Concrete permeable paving, sub bases, geotextiles and how they work

Concrete permeable paving, when combined with appropriate sub bases and geotextiles, forms an efficient way to control water drainage. This approach helps achieve SuDS compliance. Water that falls upon the surface drains through the joints between the permeable paving blocks. It then passes into the sub base where it slowly drains through the gaps between the aggregate layer. The water then either drains into the ground through a porous geotextile membrane or is retained by an impermeable membrane.  

The properties of each component, and therefore the whole design of a permeable paving system, can be adjusted. This allows control over the rate at which water drains and its final destination.  

Section 3 of the Flood and Water Management Act 2010 is designed to help with surface water management in urban areas. It was implemented in Wales in 2019 and in 2025 the government has stated it is strongly committed to requiring standardised SuDS in new developments. Regardless of the legislation, the focus on SuDS is intensifying across the board. Therefore, understanding how geotextile membranes and permeable paving systems manage surface water is more important than ever. This knowledge will help future-proof developments for SuDS compliance and evolving regulatory standards. 

How does concrete permeable paving work? 

Concrete permeable paving works by allowing surface water to drain through the joints between specially shaped permeable paving blocks. The edges of each block have carefully designed nibs that ensure controlled gaps are always left between installed blocks. These gaps are filled with jointing material that is porous. This allows water to flow off the surface of each permeable paving block, and down through the gaps into the sub base below.  

In standard paving systems, surface water is taken away through drains and gulleys. By contrast, permeable paving drains the surface water into the ground where it falls. Permeable concrete paving mimics the pattern of natural water absorption. Dealing with the water locally using permeable paving does not increase flood risk. This makes it suitable for use as part of a SuDS (Sustainable Drainage Solutions) system. 

What part does the sub base play in a permeable paving system? 

The sub base provides a stable base for the traffic loading of a concrete permeable paving area. The permeable nature of the sub base also provides an area where water can be stored and slowly released. This release can either be into the ground, or into a drainage or storage system.  

The aggregate used for a permeable block paving sub base consists of angular aggregate. This is typically 4/20mm coarse graded with no fines. The angular nature of the stones locks the sub base together to provide stability. This creates pockets where water can be stored. 

The required thickness of permeable block paving sub base will depend upon the structural strength needed to support traffic loads. It is also determined by the volume of water storage needed. The traffic loading thickness is usually the greater of the two. This increases the water holding capacity of the sub base layer. This is an advantage as it allows the permeable paving surface to drain away the runoff from surrounding impermeable surfaces. 

Where extra strength and stability is required, a layer of Dense Bitumen Macadam (DBM) can be installed. This can reduce the depth of the sub base and save on excavation and aggregate costs. 

DBM can also be used to protect the sub base from damage and clogging on site access roads that will eventually be finished with concrete permeable paving. When the time comes to lay the permeable paving, the DBM is cored. Then the laying course and permeable paving can be installed.  

What are geotextile membranes used for in permeable paving systems? 

Geotextile membranes are permeable materials that are used to separate layers within a permeable paving system. They are typically used to separate the sub base and the ground. In some instances, they can also be positioned between the laying course and the sub base. 

Geotextile membranes between the sub base and the ground in permeable paving 

Geotextile membranes are positioned between the sub base and the sub-grade soil. This contains and prevents the sub base from sinking into the soil. Geotextile membranes allow the water to pass through the sub base into the ground while retaining the aggregate layer in position. They also prevent finer particles from the soil from entering the sub base. This helps to maintain its water retention capacity. 

Geotextile membranes under the laying course of permeable paving 

The geotextile membrane layer can be used to separate the fine aggregate of the laying course from the larger aggregate layer of the sub base. This helps prevent smaller particles of the laying course from settling into the sub base layer and hampering effective drainage. However, the use of geotextile membranes in this position is often regarded as optional. If the aggregate sizes of the laying course and sub base are within specification, water drainage should not be disadvantaged. 

When are impermeable alternatives to geotextile membranes required in permeable paving systems? 

Impermeable alternatives to geotextile membranes are required when the sub base acts as a water tank. Impermeable layers are usually made of plastic. They line the entire section containing the sub base to act as a tanking layer. The contained water is then discharged to other drainage systems rather than being allowed to drain directly into the ground. This is referred to as a tanked (no infiltration) system. 

How does the infiltration rate of ground affect the choice of permeable paving system? 

Water stored in the sub base underneath concrete permeable paving – lined with geotextile membrane – can be released at different infiltration rates. These rates depend on the specific design of the system and need to consider the infiltration rate of the ground.  

There are three main types: 

1. Full infiltration (Type A) 
2. Partial infiltration (Type B) 
3. No infiltration (Type C) 

The choice of system largely depends on the permeability of the sub-grade beneath the permeable paving. High permeability rates will allow a total infiltration system to be utilised. As ground permeability rates decline, either partial or no-infiltration systems will be required.  

The exception to this is where total capture of water is required for re-use or water treatment purposes. In these cases, tanked systems will be specified.  

We will look at the design of each of these in turn. 

Full infiltration permeable paving (Type A) 

In this system, the water passing through the permeable paving is collected and temporality stored within the sub base lined with geotextile membrane. Eventually, all the water will drain through the geotextile membrane into the ground. 

Partial infiltration permeable paving (Type B) 

If the permeability of the ground is insufficient to drain the water from the sub base at the required rate, a perforated collector pipe is introduced into the system design.  When the sub base below becomes full, the pipe will remove excess water to a different part of the drainage system. This ensures that the permeable system always has sufficient capacity to meet local water management requirements. 

Tanked (no infiltration) permeable paving (Type C) 

In this system, the sub base is lined with an impermeable lining rather than a geotextile membrane. This contains all the water collected within the sub base, so none is discharged into the ground. The water is taken away to secondary drainage or storage systems by outlet pipes. The diameter of the pipes will be dependent on drainage rates determined by specific site conditions.  

This system is used when the ground has insufficient permeability and could be destabilised by exposure to water. It is also used where water is required for re-use or because it contains pollutants. The outlet pipes can then direct the collected water to be stored or treated as required. 

Selecting the appropriate permeable system for a project can deliver design benefits. It is worth noting that when comparing the whole system cost of concrete permeable systems against traditional tarmac systems, the latter tends to be more expensive. This is largely because of the infrastructure required to support tarmac systems. 

Where can I learn more about the materials required when designing permeable paving? 

Contact Tobermore’s technical team for advice and guidance when designing concrete permeable paving. We have in-depth knowledge of the materials required to deliver successful permeable paving solutions. 

Concrete permeable paving systems offer a robust, attractive and economical option to traditional systems such as tarmac when looking to achieve SuDS compliance. 

Craig McBride, Tobermore’s Technical and Product Training Manager, comments: 

“We have an excellent range of concrete permeable paving products in vibrant long-lasting colours that create robust and attractive hard landscaping for our customers. We pride ourselves on ensuring that all the materials under the paved surface are correctly specified. Our technical team offer our customers a permeable paving design service. This details the materials needed to meet their project’s structural and drainage requirements in a cost-effective manner.”