Patio Paving and Sub-Base Calculator

Mark out the proposed perimeter of your paved area using pegs, string lines, and a builder's square to ensure corners are set accurately. Measure the length and width along the perimeter. Recommended sub-base depth for domestic pedestrian paving is a minimum of 100 mm of compacted MOT Type 1 crushed stone aggregate, followed by a 30 mm to 50 mm mortar bedding layer.

Common mistakes in patio groundwork planning include:

• Ordering MOT Type 1 based on uncompacted loose volume; crushed stone settles and compacts down by 20% to 25% when compacted with a heavy vibrating plate.
• Laying porcelain slabs on dry sand or without a slurry primer; external porcelain tiles have near-zero porosity and will not bond to mortar without polymer slurry priming.
• Failing to establish a minimum 1:60 to 1:80 drainage fall away from the house, or laying patio surface levels less than 150 mm below the property damp-proof course (DPC).

This calculator covers domestic pedestrian garden patios and footpaths. It is not suitable for vehicular driveways, which require deeper engineered sub-bases (minimum 150 mm to 200 mm) and concrete or permeable block paving. Under England planning rules, paving more than 5 square metres in front gardens requires planning permission unless the surface is permeable or drains to an adjacent lawn or border.

How the estimate works

Paving = area + 5% for cuts. Sub-base tonnes = area × depth × 2.1 t/m³ compacted. The mortar bed is taken as 40mm of a 4:1 sharp-sand:cement mix.

Worked example (the default inputs)

A 6m × 4m patio needs 25.2 m² of paving, about 5 tonnes of MOT Type 1 at 100mm, 1.2 tonnes of sharp sand and 12 bags of cement.

Tips

• Keep the finished surface at least 150mm below the damp-proof course of the house.
• Compact the sub-base in layers of 50–75mm with a plate compactor.
• Use a jointing compound or slurry grout that suits your slab type.

Sources

Guidance on permeable surfacing of front gardens.