Ready-Mix Concrete C20 (GEN 3): What It's For and When You Need Something Stronger
C20 concrete costs £90–115/m³ per m³ delivered. It's for shed bases, pathways, and drainage haunching, not extension foundations. Here's how to order it right.
Your builder orders "C20" for your extension foundations. Building control inspects the delivery ticket, sees a 20 N/mm² concrete where the structural engineer specified 25, and refuses to sign off the pour. The concrete has already arrived, the truck is running, and your builder now has 6 cubic metres of the wrong grade hardening in the drum. That's £600+ wasted, a day lost, and a rebooking fee on top.
C20 concrete is one of the most commonly ordered grades in the UK. It's also one of the most commonly ordered for the wrong job.
What C20 concrete is
C20 is a designation from BS 8500 (the British Standard for specifying concrete). The "C" stands for compressive strength class, and "20" means it achieves a minimum compressive strength of 20 N/mm² (newtons per square millimetre) after 28 days of curing. In practical terms, that's enough strength to support light loads, resist moderate ground pressure, and provide a stable base for non-structural work.
The same concrete has two other names you'll encounter. Its designated mix name under BS 8500 is GEN 3, which stands for "general purpose, category 3". You'll also see it written as C20/25, where the second number (25) is the cube strength (a slightly different test method that gives a higher number for the same concrete). When you phone a ready-mix supplier, ask for "C20 GEN 3". They'll know exactly what you mean.
Do not confuse GEN 3 with ST1. Several websites incorrectly claim C20 is equivalent to ST1 (a standardised prescribed mix). It isn't. ST1 achieves only 7.5 N/mm², which is a fraction of C20's strength. The correct ST equivalent for C20 is ST4 (20 N/mm²). If your builder mentions "ST1", check they mean the right grade.
What C20 is for (and what it isn't)
This is the section most guides get wrong. Concrete suppliers list C20 as suitable for "domestic foundations" because technically, under BS 8500, it can be used for lightly loaded strip foundations. But NHBC Standards 2024 (Section 3.1.6) specify C25/30 as the minimum for foundations of habitable buildings, and most structural engineers follow that standard.
C20 is the right grade for:
- Drainage haunching (the concrete surround that holds drain pipes in position)
- Blinding (a thin layer poured at the bottom of a trench to create a clean, level surface before the structural concrete goes on top)
- Pathway and patio sub-bases
- Shed and garage bases (non-habitable outbuildings)
- Fence post foundations and similar garden structures
- Oversite concrete (the ground-floor slab in some older construction methods)
C20 is not the right grade for:
- Strip foundations for an extension
- Trench-fill foundations
- Reinforced pad foundations
- Any structural element specified by an engineer
If your structural engineer's drawings say "C25/30" (and they almost certainly will for an extension), that's what you order. The £10-15 per m³ difference between C20 and C25 is irrelevant against the cost of a failed building control inspection.
The three naming systems (and why they cause confusion)
UK concrete has three parallel designation systems, and they don't map neatly onto each other. This causes genuine confusion on building sites and in forums. Here's the definitive table.
| C-class (strength) | GEN designation | ST equivalent | 28-day strength | Typical use |
|---|---|---|---|---|
| C8/10 | GEN 0 | ST1 (7.5 N/mm²) | 8 N/mm² | Kerb bedding, non-structural fill |
| C12/15 | GEN 1 | ST2 (10 N/mm²) | 12 N/mm² | Strip foundation blinding, mass fill |
| C16/20 | GEN 2 | ST3 (15 N/mm²) | 16 N/mm² | Light-use floor slabs, drain surrounds |
| C20/25 | GEN 3 | ST4 (20 N/mm²) | 20 N/mm² | Shed bases, paths, haunching, blinding |
| C25/30 | GEN 4 | ST5 (25 N/mm²) | 25 N/mm² | Domestic strip and trench-fill foundations |
| C28/35 to C32/40 | RC28/35+ | n/a | 28-32 N/mm² | Reinforced slabs, structural elements |
The C-class tells you the guaranteed minimum cylinder/cube strength. The GEN designation is what you order from the ready-mix plant; it specifies a recipe (cement content, water-cement ratio, aggregate size) that reliably achieves that strength. The ST system is an older "standardised prescribed" mix approach that specifies ingredients by weight rather than targeting a strength outcome. ST mixes are rarely used commercially now, but the numbers still circulate online, often incorrectly.
