Key Takeaways
- A sub-base is the compacted layer of graded stone between the ground and your paving, and its job is to spread the load so the surface above it does not sink, rut or crack.
- MOT Type 1 is the sub-base material I use by default, because the 40mm-down-to-dust grading compacts harder than anything else you can buy in a bulk bag.
- Sub-base depth follows the load: roughly 100mm compacted under patios, 75 to 100mm under paths, and minimum 150mm under anything a car parks on.
- Stone compacts down by roughly 30%, so a 100mm finished sub-base needs about 130mm of loose material spread first. Our calculator does this by default.
- An 850kg bulk bag of sub-base covers roughly 4.4 square metres at 100mm compacted depth, and about 8.8 square metres at 50mm.
- Compact a sub-base in layers of 150mm loose or less, giving each layer at least eight passes with a vibrating plate, because a thick layer only ever compacts at the top.
Whenever I’ve been called to a rutted drive or a sinking patio, the surface was rarely the guilty party. Quite often it is the same picture underneath: 40mm of stone where 150mm should be, tipped straight onto wet clay, and the clay has worked up into the stone until the two are a soup. The paving on top never stood a chance, and the only honest fix is to lift it and start over.
Sub-base is one of the cheapest parts of any hard landscaping job and the one that decides whether the expensive part lasts. This page covers what a sub-base actually is, whether your project needs one, which material to buy, how deep to go and how much to order.
What Is a Sub-Base?
A sub-base is the compacted layer of graded stone that sits between the ground and your surface, spreading concentrated loads out sideways so the ground below never takes them all at one point. It is the first structural layer of the build.
The layers, from the bottom up:
- Sub-grade is the bare ground once the turf and topsoil are off. Anything organic has to come out, because it rots and the surface drops with it.
- Sub-base is the graded stone layer, compacted in lifts. This is the structure.
- Laying course is the thin cushion the paving sits on, grit sand or mortar. Never soft building sand.
- Surface is what you see: slabs, blocks, gravel, artificial grass.
The grading is what makes it work. A well-graded sub-base runs from 40mm lumps right down to dust, so the fines fill the gaps between the lumps under compaction and the whole layer knits into one slab. Single-sized stone cannot do that, however long you run the plate over it.
Do You Always Need a Sub-Base?
A sub-base is not compulsory on every job: it is essential under block paving and anything carrying a car, worth having under most patios, and genuinely optional under a light garden path on firm, undisturbed ground. Missing or inadequate sub-base is the single most common cause of block paving settling and rutting, so that is where I never compromise.
Where I will happily skip it is a narrow gravel path over firm subsoil that only ever takes feet and the odd barrow: excavate, lay a membrane, gravel straight down. What decides it is the ground you find when you dig and the weight going over the top, not the surface material. Soft or made-up ground pulls the answer back toward a proper sub-base even for a footpath.
Does Water Drain Through a Sub-Base?
Water soaks into a compacted sub-base: the layer is dense but still granular, so rain works down through it to the ground rather than sheeting off the top. But a sub-base is not a drainage system. It passes water through, it cannot carry water away, and a base that sits saturated loses strength and settles. Shedding rain is the surface’s job, through the falls built into it. Where a build needs the base itself to let water through by design, permeable paving and SuDS work, MOT Type 3 is the grade to ask for.
No sub-base will fix a waterlogged garden. Where water already stands, design the drainage in first, a soakaway at the low point or a perforated pipe run in the excavation, because retrofitting a drain through a finished base is a nightmare of a job.
What Material Should You Use for a Sub-Base?
| Material | What is in the bag | Where I would use it |
|---|---|---|
| MOT Type 1 | 40mm down to dust, certified grading, compacts hardest | Driveways, patios, paths, shed bases. The default. |
| MOT Type 2 | A finer blend, roughly 20mm down, with more fines | Light garden paths and levelling layers |
| MOT Type 3 | 40mm down to about 4mm, fines largely screened out | Permeable paving, SuDS builds, wet ground |
| Crusher run | Crushed rock screened to a top size, 50mm down to dust. A well-graded one can be sold as Type 1 | Fine for drives and patios when the blend is good |
| Scalpings | Quarry by-product, quality varies hugely | Acceptable when the load is good, unreliable by name |
| Ballast | Gravel and fines mix, little control over the ratio | A stand-in in the south-east where crushed rock is dear |
| Recycled Type 1 | Crushed concrete, brick and reclaimed stone | Anywhere Type 1 works, if it meets the grading |
| Planings | Recycled road surfacing, asphalt-rich | Farm tracks. Not the drive you park on. |
Reusing What Is Already in the Ground
Rubble from your own dig can go back in on the same terms as any other hardcore: crushed to 40 to 50mm with fines through it, not dropped in as whole flags and half bricks, which leave voids that settle later. At roughly 100 square metres of old concrete or flags, hiring a portable crusher and crushing on site starts to beat paying for skips and then buying stone in.
How Deep Should a Sub-Base Be?
