Creating the Sub-Base for Paving

Paved areas need a solid foundation to provide a stable base for the slabs or tiles. The method detailed below is suitable for the majority of projects, although any surface that will be required to support very heavy loads will need a more substantial sub-base. Some soils will shrink during periods of prolonged dry weather, which can disturb many surfaces; increasing the depth of the foundations by 5 cm (2 in) may help to minimise this effect.

Although the majority of paving materials may be laid directly onto this type of foundation, some tiles are relatively thin and brittle, so you must first spread a 10 cm (4 in) layer of concrete to act as a base. Make sure that you take this into account when working out the depth of your foundations.

Prepare the Sub-Grade

Remove any plant growth from the area, including weeds and tree roots.

Dig out the area to a depth to allow for a 10 cm (4 in) layer of hardcore, a 5 cm (2 in) layer of coarse, grit sand, plus the thickness of the paving material. Peat and heavy clay soils can shrink in dry weather; you can reduce the effect this may have on the patio surface by excavating a further 5 cm (2 in) and increasing the depth of the hardcore to 15 cm (6 in).

Tamp the base of the site down with a plate compactor.

Create a Drainage Slope

The surface of any patio or path should have a built in slope to ensure that rainwater drains efficiently away. This gradient should be approximately 1 in 70 to 80 (2.5 cm/2 m or 1 in/6 ft). This will create a slope that is steep enough to make sure that water does not pool on the surface, yet is imperceptible to the eye.

Work out the combined depth of your sub-base and surface material and mark this as a line at the same distance from the top of each of a number of levelling pegs. You will need enough pegs to form a 2x2 m (or 6x6 ft) grid across your patio or path site. You may also find it useful to also mark on the depths of each layer, perhaps in different colours to avoid confusion.

When you have excavated the area and tamped the base firmly down, insert a row of these pegs into the base where the top of the slope will be, so that the combined depth mark is at soil level. These pegs should be placed 2 m (or 6 ft) apart. The top of the peg should indicate where the top of the finished surface will be.

Insert a second row of pegs 2 m (or 6 ft) down the slope, again to the marked level. Place a 'shim' (an offcut piece of wood 2.5 cm or 1 in thick) on top of each peg in the second row in turn. Place a plank on top of the first peg and the peg with the shim. Lay a spirit level on top of the plank, and then push the second peg deeper into the soil until the top of the shim and the upper peg are level.

Remove the shim and repeat for all the pegs in the second row, and then continue adding rows down the slope in this manner until you have covered the area.

When you have finished positioning all the pegs, rake and remove the soil so that it is level with the mark on each peg.

Add the Sub-Base

Spread a 10 cm (4 in) layer of hardcore over the area (15 cm or 6 in for clay soils). Compact it so that it is level with the mark made on the levelling pegs.

Add a 5 cm (2 in) layer of coarse, grit sand and compact again to produce a level surface for laying your surface material.