This guide walks through the different types of footings, which concrete grades are best suited to each, and the key factors that affect how strong your foundations need to be.
What Are Footings and Why Do They Matter?
Footings, also called foundations, are the concrete base that sits below ground level and carries the weight of everything built above. They spread the load of a structure out across the ground beneath, which stops the building from sinking or shifting over time.
Unlike a path or a driveway slab, footings are structural. That means the concrete grade you choose must be strong enough to handle constant load, resist ground movement, and stay stable through changes in soil moisture and temperature.
Getting this wrong is costly. Getting it right starts with choosing the correct mix.
Types of Footings: What You Are Working With
Before choosing a concrete grade, it helps to understand the type of footing you are dealing with.
Strip foundations are the most common type for domestic buildings in the UK. A continuous strip of concrete runs along the line of a wall, distributing the load evenly. According to guidance from the Local Authority Building Control (LABC), Approved Document A sets minimum widths for strip foundations based on soil conditions and the load the wall carries. A standard strip footing for a single-storey extension on firm ground is typically 600mm wide and at least 200-225mm deep.
Pad foundations are individual blocks of concrete placed at specific points, usually to support columns or posts. They carry concentrated loads rather than spreading them in a line.
Trench fill foundations are similar to strip foundations but the trench is filled almost entirely with concrete, rather than leaving room for blockwork below ground. They are popular because they are faster to pour and reduce the need for bricklaying below the damp-proof course level.
Which Concrete Grade Should You Use for Footings?
Not all concrete mixes are the same. In the UK, concrete grades for general foundation work are described using either the GEN designation (a designated mix standard) or the C designation (a strength class). Here is a straightforward breakdown:
| Footing Type | Recommended Grade | Typical Use |
|---|---|---|
| Simple garden walls, low-load structures | GEN 1 | Non-structural, light domestic |
| Single-storey extensions, garages | GEN 3 concrete | Standard domestic strip footings |
| Two-storey extensions, most new builds | C25 concrete | Higher load-bearing domestic work |
| Commercial, multi-storey, reinforced | C30/C35 | Structural, engineered foundations |
GEN 3 Concrete
GEN 3 is one of the most widely used mixes for domestic strip foundations in the UK. It has a strength classification equivalent to C20/25, which means it reaches a compressive strength of around 25 newtons per square millimetre (N/mm²) at 28 days. That is more than enough for standard single-storey house extensions, garages, and similar domestic builds on firm, stable ground.
It is a reliable choice when your ground conditions are straightforward and the load above is not unusually heavy.
C25 Concrete
For two-storey extensions, most new build houses, and any project where your structural engineer has specified a higher-strength mix, C25 concrete is the go-to option. With a compressive strength of 25N/mm² and a higher cement content than GEN 3, it offers a greater safety margin under heavier loads and in trickier ground conditions.
Many engineers and building control bodies in the UK now default to C25 as their minimum for domestic strip foundations, so it is always worth checking your structural drawings or getting professional advice before ordering.
When to Go Higher
If your soil is made up of clay, has a high sulphate content, or if you are working near trees (which can cause shrinkage and heave in clay), you may need to go up to C30 or request a sulphate-resistant cement. A structural engineer should always be involved in these situations.
How Deep Should Footings Be?
Depth depends on several factors, including soil type, proximity to trees, and the number of storeys above ground. As a general guide for UK conditions:
- Minimum depth: 450mm in most stable soils (to protect against frost)
- Standard domestic depth: 600-900mm in firm ground
- Clay soils: At least 900mm, and often deeper where trees are nearby
- Near trees on clay: Can reach 1500-2500mm depending on species and distance
Your building control team or structural engineer will advise on the specific depth required for your site. Always have excavations inspected before pouring concrete.
What Else Affects the Concrete Mix You Choose?
Beyond footing type and depth, a few other site-specific factors can change what grade you need.
Sulphates in the ground can attack standard Portland cement concrete and cause it to break down over time. If a soil test shows sulphates are present, you will need a sulphate-resistant cement or a higher-class concrete mix.
Aggressive ground conditions such as acidic soils or ground with high chloride content may also require a more durable mix or additional admixtures.
Reinforcement changes the picture too. Plain mass concrete footings (unreinforced) can use lower grades such as GEN 3. As soon as steel reinforcement is added, the concrete grade typically needs to go up to ensure the two materials bond and work together effectively.
Getting the Right Mix Delivered to Site
Once you know the grade you need, volumetric delivery is often the most practical option for footing pours. Because footings are typically poured in stages, a volumetric mixer lets you order and pay for only what you use, reducing waste and cost.
Cardinalis Concrete supplies a full range of concrete grades for foundation work across Cambridgeshire, Suffolk, Norfolk, Essex, and Hertfordshire. From GEN 3 for standard strip footings to C25 and above for more demanding applications, all mixes are freshly batched and delivered directly to site. For projects where access to the trench is limited, concrete pumping services can be arranged to get the mix exactly where it needs to go.
A Quick Summary
- Footings carry the structural load of your building and must be strong enough for the ground conditions and load above
- GEN 3 concrete is the standard choice for most domestic strip foundations
- C25 concrete is often specified for two-storey builds and when structural engineers require a higher grade
- Depth, soil type, sulphate levels, and reinforcement all affect which grade you need
- Always check with building control or a structural engineer before pouring
If you are not sure which mix is right for your project, the team at Cardinalis Concrete is happy to help you work it out.
