How to…
Footing & foundation types
First things first….what is the difference between a 'footing and a foundation'?
In a nutshell, a ‘footing’ is the part of a foundation that is in contact with the ground, whereas a ‘foundation’ will often be used to describe the construction higher up, above that which is in contact with the ground (i.e. a concrete strip footing).. Think of the footing being the bit furthest down the bottom, with the foundation being that little bit higher up, but normally still underground. The two words ‘footing’ and ‘foundation’ are often used interchangeably, and for the purposes of a small domestic job you can normally use either term for any part of the construction below ground without getting into too much trouble!
Footing & foundation types – you tend not to get to choose…!
When building a house extension, new-build house or any other type of permanent structure, you will need to use one of a variety of footing & foundation types, to comply with building regulations and to ensure that you don’t have structural problems further on up the build going forward.
You won’t normally have a choice as to which method you use, some are more costly than others, and some types of footing & foundations need more expert knowledge and experience than others. The type of footing or foundation required will often be determined by a variety of factors including what is being built, the nature of the ground, the level of the water table below ground, and most often, the presence of any trees on site within about 15 metres of the build.
Something really important to be aware of….. when you get your hands on a drawing for a project such as a house extension or a house is that – unless you have paid for an extremely high level of service from your architect – the first drawing you get will normally show an ‘indicative’ concrete footing 1m deep, and either 450mm or 600mm wide. The ‘1m deep footing’ note on most drawings should be taken with extreme caution. More often than not you will be doing something to a slightly higher standard, i.e. slightly deeper footings in places, but often you will be doing a completely different type of footing or foundation altogether, than a simple 1m deep concrete footing. If you ever see ‘1m deep footing’ on a drawing, check with the architect who drew it to see if this has been agreed with Building Control* or whoever is inspecting the work.
Building Control will only ever have approved a certain footing or foundation depth and type if they have i) seen a hole dug on site to ascertain ground conditions or if ii) you have had a full ground survey done, in which the type of ground and its make-up have been detailed in a professional report. There is no other way for an architect to know that a concrete footing 1m deep will be sufficient. All too often, a 1m deep footing is dug on site, based on this ‘1m deep footing’ comment on a provisional drawing, only for the Building Inspector to want deeper footings or a different type of footing or foundation altogether. This can be particularly tricky if you already have your footings dug out to 1m deep, as it isn’t always so easy to go back on them and dig them deeper. Worst-case scenario you will be filling them all in again to be able to dig them out deeper.
*As well as Building Control needing to approve the depth and type of a foundation/footing, in the case of a new-build you normally need to satisfy a Warranty Provider as well. And their standard will often be higher than that of the Building Inspector. Just because the Building Inspector is happy with something, it doesn’t mean the Warranty provider is….
Traditional concrete footings
The most common type of footing is created by digging a trench, normally a minimum of 1 metre deep and filling it with a mixture of concrete and masonry below ground level. Many years ago in most instances, a concrete strip about 100mm -200mm deep was laid in the bottom, and the remaining trench was filled with masonry to the top, or to ground level. This is known as a concrete strip footing.
The idea here is to have as little concrete in the trench as possible and as much masonry as possible, as concrete was expensive and time-consuming to mix up by hand, and brickwork labour & brickwork materials were cheap. However, the reverse in now true – concrete is cheap (relatively speaking) and convenient when brought to site in a mixer truck, and brickwork labour in particular is expensive and of course more time-consuming. This has given rise to the popularity of the concrete trench fill footing. The trench is filled near enough to the top with concrete, and masonry is used for only the remaining 200-300mm, sometimes not even that much.
If a traditional concrete footing has been agreed upon, and if this is – for example – 1.3m deep, then the 1.3m will normally be the sum of the concrete at the bottom, and the masonry above, all of it below ground level. You will often have the choice whether to do a strip footing and the remainder in masonry, or whether you mass-fill it with concrete and do the minimal amount of masonry. If you want to do the minimum amount of concrete in the bottom, just make sure you check with Building Control as to the minimum amount of concrete required. Some of them will require a minimum of 300mm thickness, and often they will ask for a layer of mesh in the case of only a small amount of concrete in the bottom.
And remember, any masonry below ground level will often have to be solid. So if you continue the ‘cavity’ in your masonry below ground level, you will have to fill this with concrete, normally known as ‘cavity fill’.
Traditional concrete footings are generally used up to a depth of about 3m, or a bit less. If a depth of 3m or more is needed, then you will normally need to look at other options, see below.
Piled foundations
These types of foundations are becoming more and more common. Many years ago, if you were told that a foundation had to be ‘piled’, the news would often be devastating, and could sometimes even make a job cost-prohibitive. However, the cost of piling has come down relatively speaking in recent years. More companies are providing this service, which has led to an increase in competition. And there are many different types of piling solutions now for all different types of situations. Access would also be an impediment to piling in the past, as piling rigs were huge machines which needed great access and a large area of hardstanding. Nowadays there are ‘mini piling rigs’ as small as a 1.5 tonne mini-digger, and even types that will fit through a standard doorway.
There are many different types of piled foundations, and the method will often be decided on based on access, the proximity of any nearby buildings, the type of ground etc.
