Who this guide is for
If you’re involved in ordering, or placing foundation concrete in the UK, this guide is for you, whether you’re a builder, site manager, architect, or homeowner managing a project.
It explains the main foundation types used in domestic and light commercial project, how concrete is typically specified for each, across the South East.
Two important points upfront:
Foundation concrete should always be specified to match the site. Ground conditions, structural loads, and exposure all vary from project to project, and national standards exist to account for this.
EKA Concrete provides quality concrete which is mixed on site to the requested specifications. It is your contractor or structural engineer who should make the final call on mix selection, reinforcement, and design.
Why foundation concrete matters
Foundations, often called “footings”, carry the full weight of a structure and transfer it safely into the ground. The concrete used needs to perform under sustained load, resist moisture and ground chemistry, and maintain its integrity over decades – often in conditions the rest of the building never sees.
That means foundation concrete typically needs to meet higher requirements than general-purpose mixes:
Compressive strength. Foundations are commonly specified in the C10/GEN1 up-wards range, depending on the loads involved and the ground conditions. Your structural engineer or contractor will confirm the appropriate strength class for the project.
Durability. Concrete below ground is exposed to moisture, sulfates in the soil, and in some cases aggressive chemicals.
Workability. Foundation concrete often needs to flow into narrow trenches, around reinforcement, or into formwork that can’t easily be accessed for compaction. Getting the right workability – without compromising strength – is an important part of the specification.
These factors are why foundation concrete is rarely a simple off-the-shelf choice. The right mix depends on the structure above, the ground below, and the conditions on site.
Foundation types and their concrete needs
The foundation type for any project is determined by the structural engineer or designer based on the building, the ground conditions, and the site. Below is a practical overview of the main types used in domestic and light commercial work across the South East, along with the concrete commonly associated with each.
Strip foundations
Strip foundations are the traditional choice for load-bearing walls in houses and extensions. They consist of a continuous strip of concrete, typically wider than the wall above, sitting in a trench at a depth where the ground is stable and unaffected by seasonal movement.
Concrete commonly used: Strength classes in the C10 up-wards range are typical for domestic strip foundations, though the exact specification depends on the loads and ground conditions. Designated mixes such as GEN 3 are often referenced on smaller projects.
Typical depth: Usually at least 450 mm below finished ground level for most domestic work, though this increases significantly on clay soils or sloping sites. Your designer will confirm the depth required.
Reinforcement: Not always required for straightforward strip foundations, but steel mesh or rebar may be specified where ground conditions are poor, loads are uneven, or the strip spans soft spots.
Strip foundations suit most conventional house-building on reasonably stable ground. They are well understood, widely used, and relatively economical.
Trench fill
Trench fill is a variation on strip foundations where the trench is filled with concrete to within a short distance of ground level, rather than using a shallow strip of concrete with blockwork built up from it.
Why it’s widely used: Trench fill is quicker to place than traditional strip foundations, reduces the amount of below-ground blockwork, and is particularly practical on sites with deep trenches – especially in clay soils where foundation depths of a metre or more are common.
Concrete commonly used: A workable, often higher-slump mix is needed so the concrete flows to fill the trench evenly without excessive compaction. Strength classes in the C10/GEN1 up-wards range are typical. Flowable mixes are often specified to suit the placement method.
Key considerations: The mix needs enough workability to self-level in the trench, but this should not come at the expense of strength. Over-watering on site is a common problem – if a more workable mix is needed, it should be specified at the batching stage, not adjusted with a hose. If customers require additional water is added to the mix, they will need to sign a waiver provided by the operative on-site.
Trench fill is now the most common domestic foundation method across much of the South East, particularly where clay soils push foundation depths beyond what is practical for a traditional strip.
Raft foundations
Raft foundations spread the load of a building across a large, continuous slab of reinforced concrete, rather than concentrating it on strips beneath the walls. They are used where ground conditions are variable, where bearing capacity is low, or where differential settlement could be a problem.
Concrete commonly used: Raft foundations are structural elements and are typically specified with higher-strength concrete, along with designed reinforcement. The specification will be provided by the structural engineer.
Key considerations: Raft foundations involve larger volumes of concrete placed in a single, continuous pour. Planning the pour sequence, managing concrete supply, and ensuring proper compaction and finishing are all critical. Control joints and crack control detailing should be addressed at the design stage.
Where they’re common: Rafts are frequently specified on sites with made ground, variable fill, or soft clay – conditions that are not uncommon across parts of the South East.
Raft foundations are more complex to design and build than strip or trench fill, and the concrete specification, reinforcement, and pour planning should all be confirmed by the structural engineer well before ordering.
Soil conditions across the South East
Ground conditions have a direct effect on foundation design, depth, and the type of concrete specified. The South East has a wide variety of soils, and conditions can change significantly even within a single site. What follows is a general overview of common soil types by county – it is not a substitute for proper site investigation.
