Concrete & Construction News

How Much Concrete Do You Actually Need? The 10% Rule

One of the most common mistakes in DIY concrete projects is ordering the exact amount your calculations say you need. In practice, you should always order 10% more than your calculated volume. Here's why:

• Uneven ground: Even a "flat" surface has dips and variations that eat up more concrete than you'd expect
• Spillage and waste: Some concrete will inevitably be lost during mixing and pouring
• Formwork movement: Timber shuttering can shift slightly under the weight, expanding the pour area
• Subgrade absorption: Dry soil absorbs water from the concrete mix at the base

Running short mid-pour is far worse than having a bit left over. A cold joint (where fresh concrete meets partially set concrete) creates a weak point that can crack over time. Use our concrete calculator and add 10% to the result.

Understanding Concrete Mix Ratios: What the Numbers Mean

Walk into any builder's merchant and you'll see labels like "C20", "C30", or "C40" on ready-mix bags. But what do they actually mean?

The number refers to the compressive strength in N/mm² (Newtons per square millimetre) that the concrete will achieve after 28 days of curing. So C20 concrete can withstand 20 N/mm² of pressure before failing.

• C15: Foundations for small walls, bedding for kerbs. The minimum for most structural use.
• C20: Domestic foundations, garage floors, paths. The standard DIY grade.
• C25: Driveways, house foundations. Good all-rounder for load-bearing.
• C30: Reinforced foundations, commercial floors, external hardstanding with heavy traffic.
• C40+: Commercial and industrial use, multi-storey structures, bridges.

For most home projects — a shed base, patio, garden path — C20 or C25 is perfect. Don't over-spec (it costs more) and don't under-spec (it'll crack).

Why Concrete Cracks (And How to Prevent It)

Cracking is the number one complaint with DIY concrete work. Understanding why it happens is the first step to preventing it:

• Too much water in the mix: The biggest cause. Sloppy, wet concrete is easier to pour but dramatically weaker. Aim for a consistency like thick porridge, not soup.
• No control joints: Concrete shrinks as it cures. Without control joints (grooves cut into the surface), it decides where to crack. With joints, you choose.
• Curing too fast: Concrete needs moisture to cure properly. If it dries out in hot weather, it shrinks and cracks on the surface. Cover new concrete with polythene or damp hessian for at least 3 days.
• Poor subbase: Concrete poured directly onto soft or uneven ground will settle unevenly and crack. Always compact a layer of MOT Type 1 or hardcore underneath.
• No reinforcement: For slabs over 100mm thick, steel mesh or fibre reinforcement helps distribute loads and control cracking.

The golden rule: use less water, cure slowly, and always prepare the base properly.

UK Building Regulations: When Do You Need Approval for Concrete Work?

Not all concrete work needs building regulations approval, but it's important to know when it does:

• House foundations: Always require building control approval. Depth, width, and concrete grade are specified by your building control officer based on soil type.
• Garage/extension foundations: Yes, always regulated.
• Garden walls over 1m (or 2m if not adjacent to a highway): Need approval if structural.
• Shed bases, patios, garden paths: Generally don't need approval, but check if you're in a conservation area or if drainage is affected.
• Driveways: Don't need building regs, but if over 5m² with non-permeable surface, you may need planning permission for drainage (since 2008).

When in doubt, a quick call to your local council's building control department is free and can save you a costly mistake. They'd rather advise you upfront than enforce afterwards.

Ready-Mix vs Hand-Mixed: When to Call the Truck

For small jobs, mixing your own concrete from bags is fine. But at what point does ordering ready-mix delivered by truck make more sense?

The rough rule of thumb: anything over 1 cubic metre, get it delivered. Here's why:

• 1m³ of concrete = about 40 bags of 25kg cement, plus sand, gravel, and water. That's a lot of manual mixing.
• Ready-mix is machine-batched to exact specifications — more consistent than hand mixing in a belle mixer.
• Time matters: Once you start a pour, you need to finish before the first batch starts setting. A truck delivers it all at once.
• Cost comparison: Ready-mix is typically £65-£120/m³ delivered. Bagged equivalent costs more and takes hours of labour.

Most suppliers have a minimum order of 1-2m³ or charge a "small load" supplement for less. Some offer mini-mix trucks for smaller quantities.