The correct thickness of tarmac on a residential driveway is a specification detail that significantly affects how long the driveway performs before developing ruts, cracking, or surface deterioration. Understanding the recommended depths helps homeowners ensure contractors are proposing an adequate build-up.

Two-Layer Construction

A properly constructed tarmac driveway uses two layers of material applied over a compacted sub-base. The lower layer, known as the base course or binder course, provides the structural load distribution and is typically laid at a compacted depth of 50 to 60mm. The upper layer, known as the wearing course or surface course, provides the smooth finished surface and is typically laid at a compacted depth of 20 to 25mm. Together, the two tarmac layers have a total compacted depth of 70 to 85mm.

Some contractors apply a single layer of tarmac, which is a lower-specification approach that is acceptable for light use but does not provide the same durability under regular vehicle loading as a properly laid two-course construction.

The Sub-Base

The tarmac layers are laid on a compacted sub-base of MOT Type 1 granular material. For a residential driveway subject to car traffic, the sub-base depth is typically 100mm of compacted hardcore. The total construction build-up from formation to finished surface is therefore approximately 170 to 185mm, comprising 100mm sub-base, 50 to 60mm base course, and 20 to 25mm wearing course.

Applying Tarmac Over an Existing Surface

Where tarmac is being laid as an overlay on an existing sound surface, the total tarmac thickness can be reduced to a single wearing course layer of 25 to 40mm if the existing base is structurally adequate. This is a cost-effective approach for resurfacing a driveway that is structurally sound but superficially worn, and avoids the cost of full excavation and sub-base reinstatement.

Compaction Matters

Tarmac that is insufficiently compacted settles under vehicle loads and develops rutting and surface distortion. Both the sub-base and each tarmac layer must be properly compacted using a roller or plate compactor of appropriate weight for the layer thickness. A contractor who uses a hand roller or a light compactor is unlikely to achieve adequate compaction, particularly in the sub-base.

Summary

A tarmac driveway should be constructed with a base course of 50 to 60mm and a wearing course of 20 to 25mm, giving a total tarmac depth of 70 to 85mm over a 100mm compacted hardcore sub-base. Single-layer overlays on sound existing surfaces are acceptable at 25 to 40mm. Adequate compaction of each layer is as important as the specified depth for long-term performance. Always confirm layer thicknesses when comparing contractor quotes.