Sub-base depth follows the load above it: 100mm compacted under a patio, 75 to 100mm under a path, and 150mm under a driveway. Those are compacted figures, which matters when you come to order.
| Project | Compacted sub-base depth |
|---|---|
| Garden path, pedestrian only | 75 to 100mm |
| Patio | 100mm |
| Shed base | 100mm |
| Artificial grass | 75 to 100mm, plus a blinding layer on top |
| Driveway, family car | 150mm |
| Driveway, heavier vehicles or soft ground | 200mm and up, built in lifts |
Ground condition moves those numbers as much as traffic does. Heavy clay holds water and shifts with it, so on clay I go deeper and always separate the layers with a membrane. And if you find a soft spot during the dig, take it out and fill it with compacted stone: a solid bottom to the dig beats a level one.
Work your excavation depth from the top down: paving plus laying course plus sub-base gives you the dig, so 60mm blocks on a 40mm bed over 150mm of sub-base means excavating 250mm. Paving has to finish at least 150mm below the damp-proof course, so check that level before you commit. The full build sequences are in the gravel driveway guide and the gravel patio guide.
How to Compact a Sub-Base Properly
Compacting a sub-base means thin layers and plenty of passes: no more than 150mm loose per lift, at least eight passes of a vibrating plate on each, and anything over 225mm compacted built as two lifts. Tip the whole depth in at once and the plate only ever compacts the top, while the bottom stays soft and announces itself a year later. The material also compacts best slightly damp, around 8 to 13% moisture: bone dry it will not bind, soaking wet it will not compact at all.
A weighted hand tamper, like a sledgehammer with a board attached to it, in thin lifts will do for a couple of square metres of path, and the quick check is to press your full weight through your heel: if the surface gives, keep going. For anything bigger, and every drive, hire the plate for the day. If the finished surface reads open and honeycombed rather than tight, brush in fines and run the plate over it again, or the bedding sand will trickle into those voids and take your paving with it.
Do You Need a Membrane Under a Sub-Base?
A membrane under a sub-base is a separation layer, there to stop the stone punching down into soft ground and the ground working up into the stone. On clay, soft or made-up ground it earns its place many times over. On firm, undisturbed subsoil I am comfortable compacting stone straight onto the ground, so a builder telling you it is optional there is not wrong.
There is a second sheet worth knowing about, and confusing the two is where most of the arguments start. The one at the bottom separates base from soil. A second sheet laid over the finished base, under loose gravel, does a different job: it stops the decorative stone working down into the sub-base. Loose gravel surfaces want that top sheet, soft or clay ground wants the bottom one, and a drive on clay is fair reason to use both.
What it is not doing is stopping weeds: they arrive from above and germinate in the joints, and a sheet buried 200mm down does nothing about that. Decide on your ground, not on weed fear, pick the fabric by weight and weave rather than the name on the roll, and overlap sheets by 300 to 400mm. Which weight suits which job is worked through on what to put under gravel.
How Much Sub-Base Do I Need?
Working out sub-base quantities is area times finished depth, plus roughly 30% for compaction, because stone packs down as you compact it. That 30% is the bit every short order misses, and it is why the sub-base calculator builds it in and gives you the answer in bags and tonnes straight away. Treat coverage as approximate, since real spread depth varies with the rock and its moisture, but here is what one of our bags, weighed and measured on our own yard, covers:
| Compacted depth | Coverage per 850kg bag |
|---|---|
| 50mm | ~8.8m² |
| 75mm | ~5.8m² |
| 100mm | ~4.4m² |
| 150mm | ~2.9m² |
| 200mm | ~2.2m² |
Next Steps
We supply sub-base in 850kg bulk bags and artic wheeler loads with nationwide delivery: MOT Type 1 is the certified grade, the aggregates category has the rest of the range, and the sub-base calculator turns your measurements into bags before you order. If the job is unusual, get in contact and we will talk it through.
Have you ever laid a sub-base yourself? What depth did you go to, and how did you find the compacting went? Tell us in the comments.
FAQs
What is a sub-base made of?
A sub-base is made of crushed, well-graded stone, most commonly MOT Type 1, running from 40mm lumps down to dust so the fines lock the lumps together under compaction.
Can you lay a patio without a sub-base?
A patio can be laid without a sub-base only on firm, undisturbed subsoil with the slabs on a full mortar bed. On clay or made-up ground, skipping it is what causes rocking slabs a season later.
What is the difference between sub-base and sub-grade?
Sub-grade is the natural ground you are building on once the turf and topsoil are off. Sub-base is the graded stone layer you import and compact on top of it.
What is granular sub-base?
Granular sub-base is the engineering term for an unbound layer of graded stone, meaning nothing glues the particles together and the strength comes purely from compaction and interlock. MOT Type 1 is the granular sub-base most UK merchants sell.
How thick should a sub-base be for artificial grass?
A sub-base for artificial grass runs 75 to 100mm compacted, with a blinding layer of grano dust or sharp sand screeded over it for the smooth surface the turf needs. Going straight from coarse Type 1 to grass telegraphs every stone through the pile.