What Is a G5 Sub-Base — and Why Paving Fails Without One

A G5 sub-base is the compacted layer of crushed stone that sits under your paving and actually carries the load. The bricks on top are a wearing surface and a finish — they are not the structure. When a Cape Town driveway sinks, ruts under a bakkie wheel or develops a wavy line down the middle, the fault is almost always in the layer nobody can see any more.

We price and build paving across the city every week, and the single biggest difference between a driveway that still looks straight in ten years and one that fails in two winters is what went down before the first brick was laid. Here is what G5 is, how it differs from G7, how thick it should be, and how to check what your contractor actually put in the ground.

What the letters and numbers actually mean

G-classifications come from the South African road-building specifications (the COLTO / TRH material classes). They describe the quality of a gravel or crushed stone layer — its strength, grading and how much fine material it contains. Lower number means better material.

• G1 to G3 — premium crushed stone, used in national road pavements. Overkill and unnecessarily expensive for a domestic driveway.
• G4 and G5 — good-quality crushed or natural gravel with a controlled grading. G5 is the practical domestic standard for driveways and any surface carrying vehicles.
• G6 and G7 — lower-strength gravels, more fines, weaker when wet. Acceptable as a sub-base under a G5 layer on a deep build-up, or as a base for light foot-traffic paving.
• G9 / general fill — site-won material, builder’s rubble, unclassified sand. Not a structural layer, whatever anybody tells you.

The important thing about G5 is not that it is “stone”. It is that the stone is graded — a designed mix of particle sizes from roughly 37mm down to dust. The big particles interlock and carry load; the fines fill the voids so the layer locks up tight under a roller. A load of single-sized 19mm stone is not a sub-base. It has nothing to fill the gaps, so it never compacts and it lets water and bedding sand wash straight through.

G5 versus G7 — when each one is honest

Contractors quoting very low numbers often save money precisely here: they swap G5 for whatever gravel is cheapest that week, or they skip the sub-base entirely and lay bricks on bedding sand over the existing ground. It is invisible on handover day. It is very visible after the first proper Cape winter. Our honest breakdown of what work actually costs is in the Cape Town paving prices guide, and the driveway-specific version is in our 2026 driveway paving cost breakdown.

How thick should the layer be?

There is no single number, because thickness is a response to two things: what is underneath and what will drive over it. Rough working figures we use when pricing domestic work in Cape Town:

• Patios and pathways on firm ground — roughly 75–100mm of compacted G5 under the bedding layer.
• Standard car driveway on decent subgrade — roughly 100–150mm compacted.
• Driveway on loose sand or soft clay — 150–200mm or more, often built in two layers, sometimes with a geotextile separation fabric at the bottom.
• Turning circles, gate aprons, anywhere wheels turn while stationary — treat as the heavy case. Turning scrubs the surface and punishes a thin base.

Note the word compacted. A 150mm loose tip of stone becomes something closer to 110–120mm once it is properly rolled. If the spec says 150mm, that is the finished, compacted thickness, and the contractor has to bring in enough material to get there.

Compaction in lifts — the part that gets skipped

A plate compactor or small roller only densifies the top portion of what it is standing on. Dump 200mm of stone in one go and run a plate over it and you get a hard crust with loose material underneath — which is worse than useless, because it feels solid during handover and settles a year later.

The correct method is compaction in lifts: place the material in layers of roughly 100–150mm depending on the machine, water it to close to optimum moisture, and compact each layer to refusal before placing the next. On a Cape Town site in February that means actually wetting the stone — bone-dry graded material will not lock up no matter how long you run the plate. In July it can mean the opposite: waiting for the layer to drain, because saturated material pumps and shoves instead of compacting.

Signs of a properly compacted layer, on site, without any test equipment:

• The surface rings hard and does not dent under a boot heel.
• Walking on it leaves no visible footprint impressions.
• A wheelbarrow rolls across it without cutting a groove.
• The plate compactor stops sinking and starts bouncing — that is refusal.
• Water sprayed on it beads and runs to the fall rather than disappearing straight down.

The layers, top to bottom

A correctly built paved driveway is a sandwich, and each layer has one job:

Two of those get abused constantly. The first is the bedding sand — some crews thicken it to 60 or 80mm to save on stone and to make levelling easier. Sand is not a base. Thick bedding sand migrates, and you get the classic wavy driveway. The second is the edge restraint: without it, even a perfect G5 layer cannot stop bricks spreading at the open edge, and the joints open up until weeds and water get in.

