This guide explains what strip footings and pad footings are, how they work differently, and which concrete mix is right for each one.
What Are Footings?
Footings are the concrete structures that sit below ground and carry the weight of a building down into the soil. They spread the load of walls, columns, or beams across a wide enough area that the ground beneath can support it.
In the UK, two types of footing are used most often in residential and light commercial construction: strip footings and pad footings. They work in completely different ways, which is why the concrete grade required can vary between them.
For a detailed look at foundation types and their structural requirements, the Designing Buildings wiki on pad foundations is a useful reference.
Strip Footings Explained
A strip footing is a continuous band of concrete that runs beneath load-bearing walls. Think of it as a long, shallow slab that traces the outline of the walls above it.
Strip footings are used most often in:
- House extensions and new builds
- Garden walls and boundary walls
- Outbuildings and garages
- Terraced and semi-detached housing
The load is spread along the full length of the footing, so the pressure on the soil at any one point is relatively low. This makes strip footings well-suited to softer ground conditions.
Strip footings are typically 600 mm wide for a standard domestic wall, though wider footings are used on less stable ground. Depth varies depending on soil type and the risk of frost damage, but 450 mm is a common minimum in the UK.
Pad Footings Explained
A pad footing is an isolated block of concrete, usually square or rectangular, placed at a single point to support a column or post. Rather than running continuously under a wall, each pad stands alone beneath an individual structural element.
Pad footings are commonly used in:
- Steel-framed commercial buildings
- Portal frame agricultural buildings
- Timber post structures
- Decking and pergola supports
- Bridges and larger infrastructure
Because all the load from a column is concentrated on one relatively small area, pad footings need to be strong enough to resist both the downward force and the bending stresses caused by that concentrated load. This is why the concrete specification is typically higher for pad footings than for domestic strip footings.
How the Load Type Affects the Concrete Grade
The key difference between the two footing types comes down to how load is transferred.
A strip footing spreads weight over a long distance. The stresses involved are relatively modest, and for most domestic builds the concrete does not need to be exceptionally strong.
A pad footing concentrates load at a single point. The concrete must resist much higher compressive forces, and in many cases it will be reinforced with steel rebar to handle the bending forces too. This demands a higher-strength mix.
This is why the same concrete grade that works perfectly well for a strip footing under a house extension would be insufficient for a pad footing carrying a steel column in a commercial warehouse.
Which Concrete Mix for Strip Footings?
For most domestic strip footings in the UK, the minimum recommended mix is GEN 3, which is equivalent to a C15 strength class. This meets the requirements set out in BS 8500 for unreinforced foundations in standard ground conditions.
However, many self-builders and structural engineers specify GEN 3 concrete for standard strip footings on domestic projects where ground conditions are straightforward and no aggressive chemicals are present in the soil.
Where ground conditions are more difficult, the soil contains sulphates, or the structural engineer wants a more conservative specification, C25 concrete is a common step up. C25 offers greater compressive strength and is suitable for lightly reinforced strip footings and more demanding domestic applications.
As a general rule:
- Standard domestic strip footings on stable ground: GEN 3
- Strip footings on poorer ground or with light reinforcement: C25
- Always follow the specification in your structural engineer’s drawings
Which Concrete Mix for Pad Footings?
Pad footings typically require a stronger mix because of the concentrated loads involved. C30 is a widely used specification for reinforced pad footings in commercial construction, and C35 or above is often specified where loads are particularly high or ground conditions are aggressive.
For smaller pad footings under timber posts or light domestic columns, C25 may be sufficient, but this should always be confirmed with a structural engineer or building control.
Side-by-Side Comparison
| Feature | Strip Footing | Pad Footing |
|---|---|---|
| Shape | Continuous band | Isolated block |
| Supports | Load-bearing walls | Columns and posts |
| Load distribution | Along the length of the wall | Concentrated at one point |
| Typical use | Houses, extensions, garages | Steel frames, portal frames |
| Minimum domestic mix | GEN 3 | C25 |
| Commercial / heavy load mix | C25 | C30 to C35+ |
| Reinforcement | Usually unreinforced | Usually reinforced |
Does Soil Type Change the Specification?
Yes. The ground your footings sit in has a direct impact on the concrete grade you should use.
Clay soils can shrink and swell with changes in moisture, which puts additional stress on footings. Sulphate-bearing soils, which are found in parts of the UK including some areas of East Anglia, can attack standard concrete over time. In these situations a sulphate-resistant mix is required regardless of the footing type.
Always have a soil investigation carried out before specifying your footing design. Your structural engineer will assess the ground conditions and advise on the correct mix, depth, and width.
Do You Need Reinforcement?
Strip footings for standard domestic walls are usually unreinforced, which is why GEN 3 or C25 is suitable. The concrete alone has enough strength to do the job.
Pad footings almost always include steel reinforcement because they need to resist bending as well as compression. The reinforcement and the concrete work together, which is why the concrete specification tends to be higher: the mix needs to bond effectively with the steel and carry its share of the load.
If your footing design includes rebar, your structural engineer’s drawings will specify both the steel layout and the concrete grade. Do not substitute a lower grade without seeking advice.
Ordering the Right Mix
When you place your order, make sure you know:
- The concrete grade required (GEN 3, C25, C30, etc.)
- Whether the mix needs to be sulphate-resistant
- The volume you need in cubic metres
- Access to the pour location, as this may affect whether pumping is required
If you are in Cambridgeshire, Suffolk, Hertfordshire, Essex, or Norfolk, Cardinalis Concrete can supply the right mix for your footings and arrange ready mixed concrete delivery directly to your site.
Summary
Strip footings and pad footings carry loads in different ways, and that difference matters when choosing your concrete mix.
For most domestic strip footings, GEN 3 is the minimum and C25 is appropriate where conditions are more demanding. For pad footings under commercial columns, C30 or above is the standard starting point. Always work from a structural engineer’s specification and take soil conditions into account before placing your order.
