Laying a macadam driveway is one of the most durable paving solutions available to homeowners seeking a robust, long lasting surface that can handle vehicle traffic year after year. Whether you are building a new driveway from scratch or resurfacing an existing worn out area, understanding the full construction process from excavation through to the final wearing course is essential for achieving a professional result. Macadam, also known as tarmacadam or simply tarmac, has been used in road building since the early nineteenth century and remains a popular choice for driveways because of its strength, weather resistance, and relatively low maintenance requirements. This article walks through every stage of the job, explaining what each layer does and how to get the best finish. For a deeper look at traditional road building techniques, see our guide on Water Bound Macadam WBM Road Construction, which covers the historical foundations of this surfacing method.
What is Macadam, Asphalt and Bitmac?
Before starting any driveway project it helps to understand the terminology because the names vary depending on where you live. In Europe and the UK, asphalt refers to a mix of gravel, sand and bitumen the tarlike binder that holds everything together. In the United States, the word asphalt is often used to describe the bitumen itself, so using the correct local terms avoids confusion when ordering materials.
Macadam surfaces were originally made using naturally occurring tar mixed with aggregate. The name itself comes from John Loudon McAdam, the Scottish engineer who pioneered the technique of compacting layers of small stone to create a solid road surface. Later the Welsh inventor Edgar Purnell Hooley refined the process, patented the result in 1902, and the term tarmacadam was born. Because natural tar supplies were limited, the industry switched to bitumen, a byproduct of crude oil refining, and the term bitmac entered common use. Bitmac simply means a macadam style surface that uses bitumen as the binder. For more on how these methods compare with traditional road construction, read about Water Bound Macadam Surface Treated Roads.
Site Preparation and Drainage Requirements
The overall quality of a macadam driveway depends almost entirely on how well the base is prepared. Rushing this stage leads to cracks, dips and premature failure. The first job is to clear the entire area down to a level where a suitable sub base can be laid. In most cases the excavation depth should be around 175 to 190 mm depending on the thickness of the final wearing course. This depth typically consists of:
- 100 mm of sub base aggregate
- 50 mm of binder course
- 25 to 40 mm of finished wearing course
An excavator or mini digger is usually used to remove the unwanted material, but for small jobs with limited access the excavation may need to be done by hand. Once the desired depth is reached and all waste material is removed, the excavated area is smoothed over. At this stage it is wise to lay a weed membrane to prevent plants or tree roots from pushing up through the finished surface later. Tree roots are a common cause of driveway damage, so it is worth reading about Trees You Should Never Plant Near Your Driveway to avoid problems before they start.
Drainage is critical at every stage of the job. Any macadam surface must be graded correctly so that surface water falls toward a drain or soakaway. The correct fall for a driveway is between 1:50 and 1:80. When a paving machine is used the fall can be set mechanically, but with hand laying you must be very careful to avoid shallow spots where water can pool. Water that sits on the surface will eventually find its way through cracks and cause the base to fail.
Installing Edgings, Kerbs and Supports
For the best finish and the longest lifespan, every edge of a macadam driveway must be contained against a solid surface. If the bitmac is left unsupported along the edges it will begin to crack and crumble under traffic. The most common containment methods are concrete edging stones, concrete kerb stones, timber edgings, or existing solid walls.
Concrete edgings provide the best long term support but they are more expensive and take longer to install. They are laid on a concrete bedding of medium strength C20 to C30 mix, which should be around 100 to 150 mm thick. The bedding is tamped down and smoothed with a trowel before the stones are placed. A string line should be run along the route to keep everything level, and a rubber mallet rather than a lump hammer should be used to tap stones into position to avoid cracking them.
Timber edgings are a decent alternative where budgets are tight. They are typically 100 to 150 mm deep and are held in place with timber stakes knocked into the ground. However, timber tends to rot over time and can expand and contract, leaving the bitmac edges unsupported. If you choose timber, use pressure treated wood and apply an additional exterior treatment to extend its life.
Once the edgings are in place the final step is to add haunching, a section of concrete that runs along the outside of the edgings to provide lateral support. The haunching prevents the edgings from being pushed outward by the weight of the surface and traffic. The top of the haunching should sit about 30 to 50 mm below the top of the edging, and the concrete should slope away slightly for drainage. For a comparison of driveway costs and materials, read Understanding The Cost Of Asphalt Driveway Construction.
Laying the Sub Base and Binder Course
With the area excavated and the edgings set, the sub base is laid. The sub base is normally made of an aggregate known as 40 mm to dust, MOT1 or Type 1 granular sub base. As the name suggests, it contains particles ranging from dust size up to 40 mm stone. When compacted, the precise blend of particle sizes keeps voids to a minimum while still allowing proper drainage.
