What Goes Under Artificial Grass? The Cape Town Base Build, Layer by Layer

Under a properly installed artificial lawn there are four things, in this order from the bottom up: a compacted sub-grade, a compacted stone sub-base (usually crusher run, 75–100mm), a screeded laying course of crusher dust or sharp sand (25–40mm), and a weed membrane. The grass is then fixed to a perimeter edge and dressed with kiln-dried silica sand infill. On some jobs a shockpad goes in between the laying course and the grass.

That is the whole answer, and it is not complicated. What is complicated is doing it correctly on Cape Town ground, because the base that works on Cape Flats sand is not the base that works on Durbanville clay, and neither behaves the same in July as it does in February. We build sub-bases for driveways every week, so we approach turf the way we approach paving preparation — the surface layer is the cheap part, the base is the job.

The full layer stack at a glance

Layer 1: excavation and the sub-grade

Excavate to somewhere between 100mm and 150mm below your finished level. Finished level matters more than people expect: the top of the grass should sit flush with adjacent paving, a fraction below a door threshold, and slightly proud of a bed edge so water runs off rather than pooling against a wall.

Everything organic comes out. Topsoil, old kikuyu, root mat, buried builder’s rubble, the remains of a flower bed. Organic matter breaks down and the surface dips over it — that is where those unexplained hollows in a two-year-old turf lawn come from. Cut any tree roots you are entitled to cut, and if a root is structural, plan the lawn around it rather than through it.

Then compact what is left with a plate compactor. On sand this is quick. On clay, if the sub-grade is wet and rubbery underfoot, stop and wait for it to dry. Compacting saturated Durbanville clay just kneads it, and it will move later.

Layer 2: the sub-base — the layer that decides whether it stays flat

This is where turf installers and paving contractors part ways. Plenty of turf-only outfits skip straight from soil to sand. That is fine on a shaded, level, sandy patch nobody walks on. It is not fine on a family lawn, on clay, or anywhere with a slope.

Use G5 crusher run or a similar graded 19mm-and-down stone, laid in one or two lifts and compacted to 75–100mm. Graded stone matters: the mix of fines and larger aggregate is what locks together under a plate compactor. Clean single-size stone will not bind, and river sand will not either.

• Light-use, sandy, flat sites: 75mm compacted is enough.
• Clay soils, slopes, or areas taking a trampoline, a dog run or steady foot traffic: 100mm.
• Areas a vehicle may cross: do not use artificial grass. Pave it. Turf over a driveable base still ruts and flattens.

Compact in layers no thicker than about 50mm. Compacting 100mm in one pass gives you a firm top and a loose bottom, which amounts to no compaction at all six months later.

Layer 3: the laying course

On top of the compacted stone goes 25–40mm of crusher dust or sharp washed sand, screeded flat and then compacted again. This is your accuracy layer. Every ripple you leave here shows through the grass, because turf is only 30-odd millimetres thick and hides nothing.

Screed to a fall of roughly 1:80 to 1:100 away from the house. Artificial grass is permeable, but the base underneath still has to shed water somewhere, and a dead-flat lawn tight against a wall is how you end up with a damp problem in August.

Use crusher dust rather than builder’s sand. Builder’s sand is too fine, holds water and slumps. Cape Flats dune sand is worse — the grains are rounded, they do not lock, and they migrate.

Layer 4: weed membrane, and when to add a shockpad

A woven geotextile membrane over the laying course stops anything germinating from below. It does not stop weeds germinating in the infill from windblown seed on top — nothing does, and anyone who tells you otherwise has not maintained a lawn through a south-easter season. Those top-side weeds pull out by hand in seconds because they have nothing to root into.

A shockpad earns its place in two situations: under a play area where you want genuine fall protection, and on a rooftop or over a concrete slab where you cannot build a proper stone base. It also takes a couple of degrees off surface temperature and makes the lawn noticeably softer underfoot. Everywhere else the money is better spent on a higher grade of grass.

Cape Flats sand versus Durbanville clay

These two soils fail in opposite directions, and the base spec should reflect that.

Sandy soils — most of the Cape Flats, Table View, Blouberg, Milnerton, Parklands. Drainage is excellent, almost too good. The problem is bearing capacity and migration: loose sand moves under load and washes out at edges. Compact the sub-grade properly, use the full 75mm of graded stone, and pay real attention to edge restraint. Without a hard edge, a sandy-site lawn creeps and the perimeter goes wavy.

