Calculating the correct amount of Type 1 sub-base is essential for any groundworks project. This guide shows you exactly how to calculate quantities, understand compaction, and avoid costly mistakes.
What is Type 1 Sub-Base?
Type 1 sub-base (also called MOT Type 1 or roadbase) is a crushed aggregate material used as the foundation layer for:
- Driveways and car parks
- Roads and pathways
- Concrete slabs and bases
- Block paving installations
- Industrial hardstandings
It consists of crushed limestone or granite with particle sizes from dust up to 40mm, creating a dense, stable base when compacted.
Standard Depth Requirements
The depth of Type 1 you need depends on the application and expected loads:
| Application | Minimum Depth | Recommended Depth |
|---|---|---|
| Garden paths | 75mm | 100mm |
| Domestic driveways | 100mm | 150mm |
| Car parks (cars) | 150mm | 200mm |
| Commercial traffic | 200mm | 250mm |
| HGV/industrial | 250mm | 300mm+ |
How to Calculate Type 1 Quantities
The basic calculation is straightforward, but you must account for compaction:
Formula:
Volume (m³) = Length (m) × Width (m) × Depth (m) × Compaction Factor
Example: 10m × 5m × 0.15m × 1.25 = 9.4m³
Step-by-Step Calculation
Project: Domestic driveway, 10m × 5m, 150mm depth
- Calculate area: 10m × 5m = 50m²
- Convert depth to meters: 150mm = 0.15m
- Calculate base volume: 50m² × 0.15m = 7.5m³
- Apply compaction factor: 7.5m³ × 1.25 = 9.375m³
- Add wastage (10%): 9.375m³ × 1.1 = 10.3m³
- Round up: Order 11m³ (typical lorry load increments)
Understanding Compaction
Type 1 sub-base must be compacted to achieve proper density and stability. Loose material compacts by approximately 20-25%.
Compaction Factors
- Loose to compacted: Multiply by 1.20-1.25
- Compacted to loose: Multiply by 0.80-0.83
Example: You need 10m³ compacted. Order: 10m³ × 1.25 = 12.5m³ loose material.
💡 Pro Tip: Always use a compaction factor of 1.25 for safety. Running short on site costs more than ordering slightly extra!
Layering for Deep Sections
For depths over 150mm, install Type 1 in layers for proper compaction:
- Maximum layer thickness: 150mm loose (120mm compacted)
- Compact each layer before adding the next
- Total depth = sum of compacted layers
Example: 300mm Total Depth
- Spread 150mm loose Type 1
- Compact to approximately 120mm
- Spread second 150mm layer
- Compact to 120mm
- Total compacted depth: 240mm (add more if needed to reach 300mm)
Weight vs Volume
Type 1 is sold by weight (tonnes) or volume (m³):
- Loose density: 1.6-1.8 tonnes/m³
- Compacted density: 2.0-2.2 tonnes/m³
Quick Conversions
Volume to Weight (loose material):
- 1m³ = approximately 1.7 tonnes
- 10m³ = approximately 17 tonnes
- 20m³ = approximately 34 tonnes
Weight to Volume (loose material):
- 1 tonne = approximately 0.6m³
- 10 tonnes = approximately 6m³
- 20 tonnes = approximately 12m³
Poor Ground Conditions
Increase sub-base depth on poor ground:
| Ground Condition | Standard Depth | Increase By |
|---|---|---|
| Firm clay or sand | 150mm | 0mm (no change) |
| Soft clay | 150mm | +50mm (total 200mm) |
| Very soft/organic | 150mm | +100mm+ (total 250mm+) |
Consider using geotextile membrane on very soft ground to prevent Type 1 sinking into subgrade.
Cost Estimation
Type 1 sub-base costs vary by region and supplier:
Typical UK Prices (2026)
- Small loads (1-5 tonnes): £20-25 per tonne
- Bulk loads (10+ tonnes): £15-18 per tonne
- Delivery: £40-80 depending on distance
Example Cost Calculation
Project: 10m × 5m driveway, 150mm depth = 11m³ required
- Volume: 11m³
- Weight: 11m³ × 1.7 = 18.7 tonnes (say 19 tonnes)
- Material cost: 19t × £16/t = £304
- Delivery: £60
- Total: £364
Installation Tips
Ground Preparation
- Remove topsoil and vegetation completely
- Excavate to required depth plus sub-base thickness
- Level and compact subgrade
- Install edge restraints (kerbs, boards)
Laying Type 1
- Spread evenly in 150mm layers maximum
- Compact with vibrating plate (minimum 200kg)
- Add water if too dry (helps compaction)
- Check levels regularly as you go
- Fill any soft spots before continuing
Compaction Equipment
- Small areas (up to 50m²): Wacker plate (200-300kg)
- Medium areas (50-200m²): Vibrating roller
- Large areas (200m²+): Ride-on roller
Calculate Type 1 Automatically
Use our free groundworks calculator to work out exact Type 1 quantities including compaction, layers, and costs.
Try Type 1 Calculator →Common Mistakes to Avoid
- Forgetting compaction factor: You'll run 20-25% short
- Layers too thick: Won't compact properly, leading to settlement
- No edge restraint: Material spreads out over time
- Poor subgrade preparation: Type 1 sinks into soft ground
- Skipping wastage allowance: Small shortfalls cause delays
Quality Checks
Ensure proper installation:
- Visual inspection: Should look dense, no loose pockets
- Foot test: Minimal movement when walking on it
- Level check: Within 10mm tolerance
- Compaction test: No visible deflection under roller
Summary
Key takeaways for Type 1 sub-base:
- Use 100-150mm for domestic driveways, 200mm+ for commercial
- Always apply 1.25 compaction factor to loose volume
- Add 10% wastage to final calculation
- Install in 150mm layers maximum for proper compaction
- 1m³ loose = approximately 1.7 tonnes
- Increase depth 50-100mm on poor ground
Professional Groundworks Calculations
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