Subbase Compaction Tips for Durable Paving

Why the subbase carries the whole job

Paving looks like the important bit, but it is the layer underneath that decides whether your driveway still sits flat in five years' time. A properly compacted subbase spreads the load from the paving — and from the cars, caravans or foot traffic above it — across a wider area of the ground beneath. Skip the compaction and you get a lovely-looking patio that settles, dips and holds puddles by the second winter.

The good news is that compaction is mostly about patience rather than expensive kit. Layer it thin, damp it slightly, and go over it more times than feels necessary.

Choose the right material and depth

Most domestic groundworks in the UK use Type 1 granular subbase — often sold simply as MOT Type 1 — a well-graded crushed stone mix, usually 0/40mm, that knits together when compacted. It is cheap, widely available and forgiving to work with. Sharp sand or loose hardcore are not substitutes; they do not interlock the same way.

As a rough guide for compacted depth:

• Patios, paths and shed bases: 75–100mm
• Domestic car driveways: 100–150mm
• Driveways taking vans, caravans or regular deliveries: 150–200mm

On soft clay or made ground, lay a geotextile membrane first. It stops the stone migrating down into the subsoil and keeps the finished layer working as a single plate rather than a loose scatter of stones.

Thin lifts, not one thick dump

This is the single most common mistake on UK sites. Tip 200mm of Type 1 into an excavation and a plate compactor only reaches the top 50–75mm. The rest stays loose, and every bit of that looseness turns into settlement later.

• Spread in loose lifts of no more than 100mm — around 75mm is easier to work properly
• Compact each lift fully before barrowing in the next
• Rake or lute each lift level before you switch the machine on
• Keep the edge of the excavation tidy so stone does not spill into soft ground

Yes, it takes longer. It also means you are compacting stone rather than compacting the top of a pile and hoping.

Moisture, technique and passes

Type 1 compacts best when it is slightly damp — think of a crumb you can squeeze into a ball that holds together without dripping. Bone-dry stone just shuffles around under the plate; saturated stone pumps and turns to mush.

On technique:

• Use a vibrating plate compactor for patios, paths and small areas; hire a 1.5–3 tonne vibrating roller for driveways where access allows
• Aim for at least four to six passes over every lift, changing direction between passes
• Overlap each pass by roughly half the width of the plate or drum
• Work in sections rather than chasing the machine around the whole excavation
• Do not neglect the outer 300mm where the paving meets the lawn, kerb or planting bed

If the ground starts bouncing or the stone begins to pump, stop. That is usually soft subsoil rather than the stone itself. Dig out the weak patch, replace it with fresh Type 1 and start again on that section.

Prove it before you lay anything on top

A compacted subbase should feel like a firm, slightly crunchy car park surface. Walk across it and the heel of your boot should not sink. Scuff the surface with a spade and you should meet resistance, not spoon through it.

• No springy spots, soft patches or hollow-sounding areas
• Level to within about 10mm over a 3m straight edge
• Falls of roughly 1:60 to 1:80 on patios, and 1:40 to 1:60 on driveways, running away from the house
• Edge restraints in place and bedded before final compaction against them

Check the levels again after the last pass. Compaction settles stone, so a surface that looked right when loose may now be a little low — top it up, re-compact and re-check.

Protect what you have built

Once the subbase is tight and true, keep it that way. Do not let a delivery wagon reverse onto it, and do not leave it exposed through weeks of heavy rain where fines can wash out and the surface puddles. If you have to pause the job, sheet it over or rake it back, re-compact lightly and re-check the levels before laying the bedding course.

If it does get soaked, let it dry out before compacting again — working wet stone simply pushes it around. A little care at this stage is the difference between paving that still looks sharp in ten years and paving you are lifting and re-laying after two.