When self builders start planning their projects, foundations are rarely the topic that gets them excited. The focus almost always shifts to floorplans, interior finishes, kitchen designs or the latest heating technology.
In reality, however, foundations are arguably the most important part of any house. Get them right and the rest of the build has a level and stable platform to work from. Get them wrong and problems will follow.
Choosing the right type of foundation isn’t usually down to personal preference. It often depends on the ground conditions, the design of the house and sometimes practical considerations, such as access for machinery or the presence of nearby trees or other buildings.
What are foundations?
For self builders, understanding the basics of foundations and how to make foundation decisions doesn’t mean you need to become a structural engineer. It’s more about recognising what decisions you will need to make and how those decisions will affect cost, construction and long-term performance of the foundation and the house it supports.
And, it starts by knowing what, in its most basic terms, a foundation actually is and why you need one.
A foundation spreads the weight of a building safely into the ground beneath it. Everything in the house – the roof, walls, floors and its contents – transfers its load downwards through the structure and onto the foundation before eventually reaching the ground.
The foundation and the ground below it has to carry the weight of the whole building without there being excessive movement. If it compresses too much or shifts over time, the structure above can start to move as well.
What influences the type of foundations you choose?
One of the most influential factors on the type of foundation you need to build your home is the soil type you're building on. The ground beneath a site is rarely uniform. One plot may have gravel and another may sit on soft clay or loose sand that behaves very differently under load.
Clay soils are particularly challenging as they expand when wet and shrink when they dry out. If there are trees nearby, their roots can draw moisture from the soil, causing the ground to contract. Over time this movement can lead to cracks in walls and even structural failure.
Ground water also has implications and a high-water table can weaken soil strength and make excavations difficult to keep dry. Frost is another concern because freezing conditions can cause the ground to expand.
All these issues are why foundation design is so important when it comes to planning your self build. The design normally begins with understanding the ground conditions by completing trial pits or boreholes to discover what lies beneath the surface. In practice, these investigations are one of the most valuable steps a self builder can take. They remove the guesswork and allow the structure to be designed around real conditions rather than assumptions.
The foundation design must comply with the building regulations, particularly Approved Document A, which has a focus on structural safety. The document emphasises that foundations must be capable of safely supporting the building loads and that they must be designed to suit the ground conditions of the project. Once the ground has been investigated and is understood, the engineer will usually recommend the right foundations system for your home.
Types of foundation systems: Engineered vs foundation
The main types of foundation fall into one of two categories - foundation systems and engineered foundation systems. Strip and trench fill foundations are considered standard foundation systems, while engineered systems comprise of raft, pad, piled and screw pile foundations.
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While we'll cover each one of these below and provide a signpost to the article with the more detailed expert advice for each type, it's good to know from the outset that standard foundation systems tend to be cheaper than engineered ones.
But there's a caveat. You may not always be able to use the cheaper option. One example of this is when the make-up of the ground or the structural load of your build is such that you need to dig deeper foundations, thus requiring more concrete and labour. In this instance, you may be advised to consider an engineered system.
For a more in-depth look at costs, our foundation cost guide gives examples of what you can expect to pay. Either way, it's always wise to ensure there's at least a 10% contingency in your budget to allow for unexpected issues.
Conventional foundation systems
1. Strip foundations
The standard domestic build foundation solution in most parts of the UK are strip foundations, also known as strip footings.
Formed, as the name suggests, from strips of concrete laid in the ground, they are placed in the ground beneath load-bearing walls and any supporting columns specified for the main structure.
As strip foundations are one of the most common types of foundations you should find most contractors or groundworkers comfortable with using them. To build them, the topsoil is scraped back (and usually stored for reuse) and trenches dug down to a depth at least 450mm (this is often deeper depending on soil type), which is enough to put the foundations below frost action. The trenches are then filled with concrete to a minimum 150mm below the surface ground level.
The walls are then built up to just below finished ground level in masonry (concrete blockwork or engineering bricks) with the outer leaf switching to the selected external facing material for the walls or plinth, typically brick or stone, just below ground level.
Approved Document A defines the minimum widths for strip footings based on the type of ground and loadbearing wall. But, they also require foundations to reach stable ground and sit below the level affected by frost or seasonal movement. On clay soils, or where trees are present, this can mean digging much deeper than expected. This is where projects sometimes run into difficulty.
Trenches that extend well over a metre deep become harder to excavate safely and are more costly to construct. The deeper they go, the more excavation, concrete and blockwork is required. In these situations, structural engineers often switch to a slightly different approach.
2. Trench fill foundations
Similar in design to strip foundations, trench fill foundations are also a type of foundation formed from strips of concrete dug into trenches in the ground.
However, they differ from strip foundations in the fact that they are only usually placed around the perimeter of the proposed building. They are not placed underneath load-bearing walls or supporting columns. This makes them easier to set out and a popular choice for extension foundations.