When ordering, use the GEN designation. "GEN 3" is unambiguous. "C20" can mean slightly different things depending on who you're talking to. "ST4" will confuse most people on the phone.
How ready-mix delivery works
Ready-mix concrete arrives in one of two types of truck.
Barrel trucks (the classic rotating drum) carry a pre-batched load from the plant. A standard truck holds 6m³. The concrete is mixed with water at the plant and has a 90-minute window from batching to discharge, after which it starts to stiffen and lose workability. Most suppliers include 30 minutes of unloading time in the price. Go over that and you'll pay waiting charges, typically £1-2 per minute.
Volumetric trucks carry the raw ingredients (cement, aggregates, water) separately and mix on site. You pay only for what's actually dispensed. There's no 90-minute clock, no waiting charges, and no waste from over-ordering. The per-m³ cost is higher, but for orders under 4m³ a volumetric truck often works out cheaper because you avoid the short-load surcharge.
What happens if you're not ready
The concrete truck arrives at the booked time regardless of whether your site is prepared. If the driver can't discharge within the free unloading window (usually 30 minutes from arrival), waiting charges start. If you cancel on the day, you'll typically pay the full load cost because the concrete has already been batched and is setting in the drum. Some suppliers charge a rebooking fee of £50-100 on top.
The most common reason for a wasted delivery: formwork not finished, or the base of the pour area still has standing water from overnight rain. Check your site the evening before delivery day. Make sure any formwork is secure, the area is clear of debris, and you have enough helpers on hand to manage the pour. For a typical C20 shed base pour, you need at least two people: one to rake and level the concrete as it comes off the chute, and another to tamp and screed (drag a straight-edged board across the top to level it). Three is better.
If heavy rain is forecast for the morning of your pour, phone the supplier the afternoon before and reschedule. A wasted load costs more than a rescheduling fee.
A standard barrel truck needs 3 metres of clear width to access your site. Measure before you book. If the truck can't reach your trench, you're either barrowing from the road (add 3-4 extra helpers) or hiring a pump.
Ordering the right amount
Concrete is ordered by volume in cubic metres (m³). The formula is simple: length x width x depth, all in metres. For a shed base 3m x 2.4m x 0.1m deep, that's 0.72m³.
Always add 10% for wastage. Concrete sticks to the chute, splashes during pouring, overfills formwork edges, and settles unevenly. Running short mid-pour is worse than having a small surplus. You can use leftover C20 to fill a fence-post hole or pour a stepping stone.
For any pour, agree the grade and the slump class (how wet the mix is) when you book. For most C20 applications, S2 or S3 consistency is appropriate. S2 is stiffer and holds its shape better in formwork. S3 is wetter and easier to work but takes longer to set. If someone asks about slump, S3 is the safe default for domestic work.
What to check on delivery day
The truck driver hands you a delivery ticket. This ticket is your proof of what was supplied, and building control can ask to see it. Check three things:
- Grade matches your order (C20/GEN 3, or whatever was specified)
- Batch time is within the 90-minute window
- QSRMC or BSI Kitemark logo is printed on the ticket, confirming the plant has third-party quality certification
Keep the ticket. Building control inspectors don't always ask for it, but when they do, not having it creates problems.
How much it costs
C20/GEN 3 delivered (national average)
£90 – £115
London and the South East run £110–135/m³ per m³, reflecting higher transport costs and land prices. A typical small domestic job (shed base, pathway foundation, drainage work) needs 1-3m³.
The headline per-m³ price only tells half the story. If you order less than a full 6m³ truck, you'll pay a short-load surcharge of £40–150 per delivery. That surcharge can easily double the effective cost per m³ on a 1m³ order.
Pump or barrow?
If the truck can't reach within chute distance of your pour location, you have two choices.
Wheelbarrowing is free but brutal. A single barrow holds about 0.06m³. For 2m³ of concrete, that's 33 barrow runs, each weighing around 140kg. You need a minimum of three people, ideally five, and you need to work fast because the 30-minute clock is running.
A concrete pump costs £275–800 for a day. The pump-grade concrete add-on is £2-3/m³ on top (slightly finer aggregate so it flows through the hose). For anything over 3m³, or where access is tight, the pump pays for itself in time saved and stress avoided.
Site-mix: the false economy
For very small quantities (under 0.25m³), mixing on site from bags makes sense. For anything bigger, it doesn't.
