Whole System

Membrane wrap isn’t an accessory line at the bottom of the quote. It changes the system hydraulics of the attenuation crate.

For infiltration the crate will have a permeable geofabric to keep fines out but let water flow away. For sealed attenuation there will be an impermeable liner with geofabric/fleece protection to prevent the liner from being pierced. AQUA guidance shows the split in its stormwater module where detention uses HDPE/LLDPE geomembrane with fleece protection, while infiltration uses non-woven geotextile.

For a clay-soil site, adoptable drainage route or car park runoff application, the membrane mistake can fail the system because water either needs a controlled discharge route or an infiltration route that the soil can accept. AQUA RainWater provides the detention-versus-infiltration package evidence, and the supplier should confirm against liner protection, geotextile separation and the published roughly 95% storage target.

Pipe, Flow Control and Access: How the Crate Array Connects to a SuDS Strategy

In the UK, the prevailing design philosophy regarding SuDS is that it should be “integrated”. That’s, the array of crates is just one component.

It needs to connect to inlet devices, outlets, flow control devices, pre-treatment measures, exceedance routing, maintenance access, and ultimately a final discharge location, in that order.

The RBWM Local Flood Authority guidelines state, “the attenuation device required to meet the proposed restriction should be clearly specified, and strong justification should be provided if in-ground attenuation devices such as crates, tanks or oversized pipes are proposed.” They also suggest that, “the designer should consider maintenance and eventual replacement over the lifetime of the development.”

When sourcing a supplier this means the buyer shouldn’t simply provide “need 25 m3 of attenuation crates.” It requires them to specify the catchment area, required storage volume, target discharge rate, preferred location, loading condition on surface, cover depth, sizes of incoming/outgoing pipes, and whether the device is to be a sealed attenuation or an infiltration system. That data enable the supplier to quote the most appropriate crate package that a contractor can effectively install and helps demonstrate how the proposed solution will manage heavy rainfall without moving flooding risk further down the road.

During a contractor take-off or planning-condition response, missing pipe data creates a risk because the crate body is only one part of the application. AQUA RainWater can review the storage target, 100 mm or 150 mm pipe entry, flow-control assumption and inspection route, then provide a specification trail that links the product evidence to the drawing.

Bulk Supply and Take-Offs: What UK Buyers Should Confirm Before Ordering

If purchasing the system in bulk, unit price is only a small fraction of the overall risk. The larger risk is that the crates can be appropriate for the required storage volume but still incorrect for the specific load case, the associated membrane specification or piping configuration, and may lack the submission evidence needed for the regulatory authorities.

Ask for supplier details on your 9-Row Wrap Matrix and a unit schedule, before PO issue. Module length at 1000 mm, width at 500 mm and height at 500 mm; supplier-published net void at about 95%; assumed cover at 0.5 m and 2.0 m; pipe entry at 100 mm or 150 mm; vent or inspection allowance at 75 mm, 225 mm or 300 mm; edge clearance at 1.5 m; delivery time at 3 days; unloading time at 24 hr; maintenance review period at 6 months; and design life assumption at 50 years all come in from supplier on an RFQ as evidence, not as generic design limits.

AQUA RainWater is available to advise on crate take-offs, membrane packages and technical consultations for project buyers. Use the ‘contact page‘ when you know your project’s target storage volume, surface loading, preferred drainage pathway and pipe sizes.

2026 UK Specification Signals: Why Shallow, Traffic-Rated and Inspectable Crates Are Moving Up the Shortlist

We see three recurring signals across the market research. The new 2025 national SuDS standards mean surface water drainage evidence is becoming far more accessible and transparent for both developers and consenting authorities. Developers are under pressure to attenuate surface water drainage into shallow or constrained sites, or spaces that carry heavy traffic loads. And there’s growing scrutiny around below ground systems, especially regarding maintenance, sediment management and their longevity.

However, this doesn’t mean that all projects demand the highest-specification crate; rather it indicates a shift in design discussions, away from a focus solely on “how many crates will fit in the void,” and towards a project-specific evaluation of which crate, wrap, connection and maintenance solution can effectively function throughout the development’s lifecycle.

Patent records also show why buyers should keep structural details, connection points and maintenance routes in the evidence pack; for example, NZ719565A describes stormwater crate and geocellular module arrangements where the module is only one part of the working assembly.

As a result, a UK buyer will most confidently approach a shortlist of suppliers who aren’t necessarily the cheapest but who can clearly document the interplay of void ratio, load rating, membrane wrap and bulk supply data in a practical evidence package.

Specification signal: the right attenuation crate shortlist is the one that links storage volume, load case, wrap package and maintenance access into a single evidence trail.

In a 2026 procurement application, the problem is shallow land, traffic loading and maintenance access arriving together. Because those risks converge in one tank, AQUA RainWater’s published roughly 95% void ratio, 30/45/60 T/m2 load-class data and membrane-package options give buyers a confirm-against checklist rather than a generic crate list.

FAQ