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Complete Guide to Concrete for UK DIY Projects

Concrete is the backbone of almost every construction project in the UK, from garden paths and fence post bases to structural foundations. Understanding the right mix, the correct quantities and proper technique ensures your project is strong, durable and compliant with UK Building Regulations where applicable.

Understanding Concrete Mix Ratios

Concrete is a mixture of cement, sand (fine aggregate), gravel or stone (coarse aggregate) and water. The ratio of these ingredients determines the concrete's strength, measured in Newtons per square millimetre (N/mm²). In the UK, standard mixes are designated by a "C" number:

Ready-Mix Bags vs Mixing from Scratch

For DIY projects, you have two main options for obtaining concrete:

Ready-mix bags (such as Postcrete, Blue Circle Ready-Mix, or Wickes Post Fix) are pre-mixed dry bags containing cement, sand and aggregate in the correct proportions. Simply add water and mix. They are convenient for small projects but expensive for large quantities:

• A 20kg bag of ready-mix yields approximately 0.009m³ of concrete
• A 25kg bag yields approximately 0.012m³
• Postcrete (fast-setting post mix): one 20kg bag fills one standard fence post hole
• Cost: approximately £4-£6 per 20kg bag from B&Q, Wickes or Screwfix

Mixing from separate materials is significantly cheaper for any project requiring more than about 10 bags. Buy cement (25kg bags, ~£5-£7), sharp sand (25kg bags ~£2-£3 or bulk bag ~£45-£60) and 20mm aggregate (25kg bags ~£2-£3 or bulk bag ~£45-£60) individually and combine using the appropriate ratio.

UK Building Regulations for Concrete Foundations

If your concrete work involves foundations for a new structure (extension, garage, conservatory, garden room), it must comply with Part A of the Building Regulations. Key requirements include:

• Minimum foundation depth: 450mm below ground level in most soils. In clay soils, depths of 900mm-1200mm or more may be required due to seasonal shrinkage and swelling.
• Minimum concrete thickness: 150mm for strip foundations in normal conditions.
• Foundation width: Must be at least as wide as the wall above. For a standard cavity wall (approximately 300mm), the foundation should be at least 450mm wide.
• Concrete grade: Minimum C25 (Gen 3 / ST2) for strip foundations. C30 for reinforced foundations.
• Tree proximity: Building near trees in clay soils requires deeper foundations. The NHBC provides guidance tables based on tree species, distance and soil type.

How to Mix Concrete by Hand

For projects requiring less than 0.5m³, hand mixing is practical. Follow these steps for a consistent, strong mix:

1. Prepare your area: Use a clean mixing board, wheelbarrow or cement mixer. Wet the container slightly to prevent the mix sticking.
2. Measure dry materials: Use a bucket as a measure. For C20, that means 1 bucket of cement, 2 buckets of sharp sand, and 4 buckets of 20mm aggregate.
3. Mix dry first: Combine all dry materials thoroughly until the colour is even throughout. There should be no visible pockets of pure cement or sand.
4. Add water gradually: Create a well in the centre and add water slowly. The typical water-to-cement ratio is about 0.5 (half the weight of cement in water). Too much water weakens the concrete significantly.
5. Check consistency: The mix should be like thick porridge — it should hold its shape when sliced with a spade but not be so dry that it crumbles. If you squeeze a handful, it should hold together without water dripping out.

Curing Concrete

Proper curing is essential for concrete to reach its designed strength. Concrete does not "dry" — it cures through a chemical reaction (hydration) that requires moisture. Key curing points for UK conditions:

• Keep moist: Cover with polythene sheeting or damp hessian for at least 7 days. In warm weather, mist the surface daily.
• Temperature: Do not pour concrete when the temperature is below 3°C or forecast to fall below freezing within 48 hours. Frost destroys uncured concrete.
• Strength timeline: Concrete reaches approximately 65% of its design strength at 7 days and full strength at 28 days. Do not load foundations or bases before 7 days minimum.
• Rain: Light rain after initial set (2-4 hours) actually helps curing. Heavy rain on fresh concrete can wash out cement — cover with polythene if heavy rain is forecast within the first few hours.

Cost of Concrete in the UK (2026)

Frequently Asked Questions