Sub-base or capping? Specifying Type 1, 6F2 and 6F5 correctly

Sub-base and capping do different jobs

A sub-base is the load-spreading layer directly beneath your surfacing. MOT Type 1 (granular sub-base to the Specification for Highway Works, Clause 803) is the UK default: crushed rock graded 0–63 mm, so the fines lock the coarse stone together and compact to a near-impermeable, load-bearing platform.

Capping sits below the sub-base and exists to protect weak subgrade. Where the CBR of the formation is low — soft clay, made ground, wet silts — capping bridges it so the sub-base can be compacted at all. On strong ground capping is unnecessary; on poor ground, skipping it means pumping and rutting no matter how much Type 1 you tip on top.

A quick rule of thumb: if a loaded lorry leaves visible ruts on the stripped formation, you need capping before sub-base. If the formation stays firm under plant, Type 1 alone is usually the correct spec.

6F2 vs 6F5: the practical difference

Both 6F2 and 6F5 are capping materials to SHW Clause 613, and both are recycled — crushed concrete, brick and other inert demolition arisings screened to a capping grading. The difference is source and consistency.

6F2 is the coarser, more traditional capping grading (typically up to 125 mm), produced from crushed concrete and hardcore. It is the material most commonly specified where a designer has named a capping layer on the drawings.

6F5 is produced from recycled asphalt planings and crushed material, generally with a tighter, blacker grading. It compacts to a very firm, wearing platform, which is why it is popular for temporary haul roads, compound areas and site accesses as well as designed capping.

If a specification names one of them, supply that one — they are not automatically interchangeable on adopted or highways work. For temporary works, either will usually do the job and availability tends to decide it.

Layer thicknesses that actually get used

Foot-traffic paths and patios: 100 mm compacted Type 1, no capping on competent ground.

Domestic driveways: 150 mm compacted Type 1. On clay, add a geotextile separator to stop fines migrating up into the sub-base.

Estate roads, compounds and yards: 150–250 mm Type 1 over 250–600 mm capping, thickness driven by the design CBR. This is where a soils report earns its keep.

Temporary haul roads: 200–300 mm of 6F5 or 6F2 straight onto a geogrid, re-graded as it wears rather than replaced.

Compact everything in layers of no more than 200–250 mm, and damp coarse material down in dry weather so the fines bind rather than blow off.

Recycled vs primary: when each is right

Recycled Type 1 (crushed concrete to Clause 803) performs comparably to primary limestone or granite on most driveways, car parks and estate roads, and carries a much lower embodied-carbon figure — useful evidence for BREEAM and planning conditions.

Primary Type 1 is still the safer choice where a client or highways authority requires a certificate of conformity against a named quarry source, or where the surfacing is sensitive to variability.

Recycled capping (6F2/6F5) is effectively the standard — very little capping on UK sites is primary material, and there is no performance reason for it to be.

Ordering so the load arrives right

Give the material and clause reference, not just a nickname — "Type 1 to Clause 803" and "6F2 capping to Clause 613" remove ambiguity between suppliers.

Give the tonnage and the compacted depth you are building to, so the volume can be sense-checked before the lorry leaves. Our aggregate calculator does the arithmetic if you have area and depth.

Give the access: whether an 8-wheel tipper can get in and raise its body, whether a grab has to reach over a boundary, or whether the plot only takes bulk bags.

Say if muck is going out as well as material coming in — a two-way load takes your arisings to a licensed facility and returns loaded, which is one vehicle movement instead of two.

Atlas Stone Supplies supplies Type 1, 6F2 and 6F5 nationwide. Request a quote with your postcode, tonnage and access and we will come back within one working day.