Clay soils — much of Durbanville, Brackenfell, Kraaifontein and pockets of the Tygerberg slopes. Clay holds water, swells in winter and shrinks in summer. Here the stone sub-base is not optional; it is the layer that separates the grass from all that seasonal movement. Go to 100mm, and if the site is a known wet spot, add a perforated drainage pipe in a stone-filled trench running to a soakaway or the stormwater connection. It is the same logic we apply to driveway drainage in winter rain — give the water a designed route or it will choose its own.

Drainage: building for a Cape winter, not a Cape summer

Most artificial grass in South Africa is sold on a summer promise. Cape Town’s problem season is winter. Between roughly May and August we get the rain the rest of the country gets in December, and it arrives in long soaking fronts rather than short thunderstorms.

Good turf backing is perforated and drains far faster than any rainfall we get, so water almost never fails at the grass. It fails at the base: a compacted clay sub-grade with nowhere to send water, a lawn boxed in by walls on three sides, or a level that sits lower than the surrounding paving so the lawn becomes the low point of the site.

• Set the lawn level at or slightly above surrounding hard surfaces, never below.
• Fall the base away from the house, always.
• On clay or in a walled courtyard, put a French drain along the low edge in before the stone goes down. Retro-fitting one means lifting the whole lawn.
• If the area puddles in winter now, fix that first. Turf does not fix drainage; it hides it until it starts to smell.

Edges, joins and infill — the finishing layers

Artificial grass has to be restrained at the perimeter or it will shrink back, lift and curl. Options, roughly in order of how long they last: a haunched concrete edge, a soldier course of paving brick, treated timber battens fixed to pegs, or proprietary composite edging. A brick or paver edge is our default because it doubles as a maintenance-free border and ties the lawn into any adjacent patio paving.

Joins should run with the pile direction, all rolls facing the same way, seamed onto joining tape with outdoor adhesive. A visible seam is almost always a pile-direction mistake, not a glue mistake.

Finally, brush in kiln-dried silica sand infill — typically in the region of 5–8kg per square metre depending on pile height, applied in passes with a stiff broom. Infill is not filler. It weights the lawn against wind lift, which matters a great deal on an exposed Table View or Big Bay site, holds the fibres upright so the lawn does not go flat and shiny, and adds a little thermal mass. Skipping infill to save a few rand is the single most common reason a lawn looks tired within two summers.

What the base costs, and where people cut corners

Installed artificial grass in Cape Town runs roughly R350–R650 per m² all-in depending on grass grade and site, with the roll itself somewhere between R89 and R420 per m². The full breakdown sits in our artificial grass prices guide. What that means in practice: on a mid-range job a meaningful share of your money is base work — stone, compaction, edging and labour — not the green stuff.

So when one quote comes in far below the rest, the base is where it has been taken out. The tells are easy to spot in a written quote: no excavation depth stated, no mention of crusher run or G5, no edge detail, no infill quantity. Ask for those four things in writing and most of the price gap explains itself.

The five mistakes we see most

• Laying over existing lawn. The grass dies, decomposes, and the surface sinks in patches within a year.
• Sand-only base. No structural layer, so it dips wherever people walk and holds every footprint after rain.
• Compacting in one thick lift. Firm on top, loose underneath, settles later.
• No perimeter restraint. Edges lift, curl and collect leaves.
• Too little infill. The pile lies flat, the lawn shines in summer sun, and it wrinkles in the wind.

None of these are exotic. They are shortcuts that save an afternoon and cost a lift-and-relay. If you are still weighing up the decision itself rather than the spec, our comparison of artificial grass versus a real lawn covers the ten-year maths honestly, including the cases where we tell people to keep the kikuyu. And if you want a base spec written for your actual soil and levels, we quote artificial lawn installations across the city — send through the suburb and a photo or two.

Quick answers

Can you lay artificial grass directly on soil?

Only on a temporary or very light-use area, and it will not stay flat. Soil holds organic matter that decomposes and moves with moisture, so the surface dips and puddles. A compacted stone sub-base with a screeded laying course is what keeps a lawn true.

How deep should the base under artificial grass be?

Excavate 100–150mm below finished level, then build 75–100mm of compacted crusher run plus 25–40mm of screeded crusher dust. Use the deeper figures on clay soils, slopes and any area that takes regular foot traffic.

Do you need sand infill on artificial grass?

Yes, on almost every outdoor installation. Kiln-dried silica sand weights the lawn against Cape Town’s wind, keeps the fibres standing so it does not go flat and shiny, and helps the seams hold shape. Leaving it out is a false saving.