West Sussex
The northern part of the county around Horsham sits on Weald Clay – heavy, shrinkable clay that commonly requires foundation depths of a metre or more and may need sulfate-resisting mixes. Further south, greensand around Midhurst and Petworth provides variable bearing, and the coastal strip from Worthing to Chichester includes alluvial and made ground that may call for engineered foundations.
West Sussex is one of EKA’s core delivery areas, and our teams are familiar with the ground conditions across the county.
Surrey
Northern Surrey sits largely on London Clay, which is highly shrinkable and often drives foundation depths well beyond national minimums. Around Woking and Camberley, Bagshot Sands provide a different set of challenges – generally better bearing, but potentially requiring attention to drainage and water table. River gravels along the Thames and Wey valleys are generally favourable for foundations but can be variable in depth and quality.
East Sussex
East Sussex includes sandstone around Rother, chalk along the coast at Eastbourne and the South Downs, and silty alluvial soils in the river valleys inland. Coastal sites may face additional exposure considerations. Chalk areas can offer good bearing, as elsewhere, the depth and condition of the chalk need to be confirmed on site.
Kent
Kent’s geology includes Thanet Sand along the coast, Gault Clay around Maidstone and the Weald, and chalk through the North Downs. Clay soils in the Maidstone and Tonbridge areas are similar to those across the Weald in Sussex – shrinkable, often sulfate-bearing, and typically requiring deeper foundations and careful mix selection. The chalk downland generally provides good bearing at reasonable depth.
A note on soil investigation
For larger or more complex projects, a formal ground investigation with soil testing is a worthwhile investment. Even on smaller domestic jobs, a basic trial hole or hand auger can reveal a great deal about what you are building on. Your contractor or structural engineer can advise on what level of investigation is appropriate.
Choosing the right concrete for foundations
Selecting foundation concrete is not a one-size-fits-all decision. The right mix depends on several factors, and the final specification should always come from the contractor or structural engineer. The points below are intended to support those conversations.
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What the structure needs
The loads from the building above – its weight, the span of walls and floors, and any imposed loads – determine the minimum strength class for the concrete. Domestic extensions and houses typically fall within the C10/GEN1 up-wards range, while heavier or more complex structures may require C25 or above.
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What the ground demands
Ground conditions affect both the foundation type and the concrete specification. Shrinkable clay may require deeper foundations. Sulfate-bearing soils may require a sulfate-resisting mix. Waterlogged ground may influence workability requirements and placement methods. BS 8500 provides the framework for specifying concrete durability based on ground conditions, and NHBC Standards give practical guidance for domestic work.
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How the concrete will be placed
Access, trench depth, reinforcement congestion, and pour volume all influence the workability and delivery method needed. A narrow, deep trench fill may need a high-slump flowable mix. A raft foundation may need a large, continuous pour with careful sequencing. Volumetric on-site mixing gives flexibility to adjust volumes and manage timing – particularly useful on foundation work where exact quantities are difficult to predict until the trenches are open.
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Reinforcement and cover
Where reinforcement is specified, the concrete must provide adequate cover to the steel to prevent corrosion. Cover requirements are set by the designer based on the exposure conditions. Getting this wrong – either through poor placement or incorrect spacers – can compromise the foundation’s long-term durability.
Common mistakes with foundation concrete
Foundation work is not always glamorous, but mistakes at this stage tend to be expensive and difficult to fix. The issues below come up regularly on sites and are worth keeping in mind.
- Incorrect workability. Concrete that is too stiff will not flow properly into formwork or around reinforcement, leaving voids and honeycombing that weaken the foundation. Conversely, adding water on site to improve flow dilutes the mix and reduces strength. If a more workable mix is needed, it must be specified in advance.
- Loading or building too early. Concrete gains strength over time. Most mixes reach their design strength at 28 days, but significant strength development occurs in the first seven days. Building on foundations before they have gained adequate strength risks cracking and settlement. Your contractor will advise on timing.
- Ignoring ground conditions. Pouring foundation concrete onto ground that has not been properly assessed is a gamble. Soft spots, filled ground, high water tables, and aggressive soil chemistry can all undermine performance. If conditions on site differ from what was expected once the trenches are open, it is worth pausing to reassess before pouring.
- Inadequate curing. Foundations are often left exposed to the elements immediately after pouring. In cold, hot, or windy conditions, this can impair strength development and surface durability. Covering freshly placed concrete and protecting it from temperature extremes during the first 48 hours makes a meaningful difference. Our cold weather concrete guide covers this in detail.
- Skipping or misplacing reinforcement. Where reinforcement has been specified, it needs to be positioned correctly with the right cover and properly tied before the pour. Reinforcement that shifts during placement, or that is placed too close to the surface or the base of the trench, will not perform as designed.
