When installing ground or grass paving systems like Bodpave 85 or Bodpave 40, one of the most common questions is: “Do I need hardcore underneath?”. Whether you use hardcore is dependent on the ground conditions and intended use.
Understanding the role of the sub-base
Before deciding whether hardcore is needed, it helps to understand how paving systems work. As Bodpave 85 and 40 are permeable paving grids, they’re designed to reinforce grass or gravel surfaces, prevent surface damage, distribute weight and maintain natural drainage. However, the grid itself is not the structural foundation. While the grid helps to spread the weight across the surface, the sub-base provides the strength. If the ground beneath cannot support a heavy load, a grid system alone wouldn’t be able to support the load either.
The difference between Bodpave 85 and Bodpave 40
Bodpave 40 and 85 are both cellular permeable grids and install the same, however their intended load bearing differs. Therefore, their sub-base requirements can vary.
Bodpave 85 is designed for heavy-duty applications, such as driveways, parking spaces and access roads. Whereas, Bodpave 40 is more suitable for lighter-duty applications, such as pedestrian pathways and overflow car parks.
When hardcore is needed
A hardcore sub-base is typically required when the ground is soft, as weak or poorly draining soils lack load-bearing capabilities. So, without a sub-base, rutting is likely. If an area is likely to deal with car loads regularly, then they will require a stable foundation to distribute wheel loads effectively. To keep a surface level and stable over time, sufficient load bearing capacity and sub-base is essential. A layered build-up including a geotextile membrane and sub-base would be best practice.
Typical installation build-up
The exact specifications of an installation should always reflect the site conditions and loading requirements, most Bodpave installations follow a layered build-up designed to provide stability, permeability and long-term implementation.
1. Existing ground
Before installing any layers onto the existing ground, it’s important to:
- Remove all vegetation and roots
- Excavate to the required formation level
- Ensure the ground is level and properly compacted
- Identify soft spots and excavate/replace if necessary
If the subgrade is clay-heavy, waterlogged or unstable – a structural sub-base will most likely be required as the strength of the entire system depends on the quality of the existing ground.
2. Geotextile separation membrane
A permeable geotextile membrane is typically laid directly over the existing ground. Its purpose is to:
- Prevent sub-base material migrating into the soil
- Improve load distribution
- Maintain permeability
- Reduce long-term settlement
This layer is particularly important on softer soils, as it prevents intermixing of materials that can weaken the structure over time.
3. Sub-Base (Hardcore)
Where structural support is necessary, a compacted sub-base is installed. Typically MOT Type 3 or similar permeable granular material that is laid in layers (usually 100 up to 150mm lifts) and mechanically compacted in stages. The required depth depends on the soil bearing capacity, frequency and weight of traffic, and project design requirements. As a general guide:
- Light-duty pedestrian use: May require minimal or no sub-base (subject to ground conditions)
- Domestic driveways: Often 150 to 250mm compacted depth
- Heavier traffic or weak soils: Greater depths may be required
The sub-base provides the structural strength that prevents rutting and settlement under load.
4. Bedding layer
A bedding layer is installed above the sub-base to provide a level surface, allow for accurate placement of the grid and to assist with the load distribution. This is typically sharp sand or fine graded aggregate, screeded to level and lightly compacted. The bedding layer should not be excessive in depth as its role is levelling, not structural support.
5. Bodpave grid installation
The selected grid, either Bodpave 85 or Bodpave 40, is then installed over the prepared bedding layer. Installation considerations include; interlocking panels securely, ensuring full ground contact, cutting panels to fit edges where necessary and installing edge restraints where specified. Proper alignment and contact with the bedding layer is critical to avoid movement.
6. Infill material
The grid is then filled depending on application. For grass ground reinforcement: quality free-draining topsoil, seeded and irrigated and maintained during establishment. For gravel applications; clean, angular gravel and evenly distributed and brushed into cells. The infill works with the grid to provide surface stability and load distribution.
Why layer depth matters
One of the most common installation mistakes is under-specifying the sub-base depth. Even with a high-performance grid system, insufficient groundwork can lead to surface rutting, uneven settlement, poor drainage and reduced lifespans. It’s always safer to assess soil classification and long-term performance expectations. Increasing structural depth provides additional resilience.
Do you need hardcore under Bodpave 85 or Bodpave 40?
If the area will carry vehicles or the ground is weak, a hardcore layer will help to stabilise the ground, allow for proper water drainage, and protect the ground from car tyres. However, if the soil is firm and the application is light-duty, then hardcore isn’t necessarily required.
The most important factor to consider isn’t the grid, it’s the ground beneath it. Investing in proper ground preparation ensures long-term performance, reduced maintenance, prevention of rutting and settlement and compliance with best practice installation standards.
If you’re unsure, a simple site assessment of soil condition and intended use will quickly determine whether a hardcore sub-base is required. Getting the foundation right ensures the permeable paving solutions perform as intended. If you’re unsure or need further guidance, the expert team at Grassmats is more than happy to help. Contact us today.
