Permeable Paving Under the New Standards: Design Ratios That Work

The introduction of the National Standards for Sustainable Drainage Systems has brought significant changes to how we approach permeable paving design. For architects and developers working on new projects, understanding these updated requirements isn't just about compliance - it's about creating effective, long-lasting drainage solutions that protect both your development and the environment.

What's Changed in Permeable Paving Requirements

Under the new standards, permeable paving must now demonstrate compliance with enhanced water quality management and interception requirements. The emphasis has shifted from simply allowing water to drain through surfaces to ensuring that runoff is properly treated and managed according to the updated runoff destination hierarchy.

The key change lies in how we calculate design ratios. Previously, many projects relied on basic permeability rates, but the new standards require a more sophisticated approach that considers climate change allowances, water quality treatment, and long-term performance.

Understanding the New Design Ratios

The 5mm Interception Rule

One of the most significant changes affects how we size permeable paving systems. The new standards require effective management of the first 5mm of rainfall from any storm event. For permeable paving, this means your sub-base storage must be calculated to handle this initial flush, which typically carries the highest concentration of pollutants.

Practical calculation: For a 1,000m² car park, you'll need to manage 5,000 litres from this first flush alone, before considering the larger storm events.

Climate Change Allowances

Design ratios must now incorporate climate change factors, which vary by region but typically add 20-40% to rainfall intensity calculations. This directly impacts the depth and composition of your permeable paving sub-base.

For central England developments, this often means increasing sub-base depth from the traditional 150mm to 200-250mm, depending on soil conditions and downstream capacity.

Effective Design Ratios That Work

Standard Car Parks and Access Roads

For typical commercial car parks, the proven ratio that consistently meets new standards is:

• Surface layer: 80mm permeable block paving

• Laying course: 50mm sharp sand (2-5mm)

• Sub-base: 300mm Type 1 open-graded aggregate (minimum 30% voids)

• Geotextile separation layer

This configuration provides approximately 90mm of water storage per square metre, sufficient for most applications when combined with controlled discharge rates.

Residential Driveways and Paths

For domestic applications, a more cost-effective ratio works well:

• Surface layer: 60mm permeable blocks

• Laying course: 30mm sharp sand

• Sub-base: 200mm Type 3 open-graded aggregate

• Geotextile layer

This provides roughly 60mm of storage, adequate for residential loading and typical domestic runoff volumes.

Heavy-Duty Applications

For areas expecting HGV traffic or intensive use:

• Surface layer: 100mm concrete permeable blocks

• Laying course: 50mm sharp sand

• Sub-base: 400mm Type 1 aggregate in two lifts

• Enhanced geotextile protection

Water Quality Considerations

The new standards place greater emphasis on water quality treatment. Permeable paving systems must now demonstrate effective removal of suspended solids, oils, and other pollutants. This affects design ratios in several ways:

Treatment Through Filtration

Permeable paving provides water quality treatment through physical filtration as water passes through the surface, laying course, and sub-base layers. The aggregate gradation is critical - too fine and the system clogs quickly, too coarse and pollutant removal is inadequate.

Maintenance Access Requirements

New standards require clear maintenance access. Design your permeable areas with maintenance vehicle access to at least 80% of the surface area. This often means incorporating standard paving strips every 50-60 metres in large installations.

Common Design Pitfalls to Avoid

Undersized Sub-Base Storage

Many designs fail because they underestimate storage requirements. Always calculate for:

• The 5mm first flush requirement

• Climate change allowances (typically +30%)

• Safety factor for reduced permeability over time (recommend +20%)

Inadequate Edge Restraint

Permeable paving systems require robust edge details. Budget for concrete edge restraints along all perimeters - this typically adds 10-15% to material costs but prevents long-term failure.

Poor Drainage Interface

Where permeable paving connects to conventional drainage systems, the interface detail is critical. Ensure your design includes proper connections to gullies or other drainage infrastructure, with appropriate fall rates (minimum 1:80 for effective drainage).

Specification for Success

When specifying permeable paving under the new standards, include these essential elements:

Materials: Specify aggregate to BS EN 13242, with confirmed void ratios. Require manufacturer's certification for permeability rates.

Installation: Mandate specialist installation teams with demonstrated experience. Poor installation is the primary cause of system failure.

Testing: Include post-installation permeability testing. Systems should achieve minimum 270mm/hour infiltration rates when new.

Long-Term Performance

The new standards emphasise whole-life performance. Design your ratios with maintenance in mind:

• Annual inspection of surface condition

• Bi-annual cleaning of high-use areas using vacuum sweeping

• Five-yearly replacement of top 20mm surface layer in heavy-traffic zones

• Decennial sub-base inspection through trial pits

Site-Specific Considerations

Successful permeable paving under the new standards requires careful site assessment:

Soil Conditions

Infiltration rates below 1x10⁻⁶ m/s may require impermeable membrane and controlled discharge rather than full infiltration.

Groundwater Protection

Maintain 1m separation between sub-base and highest groundwater level. Consider impermeable lining where groundwater protection is essential.

Slope Limitations

Maximum gradient of 1:20 for permeable block paving. Steeper slopes require specialist products or alternative approaches.

Installation Best Practice

Proper installation is crucial for meeting new standards:

1. Excavation: Maintain consistent depths and avoid over-excavation

2. Compaction: Compact sub-base in 150mm lifts to achieve required density

3. Laying course: Ensure uniform thickness and avoid cement contamination

4. Joint filling: Use appropriate kiln-dried sand, avoiding fine materials that cause clogging

The new National SuDS Standards have raised the bar for permeable paving design, but they've also created opportunities for more effective, sustainable drainage solutions. By understanding these updated requirements and applying proven design ratios, you can deliver systems that not only meet regulatory requirements but provide long-term value for your clients.

Need expert guidance on permeable paving design for your next project? The Drainage Designers specialise in drainage solutions that meet the latest standards while delivering practical, cost-effective results. Contact us at 01256 807116 or martin.young@thedrainagedesigners.co.uk to discuss your requirements.