A Guide to Property Foundations
When you’re planning a build, whether it’s an entirely new one or just a small extension, your builder will need to perform an overview of both the site itself and the access to it. Depending on their findings, they might be able to get a mini digger on the site, but if access is limited, for example if you’re planning an extension in a terraced property, they’ll need to manually dig the foundations with a spade.
Obviously, using a mini digger will cost a few quid more but it’s far faster and easier for everybody involved, and the builder’s gonna want the digger if at all possible. On the other hand, if a manual dig is necessary, it’ll take several more days of hard work and he’ll need to do everything by hand.
Using chalk or construction paint and lines of string, the layout and measurements for the excavation of the foundations need to be marked on the ground. Once the markings have been made, double check them against the plans to make sure no mistakes have been made as you don’t want a visit from the planning department. If it turns out you need to change the plans in any way, get them changed and accepted by the planning department before you start digging.
Types of foundations
There are many types of foundations, the most common of which include:
Strip foundations are when a strip of concrete is laid to support load-bearing walls. Strip foundations need to be a minimum of 450mm deep or more, depending on the type of soil, and the concrete needs to be a minimum of grade ST1.
Sometimes, when the soil can’t provide adequate support for a standard Strip foundation, a deeper, wider ‘Deep Strip foundation’ can be used. Alternatively, the concrete can be spread over a bigger area and reinforced with steel. This is called a Wide Strip foundation.
With a Trench foundation, a trench needs to be dug deep enough for the subsoil to provide enough load-bearing support, at which point the entire trench is filled with concrete. Depending on the number and location of trees on the site, steel reinforcement might need to be used to protect the foundation.
If the condition of the ground isn’t suitable for either Strip or Trench Fill foundations, Piles can be driven into position until they are deep enough for the soil to support them. Piles are usually 3m long and may need to be reinforced with steel. They are then connected at the top by reinforced concrete.
A Raft foundation is a reinforced concrete raft that is usually only used when dealing with particularly weak soils, e.g. peat and clay. Often with these soils, it is cheaper to use a raft foundation as a Deep Strip foundation would be spread too wide for it to be economical.
Once the foundations have been built and everything checks out, you’re ready to start building the rest of your new house!