Types of Tarmac: SMA, HRA, Bitmac and Porous Asphalt in Plain English

"Tarmac" is a catch-all word. On site and in the standards, there are several distinct asphalt mixes, and a driveway uses at least two of them in different layers. SMA (stone mastic asphalt) is a tough surface mix built around a skeleton of interlocking stone; hot rolled asphalt (HRA) is a dense mortar-like mix with chippings rolled into the top; bitmac (now called asphalt concrete) is the mix most domestic drives are laid in; and porous asphalt lets water drain straight through. For a typical house drive, a dense asphalt concrete binder course with a fine 6mm surface course is the usual choice.

Why are there different types of tarmac?

Each mix is designed for a job. Some have to resist heavy lorries rutting the surface in summer, some have to drain, some have to give grip at speed, and some have to give a smooth, tight finish on tarmac driveways. Since 1 January 2008, UK asphalt has been specified under the BS EN 13108 series of standards, which groups mixes by type: asphalt concrete in Part 1, hot rolled asphalt in Part 4, stone mastic asphalt in Part 5 and porous asphalt in Part 7.

A note on names: modern "tarmac" contains no tar. The Asphalt Industry Alliance says tar has not been used in asphalt production since the 1980s; the binder is bitumen.

Base, binder and surface courses

A tarmac drive is built in layers, each with its own mix:

• Sub-base. Not tarmac at all: compacted crushed stone, usually MOT Type 1, at least 100mm on a domestic drive.

• Base course. A thick structural asphalt layer used on roads and heavy-duty areas. Most domestic drives do not need a separate base course; the sub-base and binder course do that job.

• Binder course. The load-bearing asphalt layer, typically around 50mm on a drive, using 20mm aggregate. Pavingexpert says it should be at least 40mm and preferably 50mm thick.

• Surface course. The top layer you see and drive on, typically 20-30mm thick, using 6mm or 10mm aggregate.

The binder course is often called the base course on driveways, which causes confusion. What matters is that there is a proper structural layer under the surface, not just one thin layer of fine material. Each layer is spread and rolled separately, which is why how a tarmac driveway is laid matters as much as which mix is used.

What is bitmac (asphalt concrete)?

Bitmac is short for bitumen macadam: crushed stone graded from coarse to fine and coated in bitumen. When the European standards arrived in 2008, the term macadam was dropped from the specifications and replaced with asphalt concrete, written AC. A name like AC 20 dense bin means an asphalt concrete with a 20mm maximum stone size, dense grading, used as a binder course; NBS gives the example that AC 20 dense bin replaced the old "20mm DBM" binder course.

For driveways, asphalt concrete is the workhorse. A dense binder course with a fine dense surface course on top is what most domestic tarmac drives are made of, and it is what Pavingexpert's typical driveway specification describes.

What is SMA tarmac?

Stone mastic asphalt (SMA) is a gap-graded mix: lots of coarse stone that locks together stone-on-stone, with the gaps filled by a rich mastic of bitumen, filler and fibres. The fibres stop the bitumen draining out of the mix while it is hot. The stone skeleton makes SMA very resistant to rutting and deformation, which is why it is widely used on busy roads.

On a drive, a fine SMA surface course gives a tight, durable, slightly textured finish. It is a good surface where vehicles turn a lot or where vans and heavier vehicles park, though it can cost more than a standard dense surface course and depends on what local plants produce.

What is hot rolled asphalt (HRA)?

HRA is a dense, sand-rich mix with relatively little coarse stone. On its own the surface would be too smooth for grip at speed, so pre-coated chippings are spread over the hot surface and rolled in. Pavingexpert notes HRA wearing courses are usually around 40mm, laid by machine, and mostly used on roadways. Coloured chippings can be rolled into HRA for decorative finishes.

For a domestic drive HRA is rarely the first choice: it is designed for road traffic and machine laying, and the chipped finish is coarser than most homeowners want.

What is porous asphalt?

Porous asphalt is an open-graded mix with connected voids, so rainwater drains through the surface into a permeable sub-base below. It is covered by BS EN 13108-7. Because it is permeable, it is one of the surfaces gov.uk guidance lists for front gardens in England, where a new or replacement non-permeable drive over 5m² would otherwise need planning permission or drainage onto your own land.

It needs a different build-up: an open-graded sub-base such as Type 3 or 4/20 stone rather than standard MOT Type 1, because water soaks through compacted Type 1 far more slowly. The same guidance notes porous asphalt should be supplied and laid by a specialist company, and the surface needs occasional cleaning so the voids do not clog.

6mm vs 10mm tarmac surface

• 6mm surface course. Finer and tighter, with a smoother finish. The usual choice for driveways, paths and patios.

• 10mm surface course. A coarser, more open texture than 6mm, with the same structural layers needed underneath.

Pavingexpert describes dense surface courses using either 6mm or 10mm aggregate, at least 20-25mm thick when rolled, and its typical driveway specification uses 25mm of 6mm surface course.

Which type of tarmac is best for a driveway?

For most homes: a compacted MOT Type 1 sub-base, a dense asphalt concrete binder course of around 50mm with 20mm stone, and a dense 6mm surface course of 20-30mm. That is the typical domestic build-up, laid on a properly dug-out and compacted base.

Consider SMA for the surface where heavy vehicles park or turn. Consider porous asphalt if you need a permeable front drive and prefer a tarmac finish to resin. Leave HRA to the roads.

Types of tarmac questions, answered

Does the type of tarmac change the price?

Yes, somewhat. Specialist surfaces such as SMA, porous or coloured mixes usually cost more than a standard dense surface course, but the size of the drive and the groundwork move the price far more. Typical UK tarmac driveway cost ranges are driven mainly by area, dig-out and access.

Is asphalt the same as tarmac?

In the UK, yes in everyday use. Asphalt is the technical word for bitumen-bound stone mixes; tarmac is the name most people use for them.

Can you lay a single layer of tarmac?

Some contractors lay a single thick layer on light-use areas. For a drive, two layers (binder and surface) are the norm, because each mix does a different job.

Still deciding?

If you are comparing quotes that name different mixes, Pete can explain what each one means for your drive. We have laid tarmac around Wolverhampton for over 25 years, and you can ask for a free quote with the mix spelled out.