Coarse Aggregate (20mm Gravel): The Stone for Site-Mixed Concrete

A homeowner phones the builders' merchant and orders "ten bags of ballast and five of sharp sand" for a garden wall footing. The driver tips the lot, the homeowner mixes 1:5, and the resulting concrete is so weak it crumbles under the trowel a fortnight later. The mistake was simple. Ballast already contains sharp sand. By adding more sand on top, the mix ended up cement-starved and full of fines. Anyone batching their own concrete needs to know exactly which aggregate they're holding, what's in it, and what to combine it with. Coarse aggregate is the half of that question that catches people out.

What it is and what it's for

Coarse aggregate is the stone-only component of concrete. In UK practice, it almost always means clean, washed 20mm gravel: angular or rounded stone particles ranging roughly from 4mm up to 20mm, screened and washed to remove dust, clay, and silt. Under the current British Standard, BS EN 12620 (the European concrete aggregate standard adopted in the UK), it sits in the d/D 4/20 grading band, which is the dominant size used for domestic structural concrete. Particles smaller than 4mm are classed as fine aggregate (sharp sand). Anything larger than 20mm is reserved for mass concrete and large engineering pours.

You use coarse aggregate when you're site-mixing concrete from raw ingredients: cement, water, sharp sand, and stone, batched together in a mixer or barrow. It's the bulk of the mix by volume and weight. In a standard 1:2:4 domestic concrete (1 part cement, 2 parts sharp sand, 4 parts coarse aggregate), the gravel makes up roughly half the finished concrete by weight. Its job is structural. The cement paste binds everything together, but the load-carrying skeleton inside the concrete is the interlocked stone.

Coarse aggregate is sold separately from sharp sand at every major UK builders' merchant: Travis Perkins, Jewson, Buildbase, MKM, plus Wickes and Toolstation in smaller bagged formats. Online aggregate specialists like Mainland Aggregates, TW Aggregates, and Supreme Merchants deliver bulk bags or tipper loads. It's the same product across all of them, with regional variations in stone type (river-rounded gravel versus quarried crushed limestone or granite).

Coarse aggregate vs all-in ballast: don't confuse the two

The single most common homeowner mistake on this material is buying coarse aggregate when ballast is wanted, or vice versa. Five out of six community threads reviewed during research turned on this exact confusion.

The two products serve the same end purpose (concrete) but belong to different mixing approaches:

If you're following a structural specification (foundations, a garage slab, anything reinforced), buy coarse aggregate and sharp sand separately and batch a 1:2:4 or 1:1.5:3 mix. If you're pouring a path, a fence-post collar, or a small non-structural slab and you don't care about exact strength class, ballast at 1:6 with cement is faster and easier to order.

Regional naming varies. In most of England the blended product is "ballast." In the West Country it's "half-inch to dust." In some northern merchants you'll hear "hoggin" used loosely for similar material. When ordering by phone, describe what you actually need: "clean 20mm concreting gravel, no sand mixed in" gets you coarse aggregate every time.

Types and specifications

Within coarse aggregate, three distinctions matter for a homeowner.

20mm versus 10mm. 20mm is the UK standard for slabs and foundations. 10mm aggregate exists, but you only need it where space is tight (around dense reinforcement, in narrow lifts, or where a fine surface finish matters). 10mm produces slightly weaker concrete for the same cement content because the cement-paste-to-stone ratio shifts. For 99% of homeowner use cases, the answer is 20mm. If a merchant asks "10 or 20?", say 20.

Crushed versus rounded. Crushed stone (quarried limestone, granite, or basalt) has angular faces that interlock and resist movement. Rounded river or sea gravel rolls past itself more easily. Crushed stone produces concrete with slightly higher shear strength, but it's harder to mix because the angular faces resist flow. Rounded gravel mixes more easily and gives better workability. For DIY work, take whatever your local merchant stocks. The performance difference is small at the strength classes you'll be using.

Concreting versus drainage gravel. Some merchant 20mm gravel SKUs are washed and graded specifically for drainage (French drains, soakaways), which means they may have a tighter size distribution, less fines, and a price premium. Others are washed concreting aggregate, sold for both drainage and concrete work. A few decorative gravels carry the same particle size but are sold for landscaping and may not meet BS EN 12620. When ordering, specify "concreting aggregate" or "20mm concrete gravel" to avoid getting a polished decorative product. The grey, slightly dusty, slightly damp version on a builders' merchant pallet is almost always the right one.