The sub base is spread evenly and raked level across the working area. It is then compacted using a heavy smooth wheeled single drum vibratory roller. For smaller jobs a vibrating plate compactor can be used. After compaction, take random measurements from the sub base to the top of the edgings to confirm you are still at the correct height. An allowance of 10 mm either way is reasonable.
The binder course is the first layer of actual bitmac and uses a larger 20 mm aggregate. This coarse layer builds up the structure from big particles at the base to fine particles at the surface. The binder course is delivered hot and must be kept warm to remain workable. For hand laid jobs it is dumped onto a clean surface and covered with tarpaulins immediately. Workers then shovel it into wheelbarrows, spread it with rakes, and roll it with a smooth drum roller. All tools and machinery are coated in red diesel or heated to prevent the hot bitmac from sticking. It typically takes six to eight passes of the roller to achieve full compaction. If you are weighing macadam against other options, see our article on Concrete Driveway Construction Design Materials And Best Pra for a thorough comparison.
Applying the Top Coat or Wearing Course
Once the binder course is rolled flat and compacted, the top coat also called the wearing course is laid straight on top. This is especially common on smaller jobs where speed is important. The top coat uses much finer grade material, typically 6 mm aggregate, to produce a smooth finish. There are two types to be aware of:
| Type | Aggregate Size | Characteristic |
|---|---|---|
| Open graded bitmac | Uniform 6 mm | Better drainage, slightly rougher texture |
| Close graded bitmac | 6 mm down to 3 mm and finer | Denser, smoother, more compact surface |
Laying the top coat is done the same way as the binder course, either with a paving machine or by hand. For hand laying, much more attention is paid to levelling. Rakers often use a tool called a lute, which looks like a rake without teeth and has a flat edge that creates a smooth level surface. Once spread and smoothed, the entire surface is rolled again with six to eight passes. Any small marks or depressions are corrected by spreading a small amount of fresh bitmac over the area and rolling it again.
After the top coat is finished it can be walked on right away, but it should be left for at least 24 hours before driving on it. For the first few weeks the surface is still curing, so wheel spinning, hard braking and turning on the spot can scuff and rip the fresh bitmac. For ideas on how to give your driveway a clean defined edge, check out A Better Driveway Border from This Old House.
Resurfacing, Regulating, Repairs and Aftercare
Not every driveway needs a full dig out. If the existing sub base is solid and free of cracks or movement, an overlay or resurface can be done. In the trade this is called an overlay and it simply means laying a new top coat over the existing surface. However, the existing base must be in good condition. If new macadam is laid over a cracked or moving base, the new surface will fail quickly.
Regulating is the process of filling holes and depressions in the existing base so that it is level. Holes that go down to the sub base must be filled with compacted sub base material, while depressions in the binder course are filled with binder course material. For large shallow depressions across an entire area, it may be possible to lay a regulating course no deeper than 30 mm to create a flat surface before the top coat goes on.
One critical point with resurfacing is the finished height of the new surface. The top coat needs a minimum depth of 25 mm, and any regulating courses add height on top of that, potentially raising the surface by up to 55 mm. This can affect damp proof courses which need to be at least 150 mm above the finished surface, garage door clearances, and existing drain or manhole covers. Before committing to a resurface, check these heights carefully. For a detailed look at base preparation for paved surfaces, see Concrete Driveway Construction Subgrade Preparation Reinforcement Placing And Finishing For Durable Pavement.
Before any finishing coat is laid on an existing surface, a tack coat must be applied. This is a bitumen based solution that bonds the new layer to the old one and prevents water ingress between them. Without a tack coat, water can get between the layers and freezethaw action will lift the new surface. For large jobs the tack coat is sprayed on with a machine, but for small areas it can be painted on by hand.
Patching and repairs are also possible for localised damage. Time, UV light, fuel spillages, vegetation growth and impact damage all take their toll on macadam surfaces over the years. For small patches, pre mixed patching macadam is available from most builders merchants and DIY stores. The best approach is to square up the damaged area, cut away any crumbling edges, treat the edges with jointing compound if available, fill with patching compound and tamp it down. Once filled, treat the perimeter seam with jointing compound to keep water out and extend the life of the repair.
A macadam driveway installed correctly provides decades of reliable service with minimal upkeep, making it a cost effective investment for any home. Even with the best installation, wear and tear can happen over time, and understanding how to address issues like cracks and joint displacement is part of responsible driveway ownership. For guidance on maintaining and fixing paved surfaces, read about Fixing An Expanded Isolation Joint In Your Concrete Driveway, which covers joint repair principles that apply across different paving materials.