To get a quote for any piling, you will normally need to provide a piling contractor with:
- A foundation drawing for the build
- The number of piles required – if known*
- The KN value required from each pile*
- Details regarding access, whether it be restrictions on height or width etc
- Any ground investigation information (i.e. boreholes)
*You will need a structural engineer to provide this information, and you’re always going to need one in the case of a piled foundation
The piling contractor will normally make a decision on which type of piling will be suitable, based on the information provided to them. There are two common methods of piling you will come across for domestic projects:
i) Augered piles, which are basically holes drilled into the ground and filled with concrete and often steel mesh. This is most commonly used where the ground will ‘stand up’ having been augured. So quite likely in clay, but not so likely in made-up ground, and ground with high sand/ballast content for example.
ii) Driven steel-cased piles. These are steel cases that are driven into the ground and then filled with concrete and often steel mesh. These are more likely to be used where the ground does not ‘stand up’ so easily.
For all Piling Foundation enquiries, we recommend M&D Foundations, please feel free to contact them on enquiries@manddmidlands.com and please mention JW Clark when you enquire!
Ringbeam for piling
With both methods of piling you are likely to need a ‘ringbeam’ constructing on top to take the masonry carrying on above. Depending on ability and experience, a lot of groundwork contractors will be able to construct this. It is basically formed concrete with reinforcement. We can quote for this if required, click here
If you’re going to have a go at constructing your ringbeam yourself, or if you have somebody to do it for you, you are going to need a couple of things from your structural engineer. Firstly, you will need a ‘bending schedule’, or a ‘steel schedule’, this will show the length and thickness of bars required, type of bar, quantity required, and it will also detail the shape of any bars – a lot of bars in a ringbeam will be ‘bent’ or ‘shaped’. Secondly, you will need a steel reinforcement drawing, this basically shows how all the bars go together. And beware, even if you are used to looking at and interpreting drawings, when it comes to a steel schedule and a steel reinforcement drawing, they are likely to be on a different scale to what you are used to! If it’s too much for your or your contractors and you would like a quote, click here.
Raft foundation
These are not as common as they used to be. Many years ago, they would quite often be the go-to alternative to a traditional concrete footing, but the ease of piling has made them less of an attractive method.
One of the popular misconceptions about a raft foundation, is that you simply create a concrete slab, which the masonry above sits on. This is true to an extent, but the depth of the slab and the amount of steel reinforcing mesh, often surprises many people. For a start, your raft foundation is going to normally be about 600mm deep, especially around the outside. And this 600mm normally starts a bit below ground, normally a minimum of 150mm, so this normally makes your raft a minimum of 750mm deep, but across quite a large area. The middle of your raft is likely to be a bit less, but you are still often looking at a minimum of 150mm of concrete, laid 150mm below ground, and then a minimum of 150mm of Type 1 MOT underneath all of that, but it will often be more. On a raft we constructed recently for a garage, the drawing called for 1 metre of Type 1 MOT below the concrete raft in the middle.
With the depths above in mind, you will often be looking at a huge amount of earth needing to be removed, and a similarly large amount of material needing to be brought in. Even once you have it dug out, you will also have to contend with the complexities of steel reinforcement, as mentioned above in ‘Ringbeam for piling’.
If you do need to construct a raft for any reason though, we can quote for this, please see here
Frequently Asked Questions (and some silly ones)
No you can’t. Well you can but you’ll have to wait 2 years before you build. This is how long it takes the ground to settle to its new characteristics with the trees having been cut down.
No they don’t. If the ground is clay, or has some clay content then any trees within a certain distance will have an impact on the footing/foundation
Of course if the builder priced for a 1 metre deep footing because that’s what was on the drawing, then he is right to ask for more money. To avoid this problem, find out in advance of getting quotes how deep you will need to go. Dig a hole 1m deep and ask the building inspector what depth he will want, based on the type of ground he sees, and taking into account any surrounding trees etc.
Most Building Inspectors will use NHBC guidelines for building near trees see here
It depends on the amount of concrete involved and how far you need to transport your concrete from the concrete mixer lorry, and that will be determined by how good your access is. Alternatives to moving concrete through a pump include using a dumper truck, we hire these, see here try to use the biggest dumper you can get into site - with wet sloppy concrete you often won’t get much concrete in the dumper. Also, make sure your dumper is low enough to discharge into, for this you will need to know the height it is discharged out of the mixer lorry, and the height of the bucket on the dumper truck. See here and click on Full specification and measurements.
If you can get the lorry all the way to the footings, then great. But just bear in mind that the distance the concrete will still need to travel. If your footing is 6m long or so, and the concrete lorry shoots the concrete in from one end, it should travel the full 6m okay. If you are going much further than this, then further the concrete needs to travel, the wetter you will need the concrete, and the wetter the concrete, the more it will shrink when it cures – this is where you’ll be getting in trouble with your bricklayer for making his job harder! You may also be compromising the strength of the concrete by having it too wet. One option you may have if you still have a digger on site, is to use the digger bucket to ‘drag’ the concrete along the trench. Us the thinnest bucket you have, and always wash the concrete off afterwards.
If all of the above doesn’t suit the site, then hire a concrete pump. With the concrete being pumped out exactly where you want it, it makes everything so much easier.
Of course you have the option to hand-dig these, but it’s time-consuming, and difficult work of course. And if you’re going much more than 1m deep then it’s not safe to be stood in a trench that is not properly supported. Why not hire a crane to get the digger in. See here. You may get a shock at the price of a crane, but just think about the time you will save with a digger.
