The short answer
A proper artificial grass base on soil is a compacted, free-draining build-up in layers. The standard UK base is a Type 1 MOT sub-base (crushed stone) laid in compacted layers to form a firm foundation, topped with a thinner levelling layer of sharp sand or granite (grano) dust screeded smooth. A weed membrane is laid over the soil and often a second layer above the aggregate. Everything is compacted with a plate compactor (wacker plate) so it can't sink or ripple, and the surface is set to a slight fall for drainage. Typical total base depth is around 50–75mm for a domestic lawn, deeper on heavy clay. On an existing patio or concrete, the hard surface acts as the base and you only need to ensure drainage and a smooth, clean surface, sometimes with a foam underlay.
Almost every problem people have with artificial grass — sinking, rippling, puddles, weeds — traces back to the base. Getting the layers, materials and compaction right is the part of the job that earns a flat, lasting lawn.
Base build-up at a glance
- Sub-baseType 1 MOT, compacted
- Levelling layerSharp sand or grano dust
- Weed controlMembrane over soil (and aggregate)
- CompactionPlate compactor (wacker plate)
- Typical depthAround 50–75mm on soil
Why the base matters so much
Artificial grass is only as good as what's underneath it. The base has three jobs: provide a firm, stable foundation that won't sink, give a perfectly flat surface so the grass lies smooth, and allow water to drain away. Skimp on any of these and the lawn will sink in places, ripple, hold puddles or grow weeds through.
On a soil garden, you can't just lay grass on the ground — soft topsoil compresses unevenly and drains poorly. That's why a proper base replaces the top few centimetres of soil with engineered, free-draining layers that are compacted hard. On an existing patio or concrete, the hard surface already does the structural job, so the base requirement is much simpler.
The standard layers on a soil lawn
A typical UK base on soil, from the ground up, is:
- Weed membrane: a geotextile sheet over the excavated soil to stop weeds coming up from below.
- Type 1 MOT sub-base: crushed limestone or granite spread in layers and compacted. This is the structural foundation — firm, load-bearing and free-draining. Several centimetres is typical, more on poor ground.
- Levelling (binding) layer: a thinner layer of sharp sand or granite/grano dust, screeded and compacted to a smooth, true surface that removes any remaining bumps and gives the grass a flat bed.
- Upper weed membrane (optional): a second membrane above the levelling layer for extra weed protection directly under the grass.
Builders' sand is generally avoided for the levelling layer because it can wash and move; sharp sand or grano dust hold their shape better once compacted.
| Layer (top to bottom) | Material | Role |
|---|---|---|
| Upper membrane | Weed membrane | Weed control under grass |
| Levelling layer | Sharp sand / grano dust | Smooth, flat surface |
| Sub-base | Type 1 MOT (compacted) | Firm, free-draining foundation |
| Lower membrane | Weed membrane | Stops weeds from soil |
Indicative typical base on a soil lawn. Depths vary by site and drainage.
Depths, drainage and bases on hard surfaces
How deep? For a domestic soil lawn, a total base of roughly 50–75mm (sub-base plus levelling layer) is common. On heavy clay or poorly draining ground, dig deeper and use a more generous free-draining sub-base, because drainage is the most frequent cause of trouble. The finished base should sit so the grass surface ends up just below paths and patios.
Drainage: set the base to a gentle fall away from the house where possible, so surface water runs off. The free-draining aggregate then lets remaining water soak through to the ground below.
On patios and concrete: a hard, sound, free-draining surface is already a base. You only need to clean and level it, ensure water can escape (drilling weep holes if it pools), and optionally add a foam shockpad for comfort. No Type 1 sub-base is needed over solid paving.
Materials explained and getting compaction right
It helps to understand what each base material is and why it is chosen, because using the wrong one is a common cause of trouble.
- Type 1 MOT sub-base: a graded crushed stone, usually limestone or granite, with a mix of particle sizes that lock together when compacted to form a hard, stable, free-draining layer. It is the same material used under driveways and paths, which is why it makes such a dependable foundation.
- Sharp sand: a gritty, angular sand used for the levelling layer. Its angular grains bind and hold their shape once compacted, unlike soft builders' sand which can wash and move.
- Granite (grano) dust: a fine crushed-granite dust, an alternative levelling material that screeds to a very smooth, firm surface and is popular for the binding layer.
- Weed membrane: a woven or non-woven geotextile that lets water and air through while blocking weed growth.
- Kiln-dried sand: the dried silica sand brushed into the grass as infill once it is laid, not part of the base but the final step.
Compaction is what turns loose aggregate into a solid base. The key points are to compact in layers rather than all at once, to dampen the material lightly so it binds rather than working it bone-dry or waterlogged, and to use a proper plate compactor rather than treading it down. The finished sub-base should feel firm with no soft spots, and the levelling layer should be screeded flat and true. A common error is tipping a deep layer and trying to compact it in one go — the lower part stays loose and the lawn sinks there later.
Get the materials, layering and compaction right and the base will support a flat, free-draining lawn for the long term. It is unglamorous work, but it is the single biggest factor in whether the finished grass looks good and stays that way.
Frequently asked questions
Can you use sand instead of Type 1 as a base?
Sand alone is not a substitute for a Type 1 sub-base. Type 1 MOT provides the firm, load-bearing, free-draining foundation, while a thin layer of sharp sand or grano dust on top is used only to level the surface. Using sand on its own would compress and move unevenly, leading to a lumpy, sinking lawn over time.
How deep should the base for artificial grass be?
For a domestic soil lawn, a total base of around 50–75mm of compacted Type 1 plus a sharp sand levelling layer is typical, set so the grass finishes just below paths and patios. On heavy clay or poorly draining ground, you should dig deeper and use a more generous free-draining sub-base to handle drainage.
Do you need a base if laying on an existing patio?
No, a sound, free-draining patio or concrete surface already acts as the base, so you do not need a Type 1 sub-base. You just need to clean and level the surface, make sure water can drain away (drilling weep holes if it pools), and optionally add a permeable foam underlay for comfort before laying the grass.
Sources & further reading
Figures on this page are typical UK ranges drawn from published cost guides and are intended as guidance, not a quotation.