Patios and terraces need a solid foundation to provide a stable base for the paved surface. 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.

Many terraces and patios are built close to the house; it is essential that you know where any service cables and pipes run across the property before you start any excavation work, so that you can avoid damaging them. If in any doubt, check with your supplier or local authority.

Preparing the Sub-Base

1. Remove any plant growth from the area, including weeds and tree roots.
2. Mark out the area with pegs and string, setting the string at the surface level of the patio. Use a builder's square to check that the corners are set at right angles.
3. 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 surface 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).
4. Tamp the base of the site down with a plate compactor.
5. Make the following marks on some levelling pegs, measuring from the top of the peg:
• the depth of the surface material,
• a mark 5 cm (2 in) further down from this first mark to indicate the bottom of the sand layer,
• a mark 10 cm (4 in) (15 cm or 6 in for clay soils) further down from the sand mark to indicate the bottom of the hardcore layer.
6. Insert these levelling pegs to the depth of the hardcore mark, so that they form a 2x2 m grid. Use a spirit level to make sure that they are level with your string guides. The top of the peg will indicate the ultimate level of the surface material.
7. Use these pegs to help you to form a 1:80 incline as described in Creating a Drainage Slope, to ensure that rain water does not pool on the patio surface.
8. 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 pegs.
9. Add a 5 cm (2 in) layer of coarse, grit sand and compact again to produce a level surface for laying your surface material.

The majority of paving materials may be laid directly onto the foundation described above; however, quarry 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.