The Ultimate Building Foundation Guide
What is a building foundation?
Building foundations are one of the most critical elements of any project, even though they aren’t visible when the home or structure is complete.
A foundation refers to the lower part of a structure, which is designed to distribute the weight of the new building evenly and provide a firm footing. It is vital that you choose the right type of foundation and concrete – for the soil type and application – as errors could have serious ramifications and even involve a completed project being demolished. Take a look at our guide to concrete types and, if in doubt, always seek advice from an expert such as a building inspector or structural engineer.
Building regulations and foundations
Getting the foundations of your building or extension right first time is vital if you are to have a successful outcome. This applies not just to the type of foundations you use but also to a number of other factors such as distance to boundaries, ground conditions soil type, adjacent structures, trees, drains and sewers.
As with any project, get the advice and support of experts early on to ensure your project meets all of the relevant regulations and you don’t have any nasty shocks further down the line.
Types of foundations in building construction
It is worth carrying out a soil survey before you decide which type of building foundation you require as ground conditions have an important part to play. This is usually done by digging holes at various points across the site and using the results to assume the conditions throughout.
Foundations are generally broken into two categories: shallow and deep. Shallow foundations are the most common type used for small buildings and domestic projects. The depth of these is generally less than their width and they are commonly used for house extension foundations.
Taller commercial or residential buildings, or those built on very weak ground, will require deep foundations, which carry the load of the structure through the weak soil to the stronger soil or rock below. We offer concretes suitable for all types of foundations.
Shallow foundation types
Individual footing or isolated footing
Also known as a spread footing or pad foundation, this type of foundation is used to support a single column and is square, rectangular or circular in shape. They are a uniform thickness and are designed to carry and spread concentrated loads. The size is calculated on the load and ground conditions.
Combined footing
These concrete footings are usually rectangular in shape and support two or more columns which are so close to each other their individual footings would overlap.
Strip foundation
A strip footing is used for load-bearing walls including footings for extensions and conservatories and house foundations. They are also used to accommodate a row of closely-spaced columns. The wider base of this foundation type spreads the weight over a wider area and provides better stability.
Raft or mat foundation
A raft or mat foundation is a large slab supporting a number of columns and walls. This type of foundation is spread across the entire area of a building and is used when soil pressure is low or where columns and walls are so close that individual footings would not be appropriate or cost effective.
Deep foundation types
Pile foundations
Pile foundations are used when ground conditions near the surface are not suitable for heavy loads. Piles are driven into the ground using specialist equipment and filled with concrete before a ground beam is added to provide a surface to build on.
Drilled shafts or caissons
Drilled shafts, also known as caissons, are foundations that are cast in-situ. A column is drilled to the required depth before reinforcing steel is lowered into the hole and then filled with concrete.
Building foundations: a step-by-step guide to creating concrete footings
Your finished structure will only ever be as good as the foundations it is built on so, if you are in any doubt about which type of foundations to use, get expert advice from a building inspector or structural engineer. Once you have decided, make sure you get your concrete footings right by following our step-by-step guide:
Prepare the ground
It doesn’t matter how big or small your concrete pour is, the first step – once all relevant permissions are in place – is to prepare the ground. Use wooden pegs and string to mark out the area where the concrete is going to be poured, allowing an extra 75mm to accommodate the formwork, which will hold the wet concrete in place when it is drying.
Then dig your foundations to the required depth: for strip footings this is generally to undisturbed, solid ground, while for individual footings and floor slabs you will need to allow enough depth for your sub base (100mm) and damp-proof membrane (dpm) as well as the concrete itself. This is much quicker and easier with a small mechanical excavator – providing access allows. Make sure all debris, stones and plant material are removed before levelling and compacting the soil to create a level base.
Next add your sub base and compact again. For most domestic concrete footings, such as extension foundations, 100mm of base aggregates is adequate. Then lay a dpm, ensuring the edges are turned up to form a tray and any joints are overlapped and taped. This will protect the underside of the concrete from rising damp and any chemicals ground water might bring in contact with the concrete as well as helping to prevent it drying out too quickly, due to water being drawn into the sub base, which will improve the final strength and reduce the chance of it cracking.
The next step is to build your formwork, which is usually made from well-supported 25mm thick timber planks, to keep the concrete in place until it has had a chance to set. The formwork needs to be as deep as the concrete slab will be.
It is vital that you then use a laser or spirit level to check the formwork is even as this determines the finished level of concrete.
