Tarmac Driveways: Sub-Base Depth, Edging, and Why Preparation Is Everything
A tarmac drive laid on a poor sub-base will crack and sink within two years. Here is what proper preparation looks like.
Tarmac (more correctly, bitumen macadam to BS 4987) remains the most cost-effective driveway surface in the UK. A properly laid tarmac drive on a good sub-base will last 15 to 20 years before it needs attention. A drive laid on inadequate preparation will show ruts, cracks, and puddles within two winters. The difference is entirely in what happens before the tarmac goes down.
Sub-Base Specification
We excavate to a minimum of 200mm below the finished surface level. The sub-base is 150mm of compacted MOT Type 1 (a graded crushed stone to BS EN 13285), laid in two lifts and compacted with a vibrating roller. On areas that will take regular vehicle turning (near the road entrance, for example), we increase the sub-base to 200mm. If the existing ground is soft or waterlogged, a geotextile membrane goes down first to prevent the aggregate migrating into the subsoil.
Edging and Restraint
Tarmac needs a solid edge to push against, otherwise it will creep outward and crack along the margins. We use either a concrete edging kerb (haunched with C20 concrete) or a brick border. Brick edging using engineering bricks bedded on concrete adds a smart detail that lifts the whole driveway.
Drainage and Planning
Since 2008, under the GPDO amendments, any impermeable surface over 5m2 requires either permeable surfacing or drainage to a soakaway within your property. Standard dense tarmac is impermeable, so we either specify a porous macadam (more expensive, slightly rougher finish) or ensure the drive falls towards a lawn, border, or drainage channel. See our driveway planning permission guide for the full rules.
View our driveway services or call Herriott Services LTD on 07887 895382 for a free survey and quotation.