A properly designed and constructed sub-base is essential for the long-term performance of any driveway, patio, or pathway. It provides structural stability, prevents movement and cracking, and ensures that surface water drains away effectively. In the UK, the term sub-base refers to a compacted layer of granular material — typically Type 1 MOT (graded crushed aggregate base) — placed beneath the bedding layer that supports the paving slabs or blocks.
The sub-base distributes loads evenly and prevents settlement, especially in areas exposed to traffic or heavy weight.
Depth and Material Selection
The required depth of the sub-base depends on its use and the underlying soil conditions. All measurements are compacted thicknesses. For driveways, Type 1 MOT (graded crushed
aggregate) or Type 3 MOT (open-graded aggregate) is typically specified. Type 3 provides superior drainage for permeable systems, while Type 1 remains the standard for impermeable installations.
The material should comply with BS 7533-101:2021, which defines performance and compaction standards for pavement construction.
Application : Pathway
Type Sub-Base Depth : 75–100 mm
Example Load : Pedestrian
Bedding Layer : 30–40 mm mortar or sand bed
Application : Patio
Type Sub-Base Depth : 100–150 mm
Example Load : Light foot traffic
Bedding Layer : 30–50 mm mortar or sand bed
Application : Driveway
Type Sub-Base Depth : 150–200 mm
Example Load : Vehicles (up to 3.5 tonnes)
Bedding Layer : 30–50 mm mortar or grit bed
Soil Conditions and Preparation
The performance of the sub-base depends heavily on the quality of the underlying soil (subgrade).
Different soil types require different preparation: Clay soils (expansive soils) retain water and expand during wet periods, so drainage and a geotextile barrier such as Ceram (non-woven filtration fabric) are essential to prevent migration of fine particles. Sandy soils (coarse-grained soils) drain freely but may shift under load, requiring compaction and edge restraint. Silty soils benefit from reinforcement using a stabilisation grid (geogrid) to prevent sub-base displacement. All organic material and loose soil should be removed before compaction of the subgrade using a vibrating plate or roller compactor.
Barrier Cloths and Geotextiles
A barrier cloth (geotextile membrane) such as Ceram or Terram (geotextile separator) is often installed between the sub-base and the natural ground. It prevents contamination of the sub-base, improves drainage, and reduces pumping and settlement. For driveways on soft ground, a reinforcement geogrid (tensar-type stabilisation mesh) may be used within the sub-base to improve load distribution.
Compaction and Levels
Each layer of the sub-base should be compacted in 50 mm increments using a vibrating plate compactor (plate tamper) or roller. Proper compaction prevents future settlement.
Finished levels should allow for a fall of 1:60 to 1:80 away from buildings for drainage.
Summary
A well-constructed sub-base forms the foundation of any successful paving project. Driveways typically require 150–200 mm of Type 1 MOT (graded crushed aggregate base), patios 100–150 mm, and pathways 75–100 mm. Using a geotextile separator or stabilising grid helps ensure long-term performance, particularly on poor soils. Always follow BS 7533 or equivalent good practice guidelines for compaction, materials, and drainage.