How Much Sub-Base Under a Concrete Slab? UK Depth and Quantity Guide
Published 29 July 2026 · Browse all tools
The sub-base is the layer that decides whether a slab stays flat or cracks in its first winter, and most people either skimp on it or over-order and leave a tonne of MOT Type 1 rotting in the garden. Getting it right means two numbers: how deep to go for the load you are carrying, and how much stone that works out to once you allow for compaction. This guide covers both, works through a real UK shed base, and flags the mistake that catches out more DIY slabs than any other.
To turn your own measurements straight into tonnes and bulk bags, use the Gravel Under Concrete Slab Calculator; this guide explains the depths and the reasoning behind them.
Gravel Under Concrete Slab Calculator — volume, tonnes and bulk bagsWhat the sub-base actually does
A concrete slab looks solid, but it is only as good as the ground it sits on. The sub-base is a compacted layer of crushed stone that spreads the load from the slab out across the soft ground below, gives you a firm level platform to cast against, and lets water drain away rather than sitting under the slab and heaving it in a frost. Skip it, or lay it too thin, and the slab flexes over soft spots until it cracks. The standard UK material for this is MOT Type 1, a graded crushed limestone or granite that locks together tightly when compacted.
Takeaway: the sub-base is structural, not just packing; it is the reason a thin slab can carry a heavy load without cracking.
How deep should the sub-base be?
Depth follows the load. These are the working UK ranges most groundworkers use for a domestic slab:
| Use | Compacted sub-base depth |
|---|---|
| Paths, shed bases, light patios | 75–100 mm |
| Patios and general foot traffic | 100 mm |
| Driveways, light vehicles | 100–150 mm |
| Regular or heavy vehicles | 150–200 mm |
The number that matters is the compacted depth, the thickness after you have run a plate compactor or roller over it, not the loose depth you tip out of the bag. Loose MOT Type 1 drops by roughly 15 to 25 percent when it is properly compacted, so a 100 mm finished layer starts life around 120 to 130 mm loose. On soft or clay ground you go deeper, or add a geotextile membrane to stop the stone punching down into the mud over time.
Takeaway: quote yourself the compacted depth, then order enough loose stone to hit it after it drops under the plate.
How much sub-base to order
Once you have the depth, the volume is straightforward: length times width times compacted depth, all in metres. That gives cubic metres. Convert to tonnes with the material density, and MOT Type 1 runs at about 2.0 tonnes per cubic metre. Then add a compaction and waste allowance on top, because you always lose some to the drop and some to spillage and over-dig at the edges. Ten percent is the sensible default for a tidy job, more if the ground is uneven.
The gravel under concrete slab calculator does all of this and rounds up to whole bulk bags, which is how you actually buy the stuff.
Takeaway: work in compacted cubic metres, multiply by 2.0 for MOT Type 1 tonnage, then add 10 percent before you order.
Worked example: a 3 m by 4 m shed base in Leeds
Say you are laying a base for a decent-sized garden workshop in Leeds, 3 m by 4 m, on reasonable ground. It is a shed base with the odd heavy tool, so 100 mm of compacted MOT Type 1 is right.
- Base volume: 3 × 4 × 0.10 = 1.2 m³
- Add 10 percent compaction and waste allowance: 1.2 × 1.10 = 1.32 m³
- Convert to weight at 2.0 t/m³: 1.32 × 2.0 = 2.64 tonnes
- Bulk bags to order at roughly 0.8 m³ usable stone per bag: 1.32 ÷ 0.8 = 1.65, rounded up to 2 bulk bags
So you order 2 bulk bags of MOT Type 1, about 2.6 tonnes, from a merchant like Travis Perkins or a local aggregates supplier, and you will have a little spare for the edges. A bulk bag from most UK builders' merchants in 2026 runs somewhere around £45 to £70 delivered depending on area and quantity, so the sub-base for this base is roughly £90 to £140 in material.
Takeaway: two bulk bags cover a typical 12 m² shed base at 100 mm, with enough left over to true up the edges.
The mistake that cracks slabs: skipping compaction
Here is the part the cheaper guides skip. Tipping the stone in and levelling it with a rake is not a sub-base, it is a pile of loose gravel. Uncompacted MOT Type 1 keeps settling for months under load, unevenly, and that differential settlement is exactly what cracks a slab. The stone only does its job once it is compacted in layers no thicker than about 100 mm at a time, each pass run over with a plate compactor or, for anything driveway-sized, a roller. If your sub-base is deeper than 100 mm, you lay and compact it in two lifts, not one deep dump. Skipping this saves an afternoon and costs you the whole slab.
Takeaway: compact in 100 mm layers with a plate; a single deep uncompacted dump is the most common reason a home-poured slab fails.
Do not forget the blinding and the DPM
Two more layers sit between the sub-base and the concrete on a proper job. A thin sand blinding, around 25 to 50 mm, fills the sharp voids in the top of the compacted stone so it does not puncture the next layer. Then a damp-proof membrane, a 1200-gauge polythene sheet, stops ground moisture wicking up into the slab. On a shed base you might get away without the blinding if the top of the stone is fine and tight, but the DPM is not optional if the slab is being lived on, floored, or insulated. Type 1 sub-base sizing follows BS EN 13285 for graded unbound mixtures, and the slab design itself sits under Approved Document A for structure and Approved Document C for moisture.
Takeaway: the sub-base is one layer of four; blinding and a DPM turn a rough base into a slab you can build on.
What UK groundworkers do differently
In practice, most UK groundworkers over-dig the sub-base slightly wider than the slab, not just level with it, so the compacted stone extends past the slab edge and the edges do not crumble away under load. They also check the finished level with a laser or a long straightedge before the concrete goes anywhere near it, because a sub-base that is 20 mm out gets hidden by the slab and you never find it until the floor slopes. And on clay, which is most of the Midlands and the North, they will nearly always roll out a geotextile separator first for a few pounds a square metre, because losing your Type 1 into soft clay over five years is a far more expensive problem to fix than to prevent.
Takeaway: run the sub-base past the slab edge, laser-check the level before you pour, and use a separator membrane on clay.
Work out your quantity
Put your slab length, width and sub-base depth into the Gravel Under Concrete Slab Calculator to get the volume, tonnes and number of bulk bags in one go. If you are also pouring the slab, the Concrete Slab Bag Calculator works out how many bags of concrete you need on top.
Disclaimer: This guide is for planning and estimating only. The correct sub-base depth and design depend on the ground conditions, the load, and drainage, which vary by site. For a foundation, a slab carrying a structure, or any case where the result matters, follow the project design and confirm requirements against Approved Document A and C, or take advice from a qualified groundworker or structural engineer.