Ballast

Ballast 0-20 mm: all-in aggregate for mixing concrete

Ballast 0-20 mm is a ready-blended mix of sharp sand and gravel with stones up to 20 mm. It is often sold as all-in ballast or 20 mm ballast, and it is the grade most builders reach for when mixing concrete by hand or in a mixer. Add cement and water and you have a general-purpose concrete for foundations, footings, shed bases and fence posts. The sand and gravel come from the depot nearest to you, so the colour varies from one part of the country to the next. That has no effect on the strength of the concrete.

What ballast 0-20 mm is used for

• Concrete for strip foundations, footings and floor slabs
• Bases for sheds, greenhouses, summerhouses and garages
• Setting fence posts, gate posts and washing line posts
• Footings for garden walls and cores for small retaining walls
• Haunching behind kerbs and edging stones
• Garden steps, plinths and pads for hot tubs or oil tanks
• Compacted fill in trenches and service ducts
• Levelling low spots before a slab is poured

Mixing concrete with 0-20 mm ballast

Concrete is mixed by volume, so use the same bucket for cement and ballast. As a rule of thumb:

• 1 part cement to 5 parts ballast for general work: shed bases, paths, slabs and post holes.
• 1 part cement to 6 parts ballast for mass-fill foundations and footings that carry a light load.
• 1 part cement to 4 parts ballast where you need a stronger, denser mix, such as a garage floor or a wall footing.

Add water a little at a time until the mix holds its shape without water pooling on the surface. Too much water makes weak concrete. For a load-bearing structure, follow the specification given by your engineer or building control rather than a rule of thumb.

How much ballast for a cubic metre of concrete

One cubic metre of finished concrete takes roughly 1.8 tonnes of ballast and 14 to 15 bags of cement at 25 kg, mixed 1:5. The ballast compacts as the cement paste fills the gaps between the stones, which is why you need more loose material than the finished volume.

One bulk bag of around 800 kg with six or seven bags of cement gives close to 0.45 m³ of concrete. That is enough for a shed base of 3 m by 1.5 m at 100 mm thick, or for about 25 fence post holes. Work out the volume of what you are pouring in metres, multiply by 1.8, and round up to the next bulk bag.

0-20 mm, 0-10 mm or 0-40 mm?

The number is the largest stone in the mix. Ballast 0-10 mm is finer and suits thin sections, patching and post holes where a bigger stone would get in the way. Ballast 0-40 mm has larger stones and is used for thick mass-fill foundations where the extra bulk saves cement. Ballast 0-20 mm sits in the middle and works for almost every domestic job, so choose it unless you have a reason to go finer or coarser.

Where ballast is the wrong choice

Ballast is not a drainage material. The sand content packs down and holds water, so use a clean single-sized gravel or a drainage stone for soakaways, French drains and pipe bedding. It is also not a wearing surface. Loose ballast on a driveway breaks up under tyres, and as a sub-base under block paving or tarmac it does not lock together the way a crushed MOT Type 1 does. Use ballast for what it is made for, which is concrete.

Questions about ballast 0-20 mm

Is ballast the same as all-in aggregate?

Yes. All-in aggregate, all-in ballast and ballast are different names for the same blend of sand and gravel. The grade, here 0-20 mm, tells you the size of the largest stone.

Can I use ballast on its own as a sub-base?

Only for light jobs such as a path or a small shed on soft ground, and only if it is compacted in layers. For anything that carries a vehicle, use MOT Type 1 and keep the ballast for the concrete on top.

How much ballast do I need for a shed base?

Measure length and width in metres, multiply by the thickness in metres, usually 0.1, and multiply the result by 1.8 to get tonnes of ballast. A 2.4 m by 1.8 m base at 100 mm is 0.43 m³, so one bulk bag.

Does the colour of the ballast matter?

No. Ballast is dug locally and ranges from yellow to grey to brown. The colour shows in the finished concrete but has no bearing on its strength.

Please note that the stones are measured at the narrowest point. The sizes used on the page therefore refer to the technical measurement. Because the gravel is measured at the narrowest point, the actual size may vary slightly.