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MOT Type 1 Sub-Base Calculator

Before laying tarmac, you need a rock-solid foundation. Calculate the exact tonnage or bulk bags of granular sub-base required for your paving project.

150mm is standard for domestic driveways. 100mm for footpaths.

Sub-Base Estimate

Enter your driveway area and base depth to calculate sub-base aggregates.

The Hidden Engine of Your Driveway

  • Load Distribution: Tarmac is technically a flexible material. Its core job is to provide a smooth, weather-proof wearing course. The actual weight of your parked car is borne almost entirely by the compacted sub-base beneath it. Without an adequate sub-base, the tarmac flexes into the dirt and breaks apart.
  • Compaction Rules: You cannot lay 150mm of MOT Type 1 loose and compact it all at once with a small whacker plate; the energy won't reach the bottom. It should be laid and compacted in thin layers (called 'lifts') of 75mm max to achieve optimal density.
  • Weed Membranes: It is highly recommended to lay a woven geotextile membrane across the bare soil *before* tipping your sub-base. This prevents the heavy stone from sinking into soft mud over time and adds considerable tensile strength to the foundation structure.

Frequently Asked Questions

What is MOT Type 1?

MOT Type 1 is a heavily graded aggregate (usually crushed limestone, granite, or concrete) ranging from 40mm down to dust. When heavily compacted, the dust binds the varying stone sizes together to create an incredibly strong, interlocking, load-bearing layer.

How thick should a sub-base be for a tarmac driveway?

For standard domestic vehicular traffic (cars and light vans), the sub-base should be a minimum of 150mm thick once fully compacted. If the ground is particularly soft (like clay) or the driveway will see heavy-duty delivery vehicles, 200mm to 225mm is recommended.

Why do I need to calculate compaction shrinkage?

Loose MOT Type 1 contains a lot of air between the stones. When you run a heavy vibrating roller over it, the stones interlock and compress. The level will typically drop by about 20% to 25%. You must order enough loose material to achieve your final, desired compacted depth.

Can I use shingles or gravel instead?

No. Clear gravel or shingles 'roll' against each other (they have no 'fines' or dust to bind them) and will not compact into a solid structural base. If you lay tarmac over loose gravel, the driveway will fail instantly.

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