Sub-base material sits between the ground and the finished surface — whether that is a concrete slab, block paving, tarmac or a building floor. Getting the sub-base right prevents settlement, surface cracking and drainage problems. In domestic construction, the terms “hardcore”, “Type 1” and “scalpings” are often used interchangeably, but they are distinct products with different performance characteristics and the wrong choice will cause problems within a few years.
MOT Type 1 is the specification most often required by engineers and local authorities for driveways and paths. Understanding why — and when cheaper alternatives are acceptable — saves money without compromising quality.
What MOT Type 1 Is
MOT Type 1 (more formally, Clause 803 Type 1 granular sub-base material to the Highways England Specification for Highway Works) is a crushed rock or recycled concrete product with a tightly controlled grading: a well-graded mix from 0 to 63 mm particle size. The grading curve ensures that fine particles fill the voids between coarser ones, producing a dense, interlocking matrix that compacts well and resists deformation under load.
The key properties that make Type 1 the preferred sub-base:
- Well-graded — particles from dust to 63 mm ensure good interlock and minimal voids after compaction
- Angular aggregate — crushed faces key together; rounded gravel does not compact as effectively
- Fines content controlled — maximum 9% passing a 0.075 mm sieve (limits frost susceptibility and prevents softening when wet)
- CBR after compaction — California Bearing Ratio ≥ 80%, making it suitable as a direct sub-base under flexible pavements
Type 1 can be made from limestone, granite, gritstone or recycled concrete (RC6 specification). Recycled Type 1 is typically £5–£8/tonne cheaper and is acceptable for most non-adoptable domestic uses, but check with Building Control for applications under building slabs.
Hardcore vs Type 1 vs Scalpings
| Material | Grading | Compaction | Best Use | Cost (2026) |
|---|---|---|---|---|
| General hardcore (mixed rubble) | Uncontrolled | Poor (variable) | Bulk fill only | £18–£30/tonne loose |
| Recycled concrete hardcore | 0–100 mm mixed | Moderate | Non-structural filling | £20–£32/tonne loose |
| MOT Type 1 (primary) | 0–63 mm well-graded | Excellent | Driveways, paths, slab sub-base | £28–£45/tonne delivered |
| MOT Type 1 (recycled) | 0–63 mm well-graded | Excellent | As above, non-adoptable | £22–£38/tonne delivered |
| Scalpings (quarry waste) | 0–75 mm, variable fines | Moderate | Temporary access, farm tracks | £15–£25/tonne |
| Type 2 (natural gravel) | 0–63 mm, rounded | Good | Capping layer, not a substitute for Type 1 | £22–£35/tonne |
General hardcore — broken brick, concrete rubble from demolition — is suitable only for bulk void filling where structural performance is not required. It cannot be relied on to compact uniformly or achieve a consistent bearing capacity. Never use it directly under a ground-bearing slab or as the sole sub-base for a driveway.
Scalpings are quarry by-products with a high proportion of fines. They compact reasonably well but can be frost-susceptible and may pump (fine particles migrate upward under repeated loading in wet conditions). They are acceptable for farm tracks and temporary access roads; not ideal for permanent domestic hard standings.
Compaction: Depth and Method
Compaction is the most critical factor in sub-base performance. Uncompacted material will settle, causing surface cracking, rocking pavers and ponding water.
Layer thickness for compaction:
- Maximum compacted layer depth: 150 mm (for plate compactors commonly available for hire)
- Larger vibrating rollers (≥ 1.5 t dead weight) can compact 200–250 mm layers but are rarely available on domestic sites
- Always compact in 150 mm layers; if total required depth is 300 mm, place and compact 150 mm, then place and compact a second 150 mm layer
Compaction equipment:
| Equipment | Compaction Depth | Hire Cost (day) | Best For |
|---|---|---|---|
| Plate compactor (80 kg) | 100–125 mm | £45–£65 | Tight areas, final passes |
| Plate compactor (300 kg) | 150 mm | £80–£120 | Driveways, general sub-base |
| Vibrating roller (1 t) | 150–200 mm | £150–£250 | Larger areas, tarmac prep |
Run the plate compactor in overlapping passes until there is no visible movement in the surface — typically 4–6 passes for Type 1. Check with a steel rod: if it penetrates more than 25 mm by hand pressure, the material needs more compaction or is too wet.
Do not compact material that is saturated — wet fines will pump rather than interlock. If weather is consistently wet, cover laid material with polythene sheeting overnight.
Depths Required
The required depth of compacted Type 1 depends on the applied load and the bearing capacity of the subgrade:
| Application | Compacted Type 1 Depth | Notes |
|---|---|---|
| Pedestrian path | 75–100 mm | On firm natural ground |
| Domestic driveway (cars) | 100–150 mm | Min 100 mm on good ground; 150 mm on clay |
| Driveway (vans/HGV occasional) | 200–250 mm | Two 100–125 mm compacted layers |
| Ground-bearing slab sub-base | 150 mm min | As per Part C and NHBC; 200 mm on weak ground |
| Adoptable highway sub-base | 225–350 mm | To Highways England specification |
For a domestic garage floor slab, NHBC Chapter 5.1 and Building Regs Part C require a minimum 100 mm of compacted hardcore (Type 1 preferred) above formation level, with the slab cast on a DPM above.
Cost Per Tonne and Bulk Bag
Prices vary significantly by region and by delivery quantity:
| Supply Format | Typical Cost (2026) |
|---|---|
| Loose tipped (6–20 tonne wagon) | £28–£45/tonne delivered |
| Bulk bag (approximately 850 kg) | £65–£95 per bag |
| Small bags (25 kg, builders’ merchant) | Not typically stocked for Type 1 |
Coverage: 1 tonne of Type 1 compacted to 150 mm depth covers approximately 4.5–5.0 m² (density approximately 2.0–2.1 t/m³ loose, compacted to approximately 2.1–2.2 t/m³). A 40 m² driveway at 150 mm compacted depth needs roughly 9 tonnes.
Bulk bags are convenient for small jobs or restricted access sites (suburban side return, rear garden via gate) but the per-tonne cost is 2–3× higher than loose. For anything over 10 m², a loose-tip delivery from an aggregate supplier is significantly cheaper.
Uses in Building Construction
Under a ground-bearing floor slab, the compacted Type 1 or hardcore layer serves two functions: it provides a stable working platform for casting concrete, and it distributes point loads from the slab into the subgrade. Building Regs Approved Document C requires protection against ground moisture, which is why the hardcore layer is always topped with a DPM before the slab is cast.
For driveways in Sustainable Drainage (SuDS) compliant schemes (required in England for driveways over 5 m² since the Town and Country Planning Act amendment), permeable Type 1 (open-graded, 10–40 mm clean stone) can replace standard dense-graded Type 1 in the sub-base, allowing water to infiltrate rather than run off.
Key Standards and References
- Clause 803, Specification for Highway Works (MCHW Vol 1) — the defining specification for Type 1 granular sub-base
- BS EN 13242:2002+A1:2007 — aggregates for unbound and hydraulically bound materials
- Building Regs Approved Document C — site preparation and resistance to moisture; sub-base under floor slabs
- NHBC Standards Chapter 5.1 — floors (sub-base depth requirements)
- BRE Digest 251 — assessment of damage in low-rise buildings; settlement from poor sub-base among causes cited