Site-mixing 1m³ of C20 requires 14 bags of cement at around £6.50–8.50 each, plus 2 tonnes of ballast at £35-55 per tonne delivered, plus water. That's roughly £195 in materials alone, before you hire a concrete mixer (£30-50/day) or factor in your time. Ready-mix delivered is £90–115/m³ per m³ with a professional quality-controlled mix and a delivery ticket for building control.
The only scenario where site-mix wins: you need a quarter of a cubic metre, you already own a mixer, and you're not in a hurry. For everything else, pick up the phone.
For quantities between 0.5m³ and 3m³, ask about volumetric delivery. Companies like Bell Concrete and Base Concrete run volumetric lorries that mix on site and charge only for what they dispense. No waste, no time pressure, no short-load surcharge in most cases.
Alternatives to C20
C25/GEN 4 is the step up. It costs £10-15 per m³ more than C20 and is the standard for domestic extension foundations. If there's any doubt about which grade you need, C25 is the safer choice. Over-specifying by one grade is cheap insurance. Under-specifying fails building control.
Site-mixed concrete using the 1:2:4 ratio (1 part cement, 2 parts sharp sand, 4 parts coarse aggregate) or 1:6 with ballast produces approximately C20-equivalent strength. But "approximately" is the problem. Site-mixed concrete has no quality certification, no delivery ticket, and no independently verified strength. For work that doesn't need building control sign-off (a garden path, a fence post), it's fine. For anything inspected, use ready-mix.
Postcrete and fast-set bags (Wickes, Toolstation, Screwfix all stock these) are designed for setting fence posts. One 20kg bag fills one post hole. They set in 10-15 minutes. Do not use them for anything structural.
Cold weather and pouring conditions
Do not pour concrete when the ambient temperature is below 5 degrees C or when frost is forecast within 48 hours. Concrete that freezes before it reaches 5 MPa strength (roughly the first 24-48 hours of curing) can lose up to 50% of its design strength. That damage is permanent and invisible.
Between 5 and 8 degrees C, you can pour with precautions. Cover the fresh concrete with insulating blankets or polythene sheeting immediately after placing. Keep it covered for at least 5-7 days rather than the standard 3 days. Don't strip formwork early.
Above 8 degrees C is normal conditions. Still protect from rain for the first few hours and keep the surface damp for 3 days minimum. Concrete doesn't dry, it cures, a chemical reaction with water. If it dries out too fast (hot sun, wind), the surface cracks.
The delivery ticket records the concrete temperature at dispatch, which must be at least 5 degrees C per BS EN 206. If the driver turns up with concrete below that threshold, you're within your rights to refuse the load.
Common mistakes
Ordering the wrong grade. C20 when the spec says C25. Or ordering "GEN 3" and assuming it covers everything. Always check the structural engineer's specification before phoning the ready-mix plant.
Adding water on site. The truck arrives, the concrete looks stiff, and someone sticks a hose in the drum to make it flow better. Every litre of water added on site reduces the final strength proportionally. If the mix is too stiff, the plant got the slump class wrong. Send it back. Don't dilute it.
Underestimating the team needed. A 6m³ pour by wheelbarrow needs 5-6 people working flat out for 30-45 minutes. Two people with one barrow cannot clear a full truck in time. Plan your crew before you book the concrete.
Pouring before building control has inspected. For any work under building regulations, the inspector must see the excavation before concrete goes in. Book the inspection at least 48 hours ahead. If the inspector can't come on pour day, you wait. Pouring first and asking permission later can result in being ordered to break out the concrete and start again.
Ordering too little. Running out mid-pour creates a cold joint (a weak plane between old and new concrete). A half-finished foundation is worse than no foundation. Order 10% more than your calculation says. The cost of surplus concrete is negligible compared to rebooking a truck.
Where you'll need this
- Drainage - C20 is used for haunching around drainage pipes and benching inside manholes. Typical quantities: 0.5-1m³, often small enough for site-mix or a volumetric delivery.
- Garden and external works - patio sub-bases, pathway foundations, fence post footings, and general external concrete work. Typical quantities: 0.5-2m³ depending on the scope.
These tasks appear across all stages of any extension or renovation project, not just kitchen extensions. Drainage haunching and external works use the same concrete whether you're building a rear extension, a side return, or a garden room.