All types of paving need to be laid on a structure built up of a number of layers of different materials. Each layer builds on the strength of the previous.
Firstly, it is necessary to remove existing vegetation and existing topsoil, back to the layer of more stable sub soil beneath (often referred to as the sub-grade).
The excavation must remove all vegetation such as grass, weeds and roots as this material will decompose over time, which can result in settlement of the paving laid on top.
The next layer to be formed is the most important layer, structurally speaking, in the formation of your project. This layer is the sub-base. Sub-bases may not always be required for patio projects but are vital when laying a block paved driveway, in order to prevent settlement.
There are various materials that can be used for the sub-base, and your selection will generally depend on the project being carried out.
The highest grade of sub-base is generally known as Type 1 MoT (Ministry of Transport) or DTp (Department of Transport), and is the preferred material for use under driveways or areas with heavy traffic. It is a specific mix of solids and fines that when compacted ensures there are no voids in the sub-base, but is sufficiently free draining to allow ground water to dissipate.
There are a number of other materials which are supplied for use as a sub-base, including Ballast and Crusher Run; however, there is often little control over the balance of solids and fines. These therefore, would be unsuitable for use under areas of very heavy traffic and commercial projects, but should be perfectly suitable for patios and standard driveways.
Bedding or Laying Course
The bedding or the laying course is basically the layer onto which your chosen paving is laid. This layer supports the paving, allowing for variations in thickness and any peaks or undulations to be accommodated without affecting the levels of the finished project.
The material used for this layer is determined by the type of paving to be laid. For example, for concrete block paving, unbound (without cement) sharp sand is used. This is compacted in order to prevent movement and provides a perfectly solid bed for a block pavior. Whereas for natural sandstone, a moist sharp sand and cement mix is preferred. This mortar mix allows the slabs to be bedded in and accommodates the varying thicknesses of the stone.
The final layer is that of your chosen paving choice, any colour and pattern at no extra cost.