Prepare a Patio Base

Check levels before digging

Mark out the edges and measure how the patio will meet doors, the damp-proof course, air bricks, the garden and existing drains. Decide where rain will run, so it does not head towards the house or next door. A level-access door needs a proper designed detail. Simply raising the paving to meet the inside floor can cause damp problems.

Check for buried services before you dig. HSE’s underground-services guidance explains how to plan, find buried services and dig safely. Do not assume a small garden job is clear of electricity, gas, water or drains.

How deep should you excavate?

Dig deep enough for the paving, its bedding and the sub-base, adjusted for the existing ground and finished levels. Topsoil, roots and loose fill are not firm enough to build on. Soft spots may need checking or digging out, not just another thin layer of stone.

The example just shows the sum: 20 + 40 + 100 = 160 mm. It does not allow for poor soil or a different product. Marshalls gives 100 mm as a typical patio sub-base in its porcelain paving guidance. Your ground and how the patio is used may need something different.

Build a stable sub-base

Use a suitable graded stone and lay it in layers thin enough for your compactor to compact properly. Check levels as you go. A loose heap at the right depth is not the same as a compacted layer. Keep mud and plant material out of the stone.

A fabric membrane can help where the ground and design need one. But it does not make weak ground strong, and it does not replace drainage. Paving that lets water through has its own stone and bedding needs, so do not assume a normal patio build-up will work.

Plan falls and water outlets

Give the patio a gentle slope (a “fall”) that suits the paving and the site, so water runs away from doors. Channels only collect water. They still need a suitable outlet. A soakaway needs ground that drains well and a safe position, not just spare space under the lawn.

To work out a fall, divide the distance by the gradient. For example, a 1:60 fall across 3 metres gives a 50 mm drop. This is just an example sum, not the right slope for every patio. Read our surface-water drainage guidance if water already puddles or runs towards the house.

Match bedding to the paving

Follow the paving maker’s instructions for bedding, bonding, joints and drying time. For porcelain on mortar, Marshalls describes a full bed of mortar and a bonding primer. Porcelain, natural stone, concrete flags and block paving are laid in different ways, so do not treat them as the same. Dabs of mortar leave gaps under the slab and do not give even support.

Check before covering the work

• Check the finished level at each door, air brick, step and boundary.
• Photograph the bottom of the dig and the compacted layers before the bedding covers them.
• Check drainage routes, channels, covers and access for future cleaning.
• Confirm the agreed product, laying pattern, edging and joints.
• Allow for safe access and the maker’s drying time before using the patio.

Keep photos and the agreed build details with the quote. Our excavation calculator can help estimate how much you will dig. Loose soil volume, waste type and lorry loads need working out separately.

What we photograph before a patio base is covered

You cannot tell from a finished patio whether the hidden layers were done properly. On our jobs, we photograph the bottom of the dig, the compacted layers, the drainage routes and the door details before the paving covers them. The photos back up the written specification. They do not replace testing or any required inspection.

Use the patio and driveway materials calculator to work out sizes and layer quantities. Then check the real build-up against your chosen product and the site.

Frequently asked questions

Can slabs go directly on soil?

Not for a permanent patio that gets regular use. Soil moves as it gets wet and dry, with roots and with weight on it. Choose layers that suit the paving and the ground, rather than relying on a surface that looks level.

Can I pave over an old patio?

Only after checking the old patio, its drainage, any movement and where the new finished level will end up. Covering loose slabs, or raising the patio too close to the door, can keep the old problem or make it worse.

What if the garden has clay soil?

Clay on its own does not tell you the answer. How water moves, soft areas, levels and how the ground changes through the seasons all matter. If the dig fills with water, find out why. Do not assume extra stone will fix it.

Can A Jones prepare the base?

Talk through the whole job with our patio and paving team. We can look at the digging, levels, drainage, preparation and how it all meets your chosen paving, across Woking, Guildford and Surrey.