The trade-off is the amount of concrete required. Filling the trench almost entirely with concrete uses considerably more material, which has a cost and environmental impact. Concrete is one of the most carbon-intensive materials used in construction, and the volumes required when ordering concrete are not insignificant. That said, while trench fill foundations can look expensive, the speed of construction often offsets some of the additional material and labour costs.
It's also important to realise that as they only run around the edges of a property, the structural load they are able to bear is far less, meaning while they may be suitable for some houses and smaller projects such as outbuildings, they might not be strong enough for larger properties.
As with strip foundations, it's important to make sure that they are placed deep enough below the ground frost level to avoid the ground swelling and contracting, leading to the integrity of the foundations being damaged and creating problems such as subsidence.
Engineered foundation systems
Where the required depth of foundations is more than 2.5m it becomes impractical to use conventional strip or trench fill foundations unless a basement storey is planned. In these cases it is more cost-effective to look at alternatives such as raft or piled foundations.
1. Raft foundations
Raft foundations take a completely different approach to supporting a building. Instead of concentrating loads beneath individual walls, the entire structure sits on a large reinforced concrete slab. This slab spreads the weight of the house evenly across the whole building footprint and the structure effectively ‘floats’ on the ground.
The advantage of this approach is that loads are distributed over a much larger area. On weaker soils this can significantly reduce the risk of uneven settlement. Raft foundations are often used on sites where ground conditions are less predictable or where soils have relatively low bearing capacity.
Raft foundations are usually designed by a structural engineer, taking into account the ground conditions assessed in a geotechnical survey or at least inspection of trial holes by the engineer.
The raft design usually has an ‘edge beam’ around its perimeter and under any areas carrying large point loads. This consists of a cage of steel reinforcement that will need to be carefully assembled on site. Internal stiffening beams are sometimes needed too. These beams transfer the building loads through the rest of the slab and then evenly over the ground.
Raft systems have also become increasingly popular on high-performance self build homes. Many energy-efficient, eco-led homes now use insulated raft foundations, where structural concrete and insulation are combined into a single energy-efficient foundation system.
Because the insulation layer runs continuously beneath the slab, thermal bridging is reduced and heat loss through the floor can be minimised. Done well, it also simplifies construction by combining several elements – foundation, ground floor and insulation – into one operation.
However, raft foundations require careful design and precise installation. Reinforcement placement, edge detailing and service penetrations must all be carefully planned as once the concrete is poured, changes become difficult.
There are also four different types of raft foundations, meaning each type actually looks different below the surface as follows:
- Solid slab – includes flat raft mat, wide toe rafts, slip plane rafts and blanket rafts - defined by their varying depths or the use of extra materials below them such as sand
- Slab beam – includes extra ground beams and used when there are uneven structural loads in the building
- Cellular raft – made from two layers of concrete with beams in between for when the ground is low load bearing
- Piled raft – include piles set below the raft for extra stability making this type of foundation suitable for taller buildings
2. Pad foundations
Not every building distributes loads through continuous walls. Modern steel frame structures often rely on columns to carry parts of the structure which in turn are supported by what are known as pad foundations.
A pad foundation is a block of reinforced concrete that is positioned beneath a column. Instead of spreading the load along a strip, the load which passes down the column is then spread into square pads cast below the ground. Pad foundations are often used alongside other foundation types to form large open-plan spaces where steelwork is required.
Usually cast in situ, concrete pads are placed beneath the position of each post of the frame, and the posts are linked together at ground floor level to spread the load evenly. Pad spacings, size and depth are designed according to the design load of the building and the ground conditions.
But, as with raft foundations, there are also different types of pad foundations depending on the structure they are being used to support.
- Plain pad – suitable for lighter loads and also referred to as footings
- Reinforced pad – containing reinforcement in the form of steel bars
- Combined double pad – when two columns require pads next to each other
- Continuous pad – placed in a line and joined by steel rods
Although pad foundations are generally seen as a relatively low-depth type of foundation, they can be made deeper if the soil type is low load-bearing. Nonetheless, on the whole, they are more suited to structures such as sheds or warehouses, or buildings of either a steel frame or timber post and beam, rather than a new-build home.
Pad foundations can be well suited to sites where excavation needs to be kept to a minimum and can also be a cost-effective solution when overcoming a sloping site by suspending the ground floor on a post and beam frame.
3. Piled foundations
Where ground conditions are poor, variable or unpredictable, piled foundations are usually the solution. Although from the name you'd be forgiven for thinking these may involve a stacked system, as we explain in more detail in the relevant article, piled foundations, they are actually long cylindrical like structures that are bedded deep into the ground.
To use the correct terminology, they are driven or bored down and can also be made of steel as well as reinforced concrete.
As with some of the other types of foundation, the term piled foundations includes a number of different sub-categories: end-bearing, driven, friction and bored piles are just some of the terms you may encounter. Other smaller versions include micro piles and helical and ground screw piles.
Once in place, the larger versions are capped off and usually connected via a series of ground beams. The main reason for using them? Primarily to deal with taller structures being built on weaker ground. Although in some instances, cases of them being required in extensions are known of to avoid disrupting the drains.