Ordering your concrete
Once the site is prepared, you are ready to lay your concrete. As well as the type of foundation, it is also important to know which type of concrete to use. Soils which contain sulphates, for example, can attack concrete over time and cause an expansive reaction. This can be overcome by using design chemical classes (DC), which help provide long term durability. If you are unsure, take a look at our guidance and/or seek advice from a structural engineer.
You may also find using a concrete calculator useful in deciding how much concrete to order. It will ask for the basic shape of the area – square/rectangle, right-angled triangle, parts of a circle – and dimensions (length, width and depth) to provide you with an estimate of the volume you need. If you’ve got a complicated area to estimate you can build up the calculations by adding different shapes together. The depth of concrete you need will depend on the use: footings for extensions, for example, will need to be at least 200mm thick, while around 100mm should be deep enough for a shed base.
You also need to consider if you need to order a concrete pump. Using a pump is perfect when you are dealing with large volumes of concrete or when time is tight and you don’t have the manpower to use wheelbarrows (you can pump approx. 1m3 of concrete per minute). You will also need to use a pump if the concrete mixer truck can’t get close enough to the pour area or access to your site is restricted, underground, within an existing building or at height.
Pouring the concrete
Time is not on your side as concrete typically starts to go off within two hours of being mixed. The actual time will depend on the type of concrete and the ambient temperature: in cold weather it can take twice as long for the concrete to set; in hot weather the setting time could be reduced to 30 mins.
As a result, the concrete will need to be unloaded and levelled as quickly as possible, so make sure you have all the tools you will need at hand including a rake/shovel to move the concrete around and roughly level it and a straight-edged piece of timber to tamp down the concrete and eliminate any air pockets. The concrete is usually wet enough that it can be pulled around the trench using rakes,(we would suggest an S4 consistence for trench filling) before the surface is levelled out and tamped down to remove any trapped air. This is most efficiently achieved using a suitably sized vibrating poker. When the surface has been compacted to the marked-out level it can be smoothed and levelled with a conventional hand float.
It’s important to make sure there’s enough room on site for the delivery truck: they are around 9.5 metres long, three metres wide and four metres high with a turning circle of about 17.5 metres and weigh up to 32 tonnes. Ready-mixed concrete trucks have extension ramps that can reach around three or four metres so unless the concrete can be off loaded at various positions around the site, you may need to use a concrete pump.
If you are using wheelbarrows, make sure you have the manpower available to speed up the job as 1m3 of concrete will fill around 30-40 wheelbarrows! Paths to the pour must be cleared and planks laid to deal with any slopes or uneven ground.
It is not advisable to lay concrete in any adverse weather conditions but, if it is unavoidable, there are a few things to consider. The minimum air temperature to pour concrete is 3°C to make sure that ice doesn’t form within the mix, which could compromise the strength of the concrete. You should also never pour on top of ice or frost so, if this is looking likely, protect the sub base with insulating blankets or thaw it with heaters before the pour, or ask about our Fast track concrete range.
You can pour concrete in the rain as long as the ground drains well and there aren’t any rainwater pools. Once it is poured, cover using tarpaulin or sheeting while it cures. Heavy rain will damage the surface of the concrete so, if appearance is important, make sure there is cover to keep the rain away until you are ready to apply the final finish.
The concrete will be reasonably wet when it is poured so a shovel or rake can be used to roughly level the concrete. A straight-edged piece of timber can then be used to tamp down the concrete, eliminating any air pockets. Repeated tamping will create a reasonably smooth surface but a steel float trowel can be used as the concrete starts to harden for a finer finish. It is worth noting that smooth concrete can be quite slippery so often a ‘rough’ tamped or brush finish is preferable.
Curing your concrete
It is important that the concrete doesn’t dry out too quickly as it hardens as this can cause a weak/dusty surface. The easiest way to achieve this is to keep the slab damp by covering it with plastic sheeting, making sure the edges are sealed to prevent wind tunnelling effects. This is particularly important in temperatures over 20°C or if there is a stiff breeze that could dry the surface. Also, if temperatures are likely to dip below 4°C, a frost blanket or similar should be used to insulate the slab and protect the surface from frost.
Follow-on work should be possible within 48 hours, although it is worth checking with your concrete supplier, and it is best to leave any formwork for 72 hours to avoid any possible damage to the edges of the concrete. The building inspector may also insist on examining the concrete foundations before any bricks or blocks are laid, so make sure you have checked before you start the next phase. Concrete takes 28 days to reach its full strength.