The British Standard governs all of it. BS EN 12620 sets requirements for cleanliness (clay coatings, organic matter, sulphate content), particle shape (the flakiness index must not exceed 30%), and grading. Aggregate from any reputable UK quarry-fed merchant complies with this by default. Where it goes wrong is unwashed, dry-screened material from low-end suppliers, or recycled aggregate that hasn't been tested. Clay-contaminated aggregate reduces concrete compressive strength by approximately 35%. That's not a marginal effect. A clay-coated 1:2:4 mix that should reach C20 ends up at roughly C13.

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How to work with it

Weight, handling, and storage

Coarse aggregate is heavy. Loose 20mm gravel weighs approximately 1,500 kg/m³, which means a standard 850kg bulk bag holds roughly 0.55-0.6 cubic metres. A full bulk bag will not move on a pallet trolley over rough ground; you need a forklift, a delivery lorry with a HIAB crane, or a teleporter. Site delivery normally drops bags by HIAB on the kerb or driveway. Confirm exact placement before the lorry arrives, because bags can't be repositioned by hand once they're down.

For loose tipper deliveries (typically 10-20 tonnes), you need clear access for a 26-tonne tipper truck and a hardstanding capable of receiving the load without rutting the surface. A 10-tonne tip will create a pile roughly 4m across and 1m high. Plan where it goes. Wheelbarrow-distance from where you'll be mixing matters. Twenty barrow runs across a sodden lawn is slower than ten across a hardstanding.

Store bulk bags off bare soil where possible. A few sheets of plywood underneath stop fines migrating up into the stone from below. Cover with a tarpaulin if the bag will be exposed for more than a couple of weeks; rainwater runs through gravel quickly but blown organic debris (leaves, mulch, grass clippings) sits on top and contaminates the surface layer.

In freezing weather, frozen aggregate must thaw before being added to a mix. Ice crystals in the stone melt during mixing and skew the water-cement ratio, weakening the concrete. Protect your aggregate stockpile in winter or pour in milder weather.

Cleanliness check at delivery

The five-minute job that nobody does, and that builders quietly do every time, is checking the load. When the bag or tipper arrives:

1. Look at the colour. Clean concreting aggregate is grey or grey-brown. A reddish or yellow tint suggests clay contamination.
2. Squeeze a handful. The stones should fall apart cleanly. If the stones cake together in your palm or leave a brown clay smear on your hand, refuse the load.
3. Check the silt level. Take a clean jam jar, fill it a third with aggregate fines from the bottom of the heap, top up with water, shake hard for 30 seconds, and let it settle. The fines should settle as a thin band on top of the stone within five minutes. A thick muddy band more than 5mm deep is a fail.
4. Confirm particle size. A handful should contain stones up to roughly 20mm across (about the size of a large coin). If it's all small (5-10mm) you've been sent the wrong product.

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Mixing technique

Site-mixing concrete is loud, dirty, and surprisingly easy to get wrong. The fundamentals are:

1. Batch by volume, not weight. For domestic work, use a consistent measure (a bucket, a builder's barrow, a shovel scoop), and stick to it. The Concrete Society's DIY guidance treats one 25kg cement bag as the base unit and scales sand and stone by volume from there. Don't try to weigh ingredients on bathroom scales. It doesn't work and you'll lose patience after the second mix.
2. Add water in stages, not all at once. Start with two-thirds of the water, add the dry ingredients, mix until just wet, then add water in small splashes until the mix flows but doesn't run. Over-watering is the most common cause of weak concrete.
3. Mix for at least three minutes after the last ingredient goes in. Under-mixed concrete has dry pockets and weak spots. The mixer drum should turn enough times to coat every stone in cement paste.
4. Pour within 30-45 minutes of adding water. After that, the cement starts setting in the drum. Don't add water to a stiffening mix to "loosen it up": that creates a cement-poor surface layer that weakens the pour.

A petrol or electric drum mixer (Belle Minimix 150, Altrad Belle, JCB) handles roughly 90 litres of mix per batch, which is enough for two large barrows of concrete. For larger pours, hire a 3.5/2.5 mixer or step up to ready-mix.

How much do you need

Quantities depend on the mix ratio. The two ratios you'll meet for domestic site-mix concrete are 1:2:4 (general-purpose, roughly C20 strength) and 1:1.5:3 (stronger, roughly C25, suitable for footings under heavier loads).

