How to build a concrete foundation
Whether you need to know how to build a concrete slab foundation for a house or are working on basement foundation construction for a commercial building, 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 hardening.
Then dig your foundations to the required depth. For strip footings, this is generally to undisturbed, solid ground. 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 groundwater might bring in contact with the concrete. It can also help prevent it from 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. For further guidance, view our guide:
How to prepare for a concrete pour
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.
How strong should concrete be for a building foundation?
One major consideration when ordering concrete for a building foundation is its strength, which is graded according to the compressive strength of the material after 28 days. The concrete used in your foundation must be strong enough to support the structure that will be built on top of it.
To give you an idea of concrete strength grading, C10 concrete has 10 Newtons of strength to hold approximately 1,000 kilograms, and C20 concrete has 20 Newtons of strength to hold approximately 2,000 kilograms.
Matching concrete to ground type for a building foundation
You also need to consider the ground type and which concrete will be best suited. 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. Our guide to types of concrete can offer further assistance in deciding what sort of concrete you need.
How much concrete is required for a building foundation?
You may find 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.
Remember the accuracy of your measurements will dictate the quantity of concrete you order. It can be difficult to dig a foundation in soils to an exact size so allow for this when ordering.
What tools are required to pour concrete for a building foundation?
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 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 harden 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, while in hot weather, the setting time could be reduced to 30 minutes.
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 consistency 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.
What to consider when pouring concrete for a building foundation
It’s important to make sure there’s enough room on site for the delivery truck. Ready-mixed concrete trucks have extension ramps that can reach around three or four metres so unless the concrete can be offloaded 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. Paths to the pour must be cleared, and planks must be 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 protect the sub base with insulating blankets or thaw it with heaters before the pour. Our guide to cold weather concrete pours can provide more advice on laying concrete at low temperatures.
View cold weather pour guide
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 it with 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 and increases the risk of cracking. 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.
Request a quote for the concrete you need for your building foundation with Heidelberg Materials today, and see our technical information page for more guidance on your construction project.
