Materials · Practical advice
Type 1 and open-graded sub-bases: why the difference matters
Understand the supporting stone beneath paving and why drainage changes the specification.
What the sub-base does
The sub-base is part of the supporting construction beneath the finished surface. It helps provide a stable platform and distribute loads, but its performance depends on the ground below, the material and how it is placed. It is not simply somewhere to hide leftover rubble. The proposed construction should suit the paving system and the vehicles or other loads that will use it.
Why dense-graded material is used
Type 1 is a graded crushed aggregate used for sub-base construction. Its range of particle sizes allows the material to compact into a dense layer. That can suit conventional paved construction where specified. Ask the contractor to identify the material rather than using “hardcore” as the only description. The name, source and intended application help make the quotation meaningful.
Why permeable work needs a different approach
A permeable pavement needs space for water to travel through and, where designed, be stored temporarily beneath the surface. The sub-base specification therefore has a hydraulic role as well as a structural one. A dense material cannot automatically replace the specified open-graded aggregate. The whole build-up, including bedding and joints, needs to match the selected permeable system.
Do not treat Type 3 as a universal answer
An open-graded product may be suitable for a particular design, but a label alone does not establish compliance with every paving manufacturer's requirements. Ask for the specified grading and system detail. Changing one layer while retaining unsuitable bedding or jointing can defeat the intended water route. Follow the complete design rather than substituting a familiar material by appearance.
Installation still matters
A suitable material can perform poorly if it is contaminated, placed badly or used over unresolved weak areas. Agree how the existing ground is assessed and how the construction is checked as work progresses. Layer thickness and compaction arrangements are project-specific. A photograph of a tidy pile of aggregate does not demonstrate the condition of the completed supporting layers.
What to ask before accepting a quote
Ask which sub-base is proposed, why it suits the surface and how water is intended to behave. For an overlay, ask which existing layers are being relied upon. Retain the specification and delivery information where available. This is particularly useful if later work exposes the construction and you need to understand what was originally intended.
Can recycled aggregate be suitable?
Yes, a suitably specified and controlled recycled product can have a legitimate construction use. That is different from accepting unspecified mixed rubble. Ask for evidence that the supplied material meets the agreed specification.
Your project checklist
- Identify the sub-base product.
- Match it to the paving and drainage design.
- Avoid unapproved material substitutions.
- Keep the agreed construction specification.
Further reading
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