Per cubic metre of finished 1:2:4 concrete, you need approximately:

• 6 bags of 25kg cement (around 150kg total)
• 450-500kg sharp sand
• 1,000-1,100kg coarse aggregate

Per cubic metre of 1:1.5:3 concrete, you need approximately:

• 8 bags of 25kg cement (around 200kg total)
• 400kg sharp sand
• 800-850kg coarse aggregate

Always add 10% to your aggregate quantities for wastage. Concrete spillage, mixer residue, and trench overpour add up. Coarse aggregate left over after a pour stores indefinitely if covered, so over-ordering by half a bulk bag costs you nothing but cupboard space.

Worked example: six post-hole bases for a fence or pergola

Six post holes, each 400mm diameter, 600mm deep:

• Volume per hole: π × 0.2² × 0.6 = 0.075 m³
• Total: 0.45 m³ concrete
• At 1:2:4: roughly 3 bags of cement, 230kg sharp sand, 500kg coarse aggregate
• Round up: 1 bulk bag of coarse aggregate (850kg), 1 small bulk bag of sharp sand, 4 bags cement (allows for waste)

This pour costs roughly £59 – £75 for the gravel, plus sharp sand and cement, plus the day to mix it. Compare that to a ready-mix supplier minimum charge of £500 – £750 for any delivery, and the site-mix wins easily.

Worked example: 4m x 6m extension trench fill foundation

Trench fill at 600mm wide x 850mm deep around a 20m perimeter:

• Volume: 20 × 0.6 × 0.85 = 10.2 m³ concrete
• At 1:2:4: roughly 60 bags of cement, 5 tonnes of sharp sand, 11 tonnes of coarse aggregate
• That's 13 bulk bags of gravel and 6 bulk bags of sand, plus a pallet of cement, plus a mixer running for two solid days

At this scale, hand-mixing makes no sense. A 10m³ pour from a single ready-mix truck arrives in 90 minutes for £90–130/m³ per cubic metre. Site-mixing the same volume costs more in raw materials than ready-mix once you account for cement bags at retail rates and the sheer labour involved, and risks a cold joint between batches that no building control officer will accept on a structural foundation.

The breakpoint between site-mix and ready-mix sits firmly at the volume where you can't mix and pour continuously in a single working day. For most homeowners on most jobs, that's around 1-2 cubic metres.

Site-mix vs ready-mix: the breakeven

The honest answer here is the one no concrete supplier wants to print and no DIY guide explains clearly.

Site-mix is cheaper for pours under roughly 1m³. Below that volume, the ready-mix supplier's minimum charge of around £500 – £750 outweighs the cost of the raw ingredients many times over. Six fence-post bases, a small garden-wall footing, a single shed slab: site-mix all the way.

Ready-mix wins on cost above roughly 2m³, and dominates above 4m³ where the short-load surcharge of £40–150 drops away. For an extension foundation, a garage slab, or any structural pour, ready-mix is the only sensible answer. You get certified mix design (BS 8500 compliance), a delivery ticket the building control officer can verify, and the entire pour completed in one continuous operation.

The middle (1-2m³) is the awkward zone. Site-mix is feasible but slow. Ready-mix carries a surcharge but delivers a stronger, certified product. If the work is structural or the BCO will inspect it, pay the surcharge. If it's non-structural and the access is bad, mix on site.

A volumetric truck (a "mix-on-site" truck that batches concrete to order from raw materials carried on board) splits the difference. Volumetric suppliers don't charge minimums of the same scale and only bill for what you use, but they charge a per-m³ premium of £15 – £30 over fixed-batch ready-mix. For 1.5-3m³ pours where you don't want to site-mix and a full ready-mix truck is overkill, volumetric is often the right answer.

Cost and where to buy

Coarse aggregate is sold three ways: as 25kg "handy" bags from retailers, as 850kg bulk bags from merchants and online aggregate suppliers, and as loose tipper loads from bulk-haulage specialists.

The 25kg bag is roughly twice the bulk-bag rate per tonne. Use them only for tiny patch jobs (a missing chunk of concrete around a lamppost, a single fence-post collar). For anything bigger, the bulk bag wins.

On bulk bags, the cheapest landed prices come from online aggregate specialists at the lower end of the range, with the major DIY retailers sitting towards the upper end (and often running multi-buy offers that close the gap). Builders' merchants fall between the two on bulk bags and become the cheapest option once you scale up to tipper loads, where Travis Perkins, Jewson, and Buildbase all offer postcode-priced loose aggregate from local depots.

Loose tipper delivery is 25-40% cheaper per tonne than bulk bags above about 5 tonnes ordered. The breakpoint to switch from bulk bags to a tipper sits at roughly 3-4 bulk bags (around 2.5-3.4 tonnes). If you need that much aggregate, phone for a tipper quote. The catch: a tipper drops a single heap on your driveway, and you have to clear it within a day or two to free the access.

