What Goes Under Artificial Grass (and Why)
Updated: Oct 9
A proper artificial grass base is four layers on firm ground: a weed membrane on the dug-out soil, typically 50-75mm of compacted stone sub-base, around 15-25mm of compacted granite dust as a laying course, then the grass with kiln-dried sand brushed into it. Commercial and heavily used areas go deeper, typically 75-100mm of sub-base. Each layer has a job, and skipping one is what makes lawns dip, puddle, smell or grow weeds.
Layer by layer, from the bottom up
Starting from the dug-out ground (typically 75-100mm below finished level):
Firm subgrade. The natural ground with turf, roots and loose topsoil removed. Soft spots get dug out and filled with stone.
Weed membrane. A woven or non-woven geotextile laid on the soil, overlapped and pinned.
Sub-base. Typically 50-75mm of MOT Type 1 or 10-20mm clean granite or limestone chippings for a home lawn, compacted with a vibrating plate.
Laying course. Around 15-25mm of granite or limestone dust (grano), screeded level and compacted.
The grass. Rolled out, joined, fixed to the edges and filled with kiln-dried sand.
Around all of it sits an edge (timber, steel, sleepers or paving) that stops the stone spreading sideways. This is the standard build-up behind a fitted artificial grass installation, and the same thinking as a driveway base: dig to firm ground, pick the stone for the drainage, compact it properly and hold it in.
The sub-base: Type 1 or chippings?
The sub-base is the structural layer. It spreads the load of feet, furniture and play so the ground underneath does not rut, and it lets water down.
MOT Type 1: A crushed stone graded from coarse pieces right down to dust. It compacts very hard and stays level, which is why it is the default under paths and lawns. The fines that make it compact well also slow drainage, so on its own it is not especially permeable.
Clean chippings (10-20mm): Single-size granite or limestone with no fines. It does not lock up as tight, but water runs straight through it, which makes the whole layer act like a shallow soakaway under the lawn. Specialist installers favour it for dog lawns and gardens that hold water.
On free-draining ground either works. On heavy, slow-draining soil or for pets, the chippings route is the safer choice.
Granite dust or sharp sand for the laying course?
Granite dust. Installer guides are consistent on this: compacted granite (or limestone) dust locks into a firm, smooth bed that stays put. Sharp sand moves underfoot when it is dry and forms lumps and dips when it is wet, which shows straight through the grass.
The dust needs compacting in several passes with a light dampening; each pass takes air out and reduces the chance of sinkage later. The finished surface should be smooth enough that you could lay the grass on it and see no stones or ridges.
Where does the membrane go?
The main weed membrane goes at the bottom, on the soil, under the stone. That is where weeds come from, and there it also keeps the stone from being pushed into soft ground.
A second membrane on top of the granite dust is optional. It can help against wind-blown weed seeds that get into the base, but it is a sheet of fabric right under the turf. On a lawn used by dogs leave it out, because it soaks up urine and holds the smell. Any top membrane also has to lie perfectly flat, or ripples show through.
Artificial grass drainage on clay
Artificial grass drains through holes in the backing (or a fully permeable backing) into the stone below. The stone holds the water while it soaks away. On sand or gravel soils that is quick; on heavy clay it can be very slow, and a stone base sitting on clay can fill up like a bath.
Ways to deal with it:
Use free-draining clean chippings rather than Type 1 so the base stores more water.
Give the formation (the dug-out ground) a slight fall towards the lowest side.
Add a soakaway or a land drain at the low point if the area is known to stay wet.
Check that nothing, such as a patio laid to fall onto the lawn, is sending extra water across it.
If a lawn puddles after it has been laid, the base is the problem, not the grass.
Sand infill: what it does and how much
Most domestic artificial grasses are designed to take kiln-dried sand brushed into the pile. A typical rate is 4-6kg per square metre, depending on pile height, with about 5kg a common rule of thumb. It weighs the grass down so it does not ripple, and supports the fibres so they stand up.
It must be kiln-dried. Damp building sand clumps and does not brush in. For dog lawns, odour-absorbing infills sold for pet areas can be used in the same way.
Building on something other than soil
If the area is already sound concrete or paving, you often do not need a stone sub-base at all: the hard surface is the base, with drainage holes where water sits and a foam shock pad on top. Laying artificial grass on concrete only goes wrong when the slab is cracked, lifting or traps water, and then breaking it out and building a stone base is better.
Artificial grass base questions, answered
How deep should the base be?
In one common spec for a home lawn, about 75mm of dig-out holds 50mm of sub-base and 25mm of dust; other installers dig to 100mm, and deeper on clay. Busy commercial or public areas typically use 75-100mm of sub-base, and very soft ground needs more.
Can you lay artificial grass on soil or sand alone?
Not if you want it to last. Without compacted stone, the surface moves, dips and holds water, and weeds come through.
Does the season affect the base?
Wet ground is harder to dig and compact, which is why the best time of year to install artificial grass depends more on ground conditions than on the calendar.
The base is the part of a lawn you never see and the part that decides how long it lasts. Pete and the team build lawn bases around Wolverhampton the same way they build driveways: dug out properly, a compacted stone base and solid edges.
