Most jobs involving tarmacadam installation fall into one of three main categories.
- These comprise a full excavation with the installation of a stone sub-base, a dense binding course of macadam and a wearing course of bitmac,
- A prepared area where the stone sub-base has already been installed perhaps by a builder and
- Re-surfacing, sometimes referred to as an overlay.
This is where a brand new driveway or patio is installed onto virgin land which requires excavation.
Once the ground has been excavated, the planned underground drainage systems need to be installed.
This may involve digging out a dry-well and back-filling it with crushed stone to create a soak-away, the pipework leading from proposed aco drainage to existing drains or soak-aways or preparing French drains. All new drainage should satisfy current Building Regulations.
Before the sub-base is installed and compacted, some companies will install a geo-textile membrane but there is much debate about the job it is supposed to do.
It is often referred to as a weed membrane but few installers will guarantee against weed damage to new tarmacadam driveways and patios, as their seeds are more likely to settle in small joints and sprout from above. Most weeds will struggle to grow through a new, properly constructed tarmacadam installation but where there is a known threat from particularly pernicious weeds such as Japanese Knotweed or Horse Tail, a geo-textile membrane can sometimes help.
Other claims for the benefit of a membrane include the reduced thickness of sub-base material required (possibly, but no excuse for too little) or as a separation membrane to prevent sand settling into the sub-base below (this only applies to block paving but if the layers are compacted well enough a membrane becomes irrelevant). In any instance, if a membrane is installed, it should never by polythene or any other non-permeable product as this cause all manner of problems when it starts to hold water.
The sub-base layer in a tarmacadam installation is normally the main load-bearing layer of a driveway or patio and works by distributing a specific load (such as that provided by car tyres), over a larger area.
This should prevent or at least reduce rutting over a period of time when a car makes the journey from the driveway entrance to the garage on the same length of tarmacadam, over a period of years. The thickness of sub-base material should be greater for a driveway which provides vehicular access than for a patio which is for pedestrians only.
Typically, 150 mm for a driveway and 100 mm for a patio, with both of them compacted to within a 10 mm tolerance. Dropping all of the sub-base material onto an area and then compacting it can lead to the lower section remaining too loose, especially if no more than a vibrating plate compactor is used. For a driveway at least, a double drum Bomag™ type, ride-on roller / compactor is recommended.
Crushed stone (but not old broken bricks) can be used for the sub-base but often Type 1 MOT is used – MOT refers to a range of Ministry of Transport recognised materials and Type 1 is a particular specification, there being other types of sub-base material.
On a normal tarmacadam installation for a driveway or patio area, the sub-base material is the least expensive material used per ton, but skimping on the type of material, its thickness and method of compaction, is one of the best ways of shortening the life of the newly finished surface.