Weather and timing
Foundation pours are affected by the weather in the same way as any other concrete work, but the consequences of getting it wrong can be more significant – you cannot easily rip up and re-pour a foundation.
- Cold weather. Low temperatures slow strength gain. Fresh concrete should be protected from freezing and kept above 5 °C for at least 48 hours after placement. On cold sites, this may mean insulating the pour, using temporary enclosures, or adjusting timing to avoid overnight freezing. Our complete guide to cold weather concrete covers this in full.
- Hot weather. Warm and windy conditions cause rapid moisture loss, increasing the risk of plastic shrinkage cracking and inconsistent curing. Managing evaporation – through timing, windbreaks, shading, and prompt curing – is key.
- Wet weather. Light rain during a pour is manageable with proper protection, but heavy rain can wash out the surface, alter the water content of the mix, and flood trenches. If heavy rain is forecast, it is usually better to postpone.
- Timing the pour. Foundation pours are often time-sensitive, particularly trench fill work where trenches can deteriorate if left open too long. Coordinating delivery, placement, and finishing so that the concrete is placed and protected efficiently is important.
Frequently asked questions
What strength concrete do I need for house foundations?
Most domestic foundations are specified in the C10/GEN1 up-wards range, though the exact strength class depends on the loads, ground conditions, and foundation type. Your structural engineer or contractor will confirm the specification. Higher strength classes may be needed for heavier structures or more demanding ground.
How long before I can build on new foundations?
Concrete gains strength progressively. Most mixes reach their design strength at 28 days, with significant early strength developing in the first seven days. Your contractor will advise on when it is safe to proceed with the next stage of construction based on the specification and site conditions.
What is the difference between strip foundations and trench fill?
Strip foundations use a relatively shallow layer of concrete at the base of the trench, with blockwork or masonry built up to ground level. Trench fill uses concrete to fill the trench to near ground level, reducing below-ground blockwork. Trench fill is quicker to place and is widely used where trenches are deep – particularly on clay soils.
Do I need a soil test before pouring foundations?
Soil testing is not a legal requirement for all projects, but it can be a sensible precaution – particularly on sites where the ground conditions are unknown, variable, or potentially aggressive. Even a basic trial hole can reveal useful information. For larger projects or sites with uncertain ground, a formal investigation with laboratory testing is a worthwhile investment. Your contractor or structural engineer can advise.
Ready-mix or volumetric – which is better for foundations?
Both work well. Volumetric (on-site mixed) concrete offers flexibility – you pay only for what you use, and volumes can be adjusted during the pour to match actual trench dimensions. This is particularly useful for foundation work, where exact quantities are often uncertain until the trenches are open. Ready-mix can be more practical for very large, continuous pours where a consistent high volume is needed without interruption.
Can I pour foundations in winter?
Yes, with the right precautions. The concrete must be protected from freezing and kept above 5 °C during the critical early curing period. Our cold weather concrete guide covers winter pouring in detail.
These answers are guidance only. Final decisions on mix class, foundation type, reinforcement, and curing should be confirmed by your contractor or structural engineer.
Why choose EKA Concrete for your concrete foundations pour?
EKA supplies dependable concrete, mixed on site to the agreed specification, with the flexibility and support to keep your project on track.
On-site volumetric mixing. You pay only for what you use. Volumes can be adjusted during the pour to match actual site conditions – particularly useful for foundation work where trench dimensions often differ from the drawings.
Local coverage. Serving West Sussex, East Sussex, Surrey, and Kent, with same-day or next-day delivery subject to availability.
Proven local experience. Over 20 years supplying homeowners, builders, and commercial sites across the South East. Our teams understand the ground conditions, access constraints, and practical realities of foundation work in this part of the country.
Practical support from quote to pour. Delivery planning, pump hire coordination, and straightforward advice when you need it.
Reliable materials and consistent batching. Concrete is mixed on site using quality-assured materials and calibrated equipment, giving control over quantity and workability.
South of England
Areas we cover
EKA Concrete cover a vast area of the South of England. With depots in Crawley and Littlehampton we are cover a vast area:
Arundel, Billingshurst, Brighton, Bognor Regis, Bosham, Burgess Hill, Chichester, Crawley, Dorking, Eastbourne, East Grinstead, Emsworth, Haywards Heath, Horley, Horsham, Lewes, Littlehampton, Midhurst, Petworth, Pulborough, Redhill, Reigate, Storrington, Worthing
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EKA Littlehampton
EKA Concrete
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A259, Climping
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BN17 5RX
EKA Crawley
EKA Concrete
Unit 5 Acacia Grove
Copthorne Road
Crawley
RH10 3PD
Areas covered
EMAIL: sales@ekaconcrete.com
Call us for a quotation or to place an order,
Crawley Office - 01293 883 000
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