Why patios and driveways sink: sub-bases and drainage on Wiltshire ground

Most paving that fails does so because of what is underneath it, not the slabs on top. Wiltshire has a mix of clay, chalk and greensand, and each behaves differently when wet. Here is what a properly built patio or driveway needs.

Dig out to firm ground

Topsoil and soft ground have to come out. How deep you go depends on the use: a patio needs less depth than a driveway that will carry cars. On clay the ground can soften in wet weather and shrink in dry summers, so a firm, well-compacted base matters even more.

A compacted Type 1 sub-base

The sub-base is a layer of crushed stone, usually MOT Type 1, laid in layers and compacted with a plate compactor or roller. It spreads the load and stops the surface settling. Skimping on sub-base is the most common reason patios rock and dip. On one Devizes garden we laid between 15 and 20 tonnes of Type 1 before a single slab went down. You can see more in our landscaping work.

Falls: let the water go somewhere

Paving should fall gently away from the house so rain runs off rather than pooling against the walls. The finished surface must also sit below the damp-proof course of the house, usually by at least 150mm, so water cannot bridge it.

Front driveways and planning

Paving over a front garden can need planning permission. Under national rules, if you cover more than 5 square metres of front garden with a hard surface, it should be permeable (letting water soak through) or drain to a lawn or border on your own land. Otherwise you need planning permission. The Planning Portal explains the rules, and you can check with Wiltshire Council planning if you are unsure. A new dropped kerb also needs council approval.

Drainage on heavy ground

On clay, water does not soak away easily, so channel drains, soakaways or connections to existing surface water drainage may be needed. This is groundworks as much as landscaping, which is why we treat it as part of the build.

Questions to ask

• How deep is the dig, and how much sub-base is going in?
• Where will the water go?
• Will the finished level stay below the damp-proof course?