Cape Town ground conditions change the answer

Two properties fifteen minutes apart can need genuinely different builds.

Sandy suburbs — Blouberg, Table View, Parklands, Milnerton and across the Cape Flats. The subgrade is deep, loose, uniformly-graded sand. It has reasonable bearing capacity when confined, but it will not hold a thin layer of stone in place; the stone simply mixes into it. Here we go thicker, often with a geotextile separator so the G5 and the sand stay as two distinct layers instead of blending into a useless hybrid. That build-up is worth insisting on anywhere the spade goes straight into soft white sand.

Clay suburbs — parts of Durbanville, Brackenfell and the Tygerberg side. Clay swells when wet and shrinks when it dries. Cape Town’s wet winter and bone-dry summer is exactly the cycle that drives that movement. On clay, the sub-base has to be thick enough to bridge seasonal heave, and drainage matters even more, because clay holds water against the base instead of letting it away.

Slope-heavy suburbs — Plattekloof, Hout Bay, Constantia, Vredehoek. Water arrives at the paving as sheet flow from higher up the property. If it runs under the paving instead of off it, it will scour fines out of the sub-base and you will get a localised sinkhole. Getting the falls and the cut-off drains right is inseparable from getting the base right — see driveway drainage for Cape Town winter rain.

How to check what your contractor actually laid

You do not need a lab. You need to be on site at the right moment, because once the bricks are down, everything below is a matter of trust.

• Ask for the material by name on the quote. “Base preparation” means nothing. “150mm compacted G5, placed in two lifts” means something you can hold someone to.
• Look at the excavation depth. Add up your paver thickness, bedding sand and specified sub-base. That is roughly how deep the dig should be below finished level. If they excavated 80mm total for a driveway, the numbers do not add up.
• Look at the stone itself. Graded G5 has a full range of sizes including dust. If the tipped pile is all one size, or is obviously builder’s rubble with brick fragments and plaster in it, question it.
• Watch for the compactor. A driveway base compacted with the back of a spade is not compacted. There should be a plate compactor or roller on site, and it should run over each lift several times in overlapping passes.
• Check the falls before bedding sand goes down. The sub-base should already be shaped to the finished falls — you cannot fix drainage with bedding sand later.
• Take photos. Photograph the open excavation, the stone going in and the compactor working. It costs nothing and it settles arguments.

If you are still choosing who to appoint, the questions worth asking are simple: what material, what compacted depth, and how many lifts.

What it costs to do this properly

Base work is the least glamorous line on a quote and the one most worth paying for. It is also why two quotes for the same brick can differ so much: one includes an excavation, a proper stone layer and compaction plant, and the other includes a rake and some sand.

Published installed rates in Cape Town run roughly R380–R900 per m² for most paving materials, with driveways specifically landing between about R650 and R1 600 per m² once material, base preparation, slope and access are all accounted for. A quote sitting well below that range has usually made its saving underground. If you want to see the trade-offs laid out honestly, the honest position is that base work is where you can genuinely save and where you cannot.

Redoing a failed driveway costs more than building it right the first time, because you pay twice for demolition, twice for disposal and twice for labour. There is no version of this where skipping the sub-base is the cheap option; it is just deferred.

Where we can help

We build to a stated base spec and we are happy to have you standing there while the stone goes in. If you want a quote that says what is going underneath rather than just what you will see, take a look at our driveway paving services or get in touch with the site details and we will price it properly.

Quick answers

What is G5 sub-base material?

G5 is a graded crushed stone or gravel classified under South African road specifications. Its particle sizes run from roughly 37mm down to dust, so it interlocks and compacts into a solid structural layer — unlike single-sized stone or plain sand.

How thick should the G5 layer be under a driveway?

Usually 100–150mm compacted on firm ground, and 150–200mm or more on loose Cape Flats sand or reactive clay. It must be placed and compacted in lifts of about 100–150mm, not dumped in one go.

Can I lay paving straight onto sand without a sub-base?

For a light-use garden path you can get away with a thin stone layer. For anything a vehicle crosses, no — the paving will rut and sink, and relaying it costs far more than building the base correctly at the start.