What Size Aggregate Is Best For Drainage? A Practical Guide

`Quick Summary:

• 10mm–20mm clean stone: Ideal for pipe bedding, French drains, and general drainage
• 40mm clean stone: Best for soakaways, attenuation crates, and high-flow systems
• Avoid MOT Type 1: Contains fines that can block drainage - not suitable
• Use clean, washed aggregate: No silt, dust, or recycled material
• Always follow design specs and use membranes to prevent contamination

When you’re working on a commercial build, civil engineering job or infrastructure project, drainage is never an afterthought. Getting water away quickly and efficiently protects everything around it; from sub-base layers and access roads to structures and underground services.

But while pipe gradients and layout plans get plenty of attention, the choice of aggregate often gets less thought than it deserves. That’s a problem. Because using the wrong size or type of aggregate can lead to poor flow, blocked systems, or even structural failure over time.

So, what size aggregate is best for drainage, and how do you choose the right one for your job?

Let’s break it down.

Why Aggregate Size Matters In Drainage Systems

Drainage aggregate isn’t just filler. It plays a critical role in how well a drainage system performs.

Too small, and fines can clog up water pathways, reducing flow rates or causing build-up. Too large, and you risk uneven distribution or instability. The right size aggregate will create a void between particles that allow water to move freely while maintaining support and structure.

It also:

• Reduces hydrostatic pressure
• Prevents waterlogging and erosion
• Supports long-term system stability
• Helps protect pipework and surrounding ground

This makes aggregate choice especially important in applications like SUDS, soakaways, French drains, and road drainage.

Common Aggregate Sizes Used For Drainage

10–20mm: Standard for Most Pipe Bedding and General Drainage

For most commercial drainage installs, a clean, single-size 10mm or 20mm gravel is used. These sizes provide enough void space to support free-draining flow, while being compactable enough to offer pipe stability.

This size range is ideal for:

• Pipe bedding and surround
• French drains
• Linear soakaways
• General sub-surface drainage

Note: Always use clean, angular stone, never recycled material with fines to avoid long-term clogging.

40mm: Ideal for Soakaways and Attenuation Systems

Where larger volumes of water need to drain away. such as below permeable car parks, in SUDS basins, or under yards - 40mm drainage stone is often preferred. The larger voids increase flow and capacity.

Benefits of 40mm clean aggregate:

• Greater flow rates
• Excellent permeability
• Reduced risk of silt build-up

Many contractors pair this with a geotextile membrane to prevent fines entering the system from surrounding ground.

Explore our guide to drainage aggregate solutions → (Add link once blog is live)

Clean vs Mixed Aggregate: Always Choose Clean for Drainage

Drainage systems need clean stone - that is, aggregate with minimal fines, silt, or dust content. Fines reduce permeability and can cause the drainage structure to block or ‘blind’ over time.

At Armstrongs, our drainage aggregates are:

• Washed and screened
• Free from clay, soil, or organic matter
• Compliant with BS EN standards for drainage use

You can read more about clean vs mixed aggregates in our Guide to Sub-Base Aggregates (example internal link)

What About Mot Type 1 for Drainage?

While MOT Type 1 is a popular choice for sub-base work, it’s not usually recommended for drainage.

Why? Because MOT Type 1 includes fines (particles under 5mm) which compact to form a solid layer. That’s perfect for roads and paths - but not for water flow. For drainage layers, a clean, single-size aggregate is a better choice.

If you’re layering a sub-base over drainage, it’s important to keep those layers separate and use a membrane where needed.

Factors to Consider When Choosing Aggregate Size

Choosing the right size isn’t just about flow — it’s about the wider system too. Consider:

• Load and traffic: Heavier loads may require larger, more stable aggregate sizes.
• Drainage design: Check spec sheets and plans, these often dictate required size ranges.
• Installation method: Smaller sites may be limited by what equipment is available.
• Environmental exposure: For wet, clay-heavy areas, larger clean stone can reduce risk of silting.

Working with an experienced supplier helps here. At Armstrongs, we don’t just deliver materials, we help our clients specify the right ones.

Drainage Applications We Support

We supply drainage stone to major contractors and public-sector frameworks across the North West and beyond. Typical use cases include:

• Highway drainage
• Yard and yard perimeter systems
• SUDS and attenuation tanks
• Land drainage for agricultural projects
• Pipe bedding and granular surround

Whether you need bulk deliveries to multiple sites or high-volume stock for large infrastructure projects, we’ve got the capacity to keep your schedule on track.

See how we support large-scale aggregate deliveries 

Armstrongs’ Drainage Aggregate Range

We supply a wide range of drainage aggregates, including:

• 10mm clean granite
• 20mm clean gritstone
• 40mm clean stone
• Pipe bedding materials
• Gabion stone (where structural support and drainage are both needed)

All from our own quarries: Montcliffe (gritstone), Brinscall (sandstone), and Shap (granite).

Need help specifying? Just give us a call and we’ll help match the right aggregate to your drainage design.

Frequently Asked Questions

What’s the best aggregate size for pipe bedding?
Usually 10mm or 20mm clean stone. It gives stability while still allowing free water flow.

Can I use MOT Type 1 for drainage?
Not recommended. It contains fines that can clog the system. Use clean stone instead.

What’s the difference between clean stone and recycled aggregate?
Clean stone is washed and graded with no fines. Recycled aggregates often contain silt, dust or clay,  which can lead to drainage failure over time.

What size is best for soakaways or attenuation?
40mm clean stone is ideal due to its larger voids and excellent flow characteristics.

Do I need a membrane under the drainage stone?
In most cases, yes, especially to separate stone from soil and prevent fines contaminating the drainage layer.

Talk to Armstrongs about your drainage aggregate needs

Drainage issues can bring even the best project to a standstill. Whether you’re working on highways, industrial yards, or new infrastructure, we’ll help you get the right material on-site, on time, and up to spec.

With bulk supply, flexible delivery, and decades of experience behind us, Armstrongs is the partner of choice for civil engineers and commercial contractors across the UK.