The short answer
Preparing ground for a patio means establishing safe finished levels, removing unsuitable material, creating a stable compacted sub-base and providing the right bedding and drainage for the chosen paving. Work backwards from the finished surface; do not choose an excavation depth before you know the construction build-up.
Check levels before digging
Mark the proposed edges and measure the relationship with doors, damp-proof courses, air bricks, garden levels and existing drains. Decide where rainfall will go without sending it towards the house or neighbouring land. Level-access thresholds need a designed detail; simply raising paving to an internal floor can cause moisture problems.
Check buried services before excavation. HSE’s underground-services guidance explains the need to plan, locate services and excavate safely. Do not assume that a small garden project is clear of electricity, gas, water or drainage.
How deep should you excavate?
The required depth is the paving thickness plus its bedding and specified sub-base, adjusted for the existing ground and finished levels. Topsoil, roots and loose fill cannot serve as a dependable formation. Soft spots may need additional investigation or replacement rather than another thin layer of aggregate.
| Layer | Illustrative thickness |
|---|---|
| Selected paving | 20 mm |
| Bedding | 40 mm |
| Compacted sub-base | 100 mm |
| Total below finished paving | 160 mm |
The example only demonstrates addition: 20 + 40 + 100 = 160 mm. It does not allow for unsuitable soil or a different product. Marshalls gives 100 mm as a typical patio sub-base in its porcelain paving guidance; ground conditions and loading can require another design.
Build a stable sub-base
Specify suitable graded material and place it in layers that the chosen compaction equipment can compact effectively. Check levels as the work proceeds. A loose heap at the right total depth is not equivalent to a compacted construction layer. Keep mud and organic material out of the aggregate.
A separation geotextile may be appropriate where the ground and design require it, but it does not make weak ground strong or replace drainage. Permeable paving systems also have their own aggregate and bedding requirements; a conventional patio build-up should not be assumed suitable.
Plan falls and water outlets
Set falls to suit the paving and site design, maintaining a route away from vulnerable thresholds. Channels only collect water: they still need an appropriate outlet. A soakaway needs viable infiltration and safe siting, not just spare space beneath a lawn.
For measurement, fall is horizontal distance divided by the gradient denominator. A designed 1:60 fall across 3 metres gives 50 mm of level change. This is an arithmetic example, not a universal patio gradient. Read our surface-water drainage guidance where water currently ponds or flows towards the building.
Match bedding to the paving
Follow the paving manufacturer’s instructions for bedding, bonding, joints and curing. For porcelain on mortar, Marshalls describes full-bed support and a bonding primer. Avoid treating porcelain, natural stone, concrete flags and flexible block paving as interchangeable systems. Isolated spots of mortar leave unsupported areas and do not provide a consistent bed.
Check before covering the work
- Confirm the finished level at each door, air brick, step and boundary.
- Record the formation and compacted layers before bedding conceals them.
- Check drainage routes, channels, covers and future maintenance access.
- Confirm the agreed product, laying pattern, edge restraint and joints.
- Allow for safe access and the manufacturer’s curing period before use.
Keep photographs and the agreed construction details with the quotation. Our excavation calculator can help estimate geometric volume; loose spoil, disposal classification and vehicle loads need separate assessment.
What we photograph before a patio base is covered
A finished patio cannot show whether the hidden construction was prepared consistently. During A Jones project work, useful progress evidence includes the exposed formation, the compacted layers, drainage routes and threshold details before paving conceals them. The photographs support the written specification; they do not replace testing or a required inspection.
| Record | What it helps confirm |
|---|---|
| Formation after excavation | Soft spots, roots, fill and the condition being built on |
| Sub-base during placement | Layering, coverage and levels before bedding is added |
| Channels and outlet route | That collected water has a planned destination |
| Doors, walls and air bricks | The relationship between paving, thresholds and vulnerable details |
Use the patio and driveway materials calculator to organise dimensions and layer quantities, then confirm the actual build-up against the chosen product and site conditions.
Frequently asked questions
Can slabs go directly on soil?
That is not a dependable construction for a permanent, regularly used patio. Soil changes with moisture, roots and loading. Choose a build-up appropriate to the paving and ground rather than relying on a level-looking surface.
Can I pave over an old patio?
Only after assessing the existing construction, drainage, movement and resulting finished levels. Covering loose slabs or raising the patio above a suitable threshold detail can preserve or worsen the original defect.
What if the garden has clay soil?
Clay does not identify a complete solution on its own. Water movement, soft areas, levels and seasonal behaviour all matter. A wet excavation should prompt investigation rather than the assumption that extra stone will solve everything.
Can A Jones prepare the base?
Discuss the full scope with our patio and paving team. We can assess excavation, levels, drainage, preparation and the interface with the chosen finish across Woking, Guildford and Surrey.
Continue with the actual site
Related guides & services
Use the guide to frame the first questions, then compare them with the property’s ground, levels, access and drainage.
Check the site-specific details
General guidance cannot verify your ground, structure, drainage, permissions or site access. For statutory requirements, use the official Planning Portal, your local authority, Building Control and the relevant water company or designer.
