Why You Shouldn’t Use Road Planings For Patio or Driveway Base
A question frequently raised on social media concerns the use of road planings — more correctly termed asphalt arisings — beneath patio and driveway constructions. These are scraped or milled materials removed from old road surfaces during resurfacing works. Because they are often cheaper than Type 1 scalpings, they are sometimes promoted as a substitute sub-base material.
We would advise against substituting Type 1 scalpings with road planings for several important reasons.
Although their use is technically permitted under Clause 807 of the Specification for Highway Works (SHW) Series 800 in the UK, this is subject to strict compliance with grading, contamination, and testing requirements. These SHW testing criteria are rightly onerous and are rarely applied to domestic landscaping projects.
Frost Susceptibility & Moisture Retention
Road planings often contain a high proportion of fines. These fine particles can trap moisture within the material, reducing permeability and increasing frost susceptibility.
Although bitumen is hydrophobic in nature, the overall composition of planings can still retain water within the matrix. Where moisture becomes trapped, freeze–thaw cycles can lead to rapid deterioration of both the sub-base and the overlying paving.
Issues With Compaction
The irregular particle shape of reclaimed planings makes effective compaction difficult. When dry, the material can resist consolidation. Moisture content must be near optimum to achieve satisfactory compaction — something that is difficult to control on typical domestic sites.
Poor compaction increases the risk of movement, settlement and failure. In some cases, the material may behave as a ‘mobile’ layer due to its inconsistent structure.
Reclaimed Asphalt Pavement (RAP)
Where Reclaimed Asphalt Pavement (RAP) is recycled properly — for example, re-crushed and graded for use as Type 4 aggregate — it must be:
- Carefully processed
- Free from tar contamination
- Suitably graded
- Capable of achieving adequate compaction
- Free from excessive oiliness or moisture
It should not be used in environmentally sensitive areas where hydrocarbon leaching could pose a risk.
If not stringently processed, excessive fines can encourage capillary rise, allowing moisture to migrate upward into paving layers, accelerating freeze–thaw damage.
Compliance with British Standards (BS 7533)
It is important to make clear that road planings do not comply with the requirements of BS 7533, the British Standard series governing the design and construction of pavements using modular paving units.
BS 7533 places strong emphasis on:
- Properly graded and tested sub-base materials
- Frost resistance and permeability characteristics
- Load-bearing capacity and long-term stability
- Suitability for the intended pavement classification
Typical domestic driveways and patios constructed in accordance with BS 7533 would require a well-graded, certified sub-base material such as MOT Type 1 or Type 3 aggregate, laid and compacted in controlled layers to achieve the required performance standards.
Road planings (asphalt arisings), unless fully processed, re-graded, laboratory-tested and certified to meet the relevant sub-base specification, do not satisfy BS 7533 compliance. Their variable grading, residual bitumen content, high fines proportion and unpredictable compaction characteristics make them unsuitable as a direct substitute for compliant sub-base materials.
In the event of structural failure, referencing BS 7533 would be difficult if road planings had been used, as the installation would fall outside recognised industry best practice.
Unsuitable for Permeable Construction
Road planings should not be used beneath permeable paving systems. Residual bitumen can clog voids within the material, reducing permeability and compromising drainage performance.
Limited Acceptable Uses
If considered at all, road planings may be used as:
- A capping layer beneath a properly specified sub-base membrane
- Temporary site roads (removed upon completion)
- Backfilling behind retaining walls in non-critical, non-permeable applications
In colder regions such as parts of Scotland, any frost-susceptible capping layer may need to sit significantly below finished ground level depending on ground conditions and frost depth.
Professional Risk
Any paving failure arising from the use of road planings would be difficult to defend. They are not generally recommended for use beneath patio or driveway constructions where long-term structural performance is required.
In short, short-term savings can result in long-term liability.
