The concept of using foundations when constructing a building is a simple one. In order to place a heavy load onto soft soil, you need to provide some form of footing. Whether you install one large foundation system that covers the entire area, or individual footings that each take a concentrated load very much depends on what you’re building on, and what you’re building above it.
It’s a fundamental part of your project and it pays to know the suitability of each type of foundation before you begin digging.
The difference between foundations and footings
It makes sense to address any confusion around these from the outset. They’re terms that you’ll sometime see used interchangeably, but they actually have very specific meanings; all footings are foundations, however, not all foundations are footings.
As the name suggests, footings are in contact with the ground, reinforcing support to individual columns.
Foundations, on the other hand, can be thought of as the leg of a building, transferring the load between the footing and whatever structure sits above it.
What are house foundations made of?
A concrete foundation is the most common mainly because it’s the most versatile. Concrete is poured into a wooden frame (which is later removed) to form the slab footing. This can be any shape and size depending on the nature of the soil and the building project.
Metal reinforcements are added along with cement columns, drainage membranes and concrete blocks to form a solid base on which to build.
Depending on whether the ground floor is actually the ground floor or whether there’s a basem*nt to consider will determine what happens above that level.
Not all concrete mixes are the same, and the type you choose for your foundation depends on a variety of factors including durability and cost. Each is graded according to its compressive strength after 28 days with a mix proportion ratio of cement, sand and aggregates.
For example, C10 has a strength of 10 newtons, and M10 would mean a 1:3:6 materials ratio. C20 would have a strength of 20 newtons strength and M25 has a ratio of 1:1:2.
In foundation construction, a common mix is C40/M20 but you should review the pros and cons of each before committing to a certain type before your project begins.
How deep should house foundations be?
Foundations are made by excavating earth to a required depth to the lowest point of the footing, then beginning the build from the base up.
Whether you go for shallow isolated footings, deep pile foundations, or a mat foundation is usually determined by the type of soil on your plot of land.
It’s probably the most unpredictable part of a house build. You can have as many soil investigations as you like, but you won’t really know what’s underground until you start digging.
The local knowledge of your designer, building inspector or planning authority will be valuable, but you should plan for the unexpected and factor the worst-case scenario into both your timescales and your budget.
- Rock (for example limestone, granite, sandstone or hard chalk) is a naturally solid footing and may require relatively little digging
- Chalk or damp clay soil needs to be excavated until a firm surface is found
- Gravel and sand subsoils can hold together well, however, be wary of your trench collapsing if they’re not suitably compacted
- With peat or any form of loose sandy soil, the likelihood is you’ll require a reinforced raft foundation, unless you find some suitable load-bearing ground
In terms of how much your foundations may cost, the elements you’ll need to factor in are:
- Excavation and loading
- Soil disposal and tipping charges
- Meshes and membranes
- Concrete foundation materials
It’s worth noting that as soon as your foundations need to go beyond 2 metres, the most cost-effective foundation option is a reinforced raft. This is because trenches beyond this depth will require supporting while footings are being put into place.
As the UK has a relatively cold climate, any shallow foundations need to be built below soil which can freeze and expand to avoid damage. If that’s not possible, additional insulation may be required which is another cost element to factor in.
Foundation concrete pouring techniques
Once you’ve dug down and built your forms, next comes the all-important pour.
The most typical method is to pour the footings, then to add a vertical foundation stemwall connected with some form of rebar to connect the two. One technique worth exploring is a ‘mono-pour’ where the footing and stemwall are a single structure. With no joints to waterproof and the option of integrating other structural elements within the foundation pre-pour. The additional effort required to create the form could bring big benefits later on.
However you decide to proceed, the physical act of pouring concrete needs both planning and manpower.
If you’re using a lorry to pour directly into your trench then parking needs to be considered carefully, as well as access to the site itself which can be tricky to arrange in some urban locations.
Trench collapses are the one thing you want to avoid, so put in every support and strut possible and ask your team to keep a close eye on any movement as they’re raking the mix.
Ensure even distribution by moving the chute back and forth and don’t fill the whole form before raking. Concrete has a mind of its own and you need to make sure you have both the people and tools ready to go as soon as the pour begins.
A well-executed pour generally means an easy levelling process, which is something your bricklayers are sure to thank you for later on.
How long after pouring concrete can you build on it?
Concrete curing is a vital process in the construction of your home. Typically, your foundation will have good resistance when it’s 7 days old, be better at 14 and at it’s best at 28 days. However, it’s unlikely you’ll want to wait that long.
Some types of readymix cement can be ready to build on in as little as 24 hours, but the general rule is you should leave the forms holding your concrete in place for at least two days to ensure that the concrete is completely dry before looking to deconstruct them.
There are various factors that may influence this including how well your pour went, the grade of cement you’re using and the weather. A temperature of 20°C could reduce the drying time by up to 50% compared to conditions of 10°C.
The process works by hydration. Concrete absorbs water to form crystals and the cooler it is, more time the crystals take to strengthen. So while warm weather is good for a speed of construction, it has a negative impact on strength, which is why pouring should be done when the temperature outside is moderate.
It’s a judgement call on when you consider the concrete to be set, but as soon as you believe it to be dry, add any sealants that you’re planning to use to give them the best chance to work effectively.
Rushing things at this stage will only create more problems later on, so while it’s tempting to want to hurry it along, good things come to those who wait.
