How to Prepare a Sub Base for Paving Properly

A patio can look spot on when it is first laid, then start rocking, dipping or holding water after one wet winter. The difference is usually underneath the slabs. Learning how to prepare a sub base for paving gives your patio, path or terrace the stable, well-drained foundation it needs to stay level for years.

The sub-base is the compacted layer between the ground and the bedding mortar. It spreads the load of the paving, reduces movement and helps water move away from the finished surface. Skipping it, using the wrong material or failing to compact it properly can undo even the best-quality porcelain or natural stone paving.

Start with the finished level and drainage fall

Before digging, decide where the top of the paving needs to sit. Check door thresholds, air bricks, garden borders and any existing drainage. As a general rule, finished paving should sit at least 150mm below a property’s damp-proof course. Never cover air bricks or direct surface water towards the house.

Your paving also needs a fall, sometimes called a gradient, so rainwater drains away. For most patios, aim for a fall of around 1:80 to 1:60. That means a drop of roughly 12.5mm to 16.5mm over every metre. A 4m patio may therefore need to fall by 50mm to 65mm from the house towards the garden.

Use string lines, timber pegs and a spirit level to mark the proposed finished height and fall. This early check avoids a common problem: excavating to a uniform depth, then finding there is not enough room for the sub-base, mortar and slabs while maintaining drainage.

How to prepare a sub base for paving

For a standard pedestrian patio or garden path, allow for the paving thickness, around 30mm to 50mm of full mortar bedding, and typically 100mm to 150mm of compacted sub-base. The exact excavation depth depends on slab thickness, ground conditions and the desired finished level.

Porcelain paving is often 20mm thick, while Indian sandstone varies and may be thicker. Measure the actual slabs rather than relying on a nominal size. If the site has soft, recently made-up or waterlogged ground, you may need a deeper sub-base and additional separation beneath it.

Excavate to firm, undisturbed ground

Remove turf, topsoil, roots and loose material across the whole paved area. Topsoil is not a base. It contains organic matter, retains moisture and can settle as it breaks down, causing the paving above to move.

Excavate evenly to the calculated depth, keeping the formation level broadly parallel to the intended paving fall. Do not dig a flat hole and try to create all the drainage fall in the bedding mortar. Mortar should support the slabs consistently, not compensate for an uneven excavation.

If you uncover soft patches, remove them until you reach firmer ground, then make up the depth with suitable aggregate. On clay-heavy gardens, take extra care with drainage. Clay holds water and can soften significantly during prolonged rain, so a sound sub-base and correct fall are especially valuable.

Install a geotextile membrane where needed

A geotextile membrane is not compulsory for every project, but it is a sensible addition on soft, silty or clay soils. Lay it over the excavated formation before adding aggregate. It helps stop the sub-base mixing with the soil below while allowing water to pass through.

Overlap joins by at least 300mm and extend the membrane slightly beyond the excavation edge where practical. Keep it smooth and free from folds. On firm, well-draining ground, a membrane may be unnecessary, but it is usually inexpensive insurance where ground conditions are uncertain.

Choose the right sub-base aggregate

For most domestic paving projects, MOT Type 1 is the standard choice. This graded crushed aggregate contains particles of different sizes, including fine material that binds together when compacted. It creates a dense, stable layer suitable for patios and paths.

Avoid using building sand, sharp sand, gravel or loose hardcore as a direct substitute for a proper Type 1 sub-base. Sand can shift, while rounded gravel does not lock together effectively. Recycled Type 1 can be a good value option when it meets the required specification and is clean, consistent and suitable for paving work.

For a typical patio, use around 100mm of compacted Type 1 on good ground. Increase this towards 150mm where the soil is less stable or the patio will receive heavier use. Driveways need a different specification: they commonly require 150mm to 200mm or more of well-compacted sub-base, depending on soil, vehicle weight and paving system.

Build and compact in layers

Do not tip in the full depth of aggregate and compact only the surface. A plate compactor cannot effectively consolidate a thick loose layer from above. Instead, add Type 1 in layers no deeper than about 50mm to 75mm, then compact each layer thoroughly before adding the next.

Use a suitable vibrating plate compactor, working methodically in overlapping passes. Pay particular attention to edges, corners and around drainage runs, where incomplete compaction often causes later settlement. The surface should feel solid underfoot, with no loose pockets or visible movement.

Keep checking levels as you work. The compacted sub-base should follow the planned fall and sit at a consistent depth below the finished paving level. A long straightedge, string line and spirit level make this much easier than trying to judge by eye.

Allow for edging and drainage details

Paving needs secure edge restraint. Without it, the outer slabs and bedding can creep outwards over time, opening joints and weakening the entire surface. Install kerbs, edging stones or a suitable concrete haunch before laying the paving, making sure they are set to the correct height and line.

Where water will collect near a doorway, retaining wall or low point, include a drainage channel or another planned outlet. Drainage cannot be solved by adding more jointing material later. It needs to be considered while the ground is open and levels can still be adjusted.

If your patio meets a lawn, leave enough room for a neat edge and avoid creating a raised lip that traps water. For projects using gravel grids, paths or mixed materials, each layer build-up may differ, so check the installation requirements before setting excavation depths.

Check the base before bedding the slabs

A quick inspection now can prevent costly lifting and relaying later. Before mixing mortar, confirm that the base is firm, evenly compacted and follows the correct fall. Check that the excavation has not left shallow areas where the mortar bed would be too thin, or low areas where it would become unnecessarily deep.

The surface does not need to be perfectly smooth, but it should be consistent. Remove loose aggregate, fill and recompact small hollows, and ensure there is no standing water after rain. If water remains on the sub-base, find out why before continuing. It may indicate a local low spot, poor drainage or unsuitable ground below.

For porcelain paving, use a full mortar bed and an appropriate slurry primer on the back of each slab. Natural stone also benefits from full mortar support, rather than the old practice of dot-and-dab bedding. Full coverage prevents voids that can lead to rocking slabs, cracking and frost damage.

Common sub-base mistakes to avoid

The most frequent mistake is underestimating depth. A patio laid too high can bridge the damp-proof course or interfere with doors, while one with too little Type 1 is more likely to settle. Measure each layer before excavation starts.

Poor compaction is another regular cause of failure. Hiring or using a plate compactor is worthwhile for all but the smallest repairs. Hand tamping may be adequate for a very narrow path in excellent ground, but it rarely delivers the same reliable result across a full patio.

Finally, do not assume every garden needs the same build-up. A sheltered path on firm sandy soil is different from a large porcelain terrace on clay, and both are different again from a driveway. If you are unsure about quantities, ground preparation materials or the right paving system, Save & Pave can help you plan the essentials before delivery.

Take the time to get the sub-base right while it is still accessible. Once the slabs are down, the foundation is out of sight, but it remains the part of the project doing the hardest work every day.