How Deep Should Patio Sub Base Be? A UK Method

How deep should patio sub base be?

A typical UK patio for foot traffic commonly uses about 100 mm of compacted MOT Type 1 beneath the bedding layer. This is a practical starting point rather than a universal legal minimum: the required depth depends on the soil, expected loading, paving material, drainage and the construction guidance for the particular paving system.

Use a greater sub-base depth where the ground is soft, filled, wet or prone to movement, or where the area may carry vehicles, wheeled bins or heavy garden structures. A driveway is a different construction from a pedestrian patio and should be designed to the relevant paving or highway guidance rather than using the patio rule. The sub-base depth is the compacted thickness, not the loose depth tipped from a delivery bag.

• Pedestrian patio on firm, well-drained ground: start by checking whether 100 mm compacted Type 1 is suitable.
• Soft, disturbed or poorly drained ground: allow for deeper excavation and seek a design suited to the conditions.
• Vehicle loading: use a driveway-specific pavement design.
• Keep the depth of bedding and paving separate from the sub-base measurement.

Check the ground before choosing a depth

Mark the patio area and remove turf, roots, topsoil and other organic material. Organic soil decomposes and changes volume, so it is not a dependable foundation. Also remove loose fill, buried timber and obvious construction debris. The excavation should reach firm, evenly supported ground across the whole patio rather than stopping at the same nominal depth regardless of what is found.

Inspect the exposed formation after rain if possible. Firm mineral soil should resist moderate pressure from a heel without pumping or forming a deep indentation. Soft clay, saturated ground, made-up ground and areas that retain water need more investigation. If the formation moves under a plate compactor or water continues to collect, increasing the Type 1 thickness alone may not solve the problem; drainage improvement, removal of unsuitable material or professional advice may be needed.

• Probe several points, including the edges and any former beds, paths or trenches.
• Mark soft spots rather than hiding them beneath aggregate.
• Check for buried services before excavation.
• Confirm that rainwater will not be directed towards the house.

Calculate the excavation and finished levels

Work backwards from the finished patio surface. Allow for the paving thickness, the bedding layer specified for the paving, and the compacted sub-base. For example, a 20 mm stone slab with a 30 mm bedding layer and a 100 mm compacted sub-base requires approximately 150 mm from the finished surface to the formation before allowing for any additional treatment of weak ground.

Set a level reference using a straightedge, laser level or water level. The patio should normally fall away from the building to shed surface water, while the paving remains below damp-proof course and other thresholds by an appropriate amount. Do not guess the relationship to doors: check the existing threshold, air bricks, damp-proof course and any drainage channel before digging.

• Write down the paving, bedding and sub-base thicknesses before ordering materials.
• Measure from a fixed benchmark rather than from loose soil.
• Check door thresholds, steps, air bricks and inspection covers.
• Plan the fall and its discharge point before compacting the base.

Use and compact the right sub-base

For many UK patio projects, a well-graded granular sub-base such as MOT Type 1 is used because its mixed particle sizes interlock when compacted. It is not the same as decorative gravel, single-sized stone or ordinary topsoil. Buy material suitable for the intended construction and keep it clean and reasonably dry during placement.

Place the aggregate in even layers rather than tipping the full depth at once. Compact each layer with a suitable vibrating plate compactor, making enough passes to produce a tight, stable surface. The achievable layer thickness depends on the material, machine and site conditions; follow the aggregate supplier’s and compactor manufacturer’s guidance. A sub-base that looks level but still ruts or rocks under the plate needs further work.

• Use a plate compactor appropriate to the area and material.
• Keep the surface evenly graded so low spots do not become water traps.
• Check levels after each compacted layer.
• Do not rely on hand tamping for a substantial patio sub-base.

Allow for drainage and UK site conditions

A patio should not create a route for water to collect against the house. Establish the fall before laying the bedding layer and ensure the lower edge has somewhere suitable to discharge. Depending on the site, this may involve a planted area, a drainage channel connected to an approved drainage arrangement, or another solution designed for the property. Surface water should not simply be sent onto a neighbour’s land or a public footway.

Front gardens and larger areas of hard surfacing may have planning and drainage implications. The Planning Portal explains when paving a front garden may require permission, particularly where surface water could run onto the highway. For significant changes, check the current guidance from the local planning authority and water company. Existing public drainage, tree roots, flood risk and conservation-area constraints can change the appropriate design.

• Direct the fall away from walls and towards the planned drainage point.
• Keep gullies, channels and inspection covers accessible.
• Check local planning guidance for front gardens and larger hardstanding areas.
• Do not block air bricks or bridge the damp-proof course.

Adjust depth for paving and expected use

The paving material affects the bedding and surface construction, but it does not automatically make a weak formation suitable. Concrete flags, porcelain paving, clay pavers and natural stone each have installation requirements for bedding, jointing and handling. Porcelain in particular may require a specified full mortar bed or proprietary system, so use the manufacturer’s method rather than assuming a dry or thin bed.

For a normal seating or walking patio on firm ground, a compacted 100 mm Type 1 layer is a reasonable point for assessment. Consider deeper construction or a professional design where the patio crosses former building works, remains wet, supports a hot tub or masonry structure, receives frequent heavy barrow traffic, or may later become a parking area. If the area is intended for a vehicle, design the whole pavement for that load, including the edge restraint and surface course.

• Confirm the paving supplier’s required bedding and base construction.
• Treat a hot tub, shed base or masonry feature as an additional load.
• Use edge restraints where the paving could spread or move.
• Do not reduce the base because the paving slabs are thick.

Verification checklist before laying paving

Walk the compacted sub-base and inspect it in several directions. It should be stable, evenly supported and free from soft patches, standing water and loose heaps. Recheck the planned fall with a long straightedge and level. Measure down from the finished reference at the corners, centre and drainage edge to confirm that the compacted depth is consistent.

Record the actual measurements before covering the work. Confirm that the planned bedding thickness will bring the paving to the intended finished level without burying air bricks or obstructing thresholds. If measurements do not agree, correct the base now; adjustments are harder after bedding and slabs have been installed.

• Sub-base is the planned compacted depth, measured at several points.
• No area pumps, ruts or shifts under compaction or foot traffic.
• The fall leads to a suitable drainage destination.
• Thresholds, damp-proof course, air bricks and covers remain correctly arranged.
• Paving, bedding and edge-restraint instructions have been checked.

Scope and limitations

This note helps householders organise measurements and understand the calculation. It does not replace an on-site decision by a designer, installer, contractor, or manufacturer. Fabrication, structure, electrical work, gas, waterproofing, and personal safety require suitably qualified verification.