Smaller versions such as ground screws are also being used more widely in pre-fab extensions and structures such as modular garden rooms.
4. Screw pile foundations
Local planning authorities will often allow structures to be built around trees within a root protection area on the proviso that the structures are supported by screw piles. These are a relatively new foundation solution consisting of slender, hollow steel shafts with a small number of steel helices (or screw threads) welded to them. The piles are screwed into the ground until they achieve sufficient friction to support the required load.
This can be a cost-effective way of building foundations on sloping ground too, as the steel piles can be left above ground and linked together with tension wires or rods, and topped with a steel ringbeam or grillage to build from.
Chris Willis, co-director of OffPOD, a modular build company, explained how they use ground screws where possible in their builds. "As a company, we've always tried to use them since we started," says Chris. "Not only because they're innovative and offer a more eco-conscious build option, but because they also be a much quicker way of building and can help when you're dealing with tree root systems that would be otherwise problematic with a more traditional type of foundation."
Budgeting for your foundation costs
The cost of building foundations is one of the biggest variables, so anyone who wants to keep control of their budget and schedule needs to work out the most appropriate foundation system to use as early as possible.
However, as foundation costs can vary dramatically depending on what lies beneath the site. Variables, from ground conditions to foundation types, mean that it’s simply not possible to quote a cost, and reliably use this for most projects.
When it comes to foundations, cost is almost always site specific, although as a ballpark, you should expect to pay around 10% of your project budget on the substructures (everything up to your damp-proof course level), including the foundations.
This cost can range from £250/sqm (gross internal floor area) for simple strip foundations to well in excess of £350/sqm for piled foundations with ground beams.
On straightforward ground with good bearing capacity, traditional strip or trench fill foundations are usually the most economical. Once excavation becomes deeper or specialist systems such as piling are required, costs can rise quickly.
Depth is one of the biggest cost drivers. Deeper excavations mean more digging, more soil to remove and greater volumes of concrete. If groundwater is encountered, pumps and dewatering may also be required which obviously will cost more.
Access can be a major influence on costs as well. Restricted sites may prevent large machinery from reaching the excavation, slowing the work down considerably. Pumps can overcome access problems but again at a cost.
Another factor that people often overlook is the cost of removing excavated material. Soil has to be transported away and disposed of safely. This typically costs about £300 for a 16-tonne lorry, but if contaminated ground is discovered, disposal costs can increase by a factor of four.
The best way to identify the most cost-effective solution prior to starting work on site is to instruct a geotechnical study. This involves digging or boring trial holes around the site of a proposed new building, principally to establish the load bearing capacity of the soil at differing depths.
Samples are collected and tested for plasticity (shrinkability), pH levels, sulphates, moisture contact (seasonal water table) and other factors that will allow the foundation solution best suited to the site to be determined.
Neglecting to investigate the ground conditions on your site is one of the earliest and costly mistakes a self builder can make.
FAQs
What other factors might affect my choice of foundation system?
Where the foundations are affected by tree roots (or their previous removal), you may be required to employ a fairly deep trench filled with concrete but with a compressible material to one or both sides of the external trenches to counteract any heave or expansion in the ground.
Water pipes must enter the building at a depth of at least 750mm but no more than 1.35m below ground. If that means that they pass through a concrete foundation then they must either be laid prior to pouring or, better still, a duct installed for them to be pushed through later.
If sewage pipes leaving the building have to be deeper than the top of the foundation concrete then they should also be ducted; they cannot be trapped within the concrete and must be able to move freely.
Bringing electricity to site or bringing gas to site doesn't usually need to be ducted or installed at this point as they are normally surface mounted. Finally, the building and warranty inspectors will have to approve the excavated foundations prior to any concrete being poured.
How do I choose the right type of foundations?
Choosing the right foundation isn’t something most self builders should attempt to decide in isolation. Foundation design usually involves input from structural engineers, geotechnical specialists and some degree of oversight from building control inspectors and practical advice from your project manager and groundworker.
Ground investigations are essential to understand the soil conditions before detailed design begins. By having a ground investigation report, the engineering team will be able to fully consider the ground conditions and recommend an appropriate foundation system. This design can also be used to tie down the contractor’s prices which, is absolutely essential.
It’s not unusual for self-builders to try to avoid the cost of ground investigations, assuming it’s an unnecessary expense, but this couldn’t be more wrong. In my experience this rarely saves money and the uncertainty often leads to conservative designs or costly changes once excavation begins.
Strip foundations, trenchfill systems, rafts and piles all have their place. None is inherently better than the others and each just suits different ground conditions and structural requirements. With the right surveys, engineering advice and a realistic groundworks budget, there’s no reason why self-builders should worry about getting their foundations right.
On a final note, it's also important to understand that the choice of foundation influences other parts of the project. Drainage runs, service connections and floor construction all depend on the chosen foundation system.
Even the construction schedule for building a house can change depending on how the foundation is designed. Which is why when it comes to foundations, careful decision making and planning is essential in order to get it right.
