Property Surveying Blog
In this series of blog posts, I have been looking at properties through the ages. In this week’s edition to the series I will be looking at the most common foundations employed in period residential buildings and some of the more modern techniques that have been developed and are widely used today.
There are many different types and capabilities of foundations, the most common of which were used in period or modern houses are further described below.
What is a Foundation?
A foundation supports and transfers the weight of the building onto the soil below.
Bearing Capacity is the amount of weight that soil can support easily without the building moving in a way that will cause damage to the property or structure.
For example, in a supportive soil that is able to carry a higher weight load, a straight forward shallow simple foundation design such as a strip foundation would be employed.
A foundation design that would be used for a less supportive soil would generally speaking be deeper and more complex, such as a piled foundation.
Soils have different layers, known as strata, as they are typically formed in lines or layers. This will consist of a topsoil, which is made mostly from organic matter such as plants at the surface, below this there will be a layer of subsoil, which is the layer which provides the support, and below this the parent rock or bedrock, which is the layer of the earth’s crust.
Topsoil on it’s own is not able to support the weight of the building. Therefore, the foundation will either need to be deeper to rest on the firmer subsoil beneath, or be able to spread the weight of the building over a larger area of ground, so that each section of soil has to support less weight.
Period House Foundations
The first recorded literature to govern the design and sufficiency of a foundation were the Model By-Laws which were introduced under the Building Act of 1878.
These gave a specified depth and construction type for the foundations of a house amongst other construction details which eventually developed into the Building Regulations Approved documents that are used today.
Broadly speaking, most period houses built before the 1930’s were on footings of bricks laid in a stepped fashion known as corbels.
These were built directly at the bottom of the walls, and would differ in height and thickness based on the height of the building being constructed. A taller building would have a wider spread of foundation, allowing it to support the greater load of the wall above, and vice versa.
Generally speaking the corbels foundation would be between 150mm (6 inches) deep for a wall up to 25 feet high and approximately 300mm (12 inches) deep for anything between 25 to 40 feet in height. Although concrete was sometimes used underneath corbel foundations, for the most part they were laid directly onto the ground.
Brick corbel foundations were both cost effective and easy to construct, needing no more than a knowledge of the final height of the building and the simple rules above to construct.
The disadvantages associated with them were due to their depth, unless the house had a basement, they were often built on the topsoil, which is prone to movement while also being able to support less weight due to it the affect that the water table and seasonal weather changes can have upon it.
This oversight has caused a number of properties with foundations of this type to to and move and subside, eventually requiring underpinning.
Underpinning is the practice of retrospectively excavating beneath the corbel foundation and laying concrete, this then safeguards the property from movement by transferring the load of the property onto the stronger soil beneath the topsoil.
For all their failure, corbel foundations have certainly stood the test of time, as I speak the foundations of the property that I am writing this blog post from is supported by corbel foundations.
Foundations as we know them today, became widespread in the 1940’s after the Second World War.
The majority of the houses built were on concrete strip foundations, or in the case of loose ground conditions or a previously ‘backfilled’ site, a raft foundation would be used.
Trench Fill or Strip Foundations
In my experience, this is the typical idea of what people think of when they hear the word foundations.
This type of foundation is achieved through the method of pouring cement, in an excavated trench beneath the location of the property walls. Once the concrete has cured, the walls of the property will then be built up directly off the foundation.
This type of foundation is typically around 600mm in width and usually a minimum of 1 metre deep. They are both quick and inexpensive to construct and it is certainly one of the most common foundation types that we see here at Berry Lodge Surveyors on the construction projects we are involved in within the Greater London area.
For all of the benefits associated with this type of foundation, they can be prone to damage from tree roots and in an area which is heavily forested, they will need to be of a greater depth than usual to avoid the layer of soil which is effected by the seasonal change of moisture in the soil.
This type of foundation is less commonly seen in Britain than in countries such as North America and Australia, where it is called a ‘Waffle Foundation’.
Raft foundations need to be individually designed depending on the type of ground and it’s qualities and properties, however generally speaking they tend to be 300mm deep and made from a reinforced concrete slab on a hardcore or sand base. They are spread below the entire footprint of the property, laying or floating on the soil like a raft.
Raft foundations by having a lower ‘contact pressure’, which is the amount of weight which is transferred to the soil.
Imagine lying on an inflatable lilo in a swimming pool, compared to trying to kneel in one position on the same inflatable lilo. This is the same effect that the raft foundation gives, allowing a greater amount of weight to be constructed on weak soil.
The main disadvantage associated with Raft foundations is that the top edge of the foundation which sits above ground can be prone to corrosion of the steel reinforcement within the raft. Over time this corrosion can lead to the failure of the foundation.
However, as with any foundation if correctly treated during constructed, this should not be an issue that most home owners find themselves confronted with.
Foundations form part of every structure you will have ever been inside of, therefore having a comprehensive understanding of them is key.
Here at Berry Lodge we have seen thousands over the years, from those that have failed, to those that are freshly laid, if you are about to embark on a new extension that will see new foundations laid, or think your foundations may be failing leading to property movement. Give our experienced team of Building Surveyors a call now and they will be happy to assist.