What about delivery surcharges

Most online aggregate suppliers add a delivery surcharge on small orders (one or two bulk bags), with the surcharge falling away on larger orders. Wickes and Travis Perkins normally include delivery in the bulk-bag price within a regional radius. Loose tipper deliveries always include delivery to a stated radius. Beyond that radius, expect a per-mile surcharge.

London and the South East carry a 15-25% premium across all aggregate products, partly because of distance from quarries and partly because of access constraints in urban areas. If you're in Greater London and a quote feels high, that's why.

Alternatives

All-in ballast. Pre-blended sharp sand and 20mm coarse aggregate, sold as a single product. Mix at 1:5 or 1:6 with cement for general-purpose concrete. The right choice when you don't need precise control over the strength class, when access doesn't allow two separate piles of material, or when you're pouring small amounts and don't want the faff of measuring two aggregates separately. Slightly less control over the final strength than batching from separate ingredients, and unsuitable for structural work where the engineer has specified a particular grade.

Ready-mix concrete. Ordered by the cubic metre, delivered by a truck-mounted mixer, certified to BS 8500. The right answer for any pour above about 2m³ or any structural element where the BCO needs to see a delivery ticket. Materially more expensive per m³ than site-mix raw ingredients, but the labour saving and the compliance paperwork make it the dominant choice for extensions and reinforced foundations. See the C20 and C25 knowledge pages for grade selection.

10mm aggregate. A finer grade of the same product, used where space is tight, where reinforcement is dense, or where a smoother surface finish matters. Slightly weaker concrete for the same cement content. Don't substitute 10mm for 20mm just because the merchant has it in stock.

Where you'll need this

These pages don't exist as published tree leaves yet. When the groundwork branch ships, coarse aggregate will be referenced in:

• Foundations and footings (small repair pours, fence-post bases, non-structural infill where site-mix wins)
• Drainage (20mm clean stone is the standard fill for French drains and soakaways under Approved Document H)
• Damp-proof course preparation (small site-mix pours around DPC junctions)
• External works (paths, paving sub-bases, garden walls, post bases for fences and pergolas)

The material is used across all stages of any extension, renovation, or external-works project where small-volume site-mixed concrete makes sense. It's not specific to kitchen extensions; the same 20mm gravel goes into a garden room base, a loft conversion's reinforced lintel bedding, and a garage slab.

Common mistakes

Buying ballast and sharp sand together. The classic over-sand mistake. Ballast already contains sharp sand, so adding more produces a cement-poor, fines-rich mix that's weaker than either pure ratio would give. If you want to mix from separate components, buy coarse aggregate and sharp sand. If you want the convenience of pre-blended material, buy ballast and don't add anything except cement.

Ordering decorative gravel for concrete work. Some merchant 20mm gravels are sold for landscaping with a colour and finish premium. Decorative gravels haven't necessarily been tested to BS EN 12620 and may have polished particles that bond poorly with cement paste. When in doubt, specify "concreting aggregate" or read the product description for "BS EN 12620 compliant" before ordering.

Skipping the cleanliness check at delivery. Five minutes of squeezing a handful and shaking a jam jar of fines and water saves a foundation pour. Clay-contaminated aggregate is the single biggest known cause of weak DIY concrete and is the reason a clay-coated 1:2:4 mix that should reach C20 ends up nearer C13. Refuse contaminated loads at the kerbside, before the lorry leaves and you sign the ticket.

Hand-mixing structural concrete. Site-mixed volume-batched concrete is fine for fence posts and garden walls, not for structural elements. If your engineer specified RC 25 or C25/30 on the drawings, order ready-mix. Trying to argue volume-batched concrete past the BCO at the pour-day inspection ends with the BCO refusing to sign the foundation off.

Using 25kg bags for anything bigger than a patch. The unit rate per tonne is roughly double the bulk-bag rate. For any pour above about 100kg of stone, the bulk bag is the correct format. For very small patches, fine. For a garden wall footing, you've paid two bulk bags' worth of money for a third of a bulk bag's worth of stone.

Ignoring the wastage allowance. A 10% over-order on aggregate costs you almost nothing and absorbs the inevitable spillage, trench bulge, and mixer residue. Running short mid-pour on a structural element forces a cold joint, which on a foundation means breaking out and starting again. Order extra. Worst case, you have a pile of clean stone left over for next year's path repair.