Best Base Preparation for Stone Setts and Cobbles

Ask any experienced landscaper what separates a driveway that still looks great after twenty years from one that's already sinking and cracking after five, and they'll give you the same answer: the base.

It's not exciting. It's invisible once the job is done. But getting your base preparation right is the single most important thing you can do when laying granite setts, sandstone cobbles, or any other natural stone paving. This guide covers everything you need to know — the right depths, the right materials, and the common mistakes that cause problems down the line.

Why Base Preparation Matters So Much

Stone setts and cobbles are rigid units. Unlike flexible paving like block paving or gravel, which can shift and settle without looking too bad, rigid natural stone is unforgiving. Once it sinks or heaves, the whole surface looks wrong — and fixing it properly usually means taking it all up and starting again.

The base serves three functions:

1. Load distribution — spreading the weight of vehicles or foot traffic across a wider area
2. Drainage — preventing water from pooling beneath the paving and undermining the bedding
3. Stability — preventing frost heave and ground movement from lifting or cracking the surface
4. 

What Base Do You Need?

The base specification depends on the intended use:

For pedestrian paths and light garden use:

• 75–100mm compacted MOT Type 1 sub-base
• 25–40mm bedding (sharp sand or semi-dry mortar)

For domestic driveways (cars and light vans):

• 150mm compacted MOT Type 1 sub-base
• 30–40mm semi-dry mortar bedding (4:1 sand/cement)

For driveways with frequent heavy vehicle access:

• 200mm compacted MOT Type 1 sub-base
• Full wet mortar bed
• Consider a concrete slab base if very heavy loads are expected

For detailed depth specifications and product guidance, our size and thickness guide is a helpful reference.

Excavation

Before any stone or sub-base goes down, you need to remove all topsoil and organic material from the area. Organic material rots and compresses, which causes settlement. Get it all out.

Your excavation depth is calculated as: sub-base depth + bedding depth + sett/cobble depth + 10mm for compaction allowance

For a standard domestic driveway using 100mm granite setts, that typically means excavating around 290–310mm below your finished paving level.

Mark your finished level on a fixed reference point before you start excavating. It's remarkably easy to lose track once you're down in a hole.

Sub-Base Installation

MOT Type 1 is the standard sub-base material in the UK. It's a well-graded crushed stone that compacts well and provides excellent load-bearing capacity. Avoid using recycled concrete or soft materials — they're variable in quality and can contain soluble sulphates that attack cement.

Lay Type 1 in layers of no more than 100mm at a time. Compact each layer thoroughly before adding the next. Use a plate compactor for most areas, and a wacker plate in corners and confined spaces.

After compaction, check levels carefully with a long straight edge. Variations in the sub-base translate directly into variations in your finished surface.

Drainage Considerations

Water is the enemy of any paved surface. Standing water beneath paving leads to frost damage, sub-base erosion, and bedding failure.

There are two approaches:

Permeable construction: Leave the joints open and filled with grit or polymeric sand, so water can drain through. This approach is ideal for areas that don't require mortar joints and is generally the preferred option for driveways under UK planning rules for front gardens.

Impermeable construction with drainage: If you're using a full mortar bed and mortar joints, you need to ensure surface water is directed to a drain, a soakaway, or a planted area. Gradients of 1:80 to 1:60 are typical for driveways.

Read more about this in our dedicated drainage guide — it covers SuDS requirements and what you need to know for front garden driveways specifically.

Bedding Layer Options

Sharp sand (dry): Only suitable for pedestrian areas with open joints. Will wash out on slopes. Not appropriate for vehicle access.

Semi-dry mortar (4:1): The best all-round option for most driveway and patio applications. Spread dry, screed level, then lay setts. The moisture in the setts and from rainfall gradually hydrates the cement over time.

Full wet mortar: Used for complex or decorative projects where precise level control is needed. Requires more skill and faster working, as the mortar has a limited working window.

For setts and cobbles specifically, a semi-dry mortar bed at 30–40mm depth is the standard recommendation for most domestic projects.

Common Mistakes to Avoid

Insufficient excavation depth: Shallow bases lead to sinking. Every millimetre of saving on excavation costs is paid back tenfold in remedial work.

Inadequate compaction: Running a compactor over once isn't enough. Multiple passes, in different directions, are needed for proper consolidation.

Organic material in the sub-base: Even small amounts of retained topsoil or turf will cause localised settlement.

Laying on frozen ground: Never lay setts or cobbles when the ground is frozen or frost is forecast within 24 hours.

Skipping edge restraints: Without a fixed edge, lateral spread will undo even a perfectly laid surface within a few years.

If you're also choosing your setts or cobbles and want to compare options, take a look at our granite paving and setts and cobblestones ranges. And if you're working on a larger project that includes patio slabs alongside your sett areas, our paving slabs collection covers everything from porcelain to sandstone to granite in a wide range of sizes.

Get the base right, and everything else is relatively straightforward. Skip it, and no amount of beautiful stone will save the finished result.