Choosing the correct aggregate for Permeable Paving: Why it matters

One in four properties in England will be at risk of flooding by 2050, according to a report by the UK’s National Infrastructure Commission (NIC) and the Environment Agency. Urban areas are particularly vulnerable due to the dominance of impermeable surfaces like concrete and asphalt, which prevent rainwater from naturally infiltrating the ground. As climate change intensifies and extreme weather events become more frequent, the need for effective Sustainable Drainage Systems (SuDS) is greater than ever. Permeable paving plays a critical role in managing stormwater, but its success depends on using the right aggregates to ensure proper infiltration, storage, and filtration.

Why the correct aggregates are essential for permeable paving 

Sustainable Drainage Systems (SuDS) are innovative water management techniques designed to replicate natural drainage patterns. They address challenges associated with urbanisation, such as surface water flooding and the strain on drainage networks. By managing rainwater at its source, SuDS minimise the volume and speed of water entering drainage systems and waterways. This reduces flooding risks and enhances water quality by filtering pollutants from runoff. Permeable paving is one of a range of SuDS methods and its effectiveness heavily relies on specific aggregates which facilitate water infiltration, storage and promote natural filtration which support groundwater recharge, making them a crucial component in permeable paving as part of a SuDS solution.  

Permeable aggregates help to restore the natural water cycle disrupted by urban surfaces like concrete and tarmac by facilitating the infiltration of water into the soil. These aggregates are designed to allow water to soak into the ground slowly, emulating the natural process of water recharge which is central to the function of SuDS. 

The correct aggregate sizes for permeable paving projects 

When designing permeable paving systems, choosing the right aggregate size is essential for ensuring adequate water storage and creating a structurally durable sub-base to support the intended traffic use while allowing water to drain efficiently. Following industry guidelines such as BS 7533-13:2009 (Guide for the design of permeable pavements constructed with concrete paving blocks and flags) ensures your project meets UK standards for performance and sustainability. Below, we’ll break down the correct aggregate sizes for each layer of a permeable paving system. 

Sub-base: 4/20mm aggregate 

What to use: Clean, free-draining aggregate with a size range of 4mm to 20mm (BS EN13242: 2002 -Coarse graded crushed rock or gravel.) The aggregate must have a void ratio of between 30- 32% for water storage purposes. Aggregate with this void ratio provides approximately 1m3 of water storage for every 3m3 of aggregate. 

Why it’s important: The sub-base layer provides both structural integrity and hydraulic capacity to store water. 

How to prepare: Once the 4/20mm aggregate is in place, compact it using a non-vibrating roller or plate compactor; this ensures that the void ratio of the material remains at 32% and avoids over compaction which can result in the aggregate being crushed. 

Laying course: 6.3-2mm aggregate 

What to use: An angular crushed aggregate with particles ranging from 6.3mm to 2mm. 

Why it’s important: This layer creates a smooth surface for laying paving blocks while maintaining good drainage. 

How to prepare: Spread and screed the 6.3mm to 2mm aggregate evenly over the compacted 4/20mm sub-base. The evenness of this layer is crucial for proper block placement. 

Paving blocks 

What to do: Lay your chosen permeable pavers in the desired pattern. 

How to compact: Use a plate vibrator to ensure the blocks are securely positioned and aligned before moving on to the joint-filling stage. 

Joint Filling: 6.3-2mm aggregate 

What to use: Angular crushed aggregate with particles ranging from 6.3mm to 2mm. 

Why it’s important: Properly filled joints prevent block movement and ensure long-term stability of the surface. 

How to fill: Brush the grit into the joints until they are completely filled. After initial use, inspect the paving and top up any settled joints with more grit to maintain a fully filled surface. 

How much to use: You’ll need approximately 1 ton of grit for every 100m² of 80mm paving. 

Note: It’s important to ensure that the permeable joints remain free from contamination during the completion of the project. Extra caution is needed during soft landscaping to prevent soil from entering the joints. To protect the surface, use an appropriate cover over the permeable paving while this type of work is being carried out. This simple step will help maintain the functionality of the system. 

Benefits of correct aggregate selection for permeable paving  

Choosing the right type and size of aggregates for your permeable paving project is crucial. It’s not just about creating a stable surface; it’s about ensuring your paving performs well over time. Here are the key benefits of getting your aggregate selection right: 

Improved drainage 

Using the right aggregates ensures that rainwater can flow through the paving system and into the ground below. This helps prevent water pooling on the surface, reduces the risk of flooding, and supports sustainable drainage solutions. 

Long-lasting stability 

Properly graded and compacted aggregates create a strong foundation for your paving, making it durable enough to handle foot and vehicle traffic without sinking or shifting over time. 

Better load baring capacity  

Using the correct aggregate sizes distributes weight evenly. This ensures that the paving can handle heavy loads without cracking or deforming. 

Low maintenance 

Proper aggregate selection reduces the likelihood of issues like joint movement or surface damage, meaning fewer repairs and less maintenance over time. 

Tips for maintaining permeable paving and aggregates 

Proper maintenance of permeable paving aggregates is essential for keeping your paving system functioning effectively and looking its best. Here are some practical tips to help you care for your permeable paving: 

Initial cleaning: After installation, allow the paved area to settle for a few weeks. Once settled, lightly hose down the surface to remove excess dirt or debris. Treat the area with a weed suppressant two or three times a year, depending on need, to prevent weeds from taking root. 

Managing general dirt: Sweep the surface regularly to prevent the build up of debris that could clog the paving joints. Do not use high-pressure washers or vacuum sweepers as they can remove jointing grit or damage the surface. Always test a small area first if using a power washer. 

If jointing grit is removed during cleaning, ensure it is replaced immediately to maintain the structural stability of the paving. 

How to clean clogged joints: After several years use, joints may become clogged with detritus and it may be necessary to clean them. To clean clogged joints, remove the old jointing grit and debris with a high-pressure water jet. Reset any displaced pavers and replace damaged ones. Refill the joints with 6.3-2mm grit, sweeping it in with a stiff brush to ensure they are fully filled  

Maintaining permeable paving aggregates is key to preserving both their appearance and functionality. You can find all maintenance information in our Hydropave Maintenance document. (link) 

Final thoughts: Why aggregate selection matters for permeable paving 

The right choice of aggregates is not just a technical detail – it’s the foundation of a successful, long-lasting permeable paving system. With the growing risk of flooding in urban areas, getting this right has never been more important. By ensuring the correct aggregate sizes are used for each layer, you create a system that efficiently manages stormwater, supports structural stability, and reduces maintenance over time. 

Permeable paving isn’t just about preventing surface water runoff; it’s about contributing to a sustainable future. When designed and installed correctly, it can enhance urban resilience, protect properties from flooding, and improve the overall environmental performance of a project. 

If you’re planning a permeable paving project and need guidance on the right aggregates to use, our technical team is here to help. Contact us for expert advice and product recommendations to ensure your system meets both performance and sustainability standards.