What is a raft foundation? Pros, cons & types

Foundations are the first part of a construction project and one of the most critical elements. One type of foundation used for larger buildings is a raft foundation.

Find out more about raft foundations below but please just note – this is general info. Every project is different so if you are at all unsure about your particular project it’s best to seek appropriate specialist advice. Where applicable work must be carried out by a qualified professional. Always follow manufacturer’s instructions advice and ensure compliance with any applicable laws or regulations.

What is a raft foundation?

A raft foundation is a strong concrete base that holds up buildings, bridges, tanks and other structures. The flat block helps spread the weight of the building out over a big area so it doesn't sink into the ground.

Raft foundations are normally used in places without strong soil, like soft clay or sand - they float on the soil and can be good to use where other foundation types wouldn’t work.

A raft foundation is made of a thick, strong piece of concrete held up by a grid of steel bars. This helps stop it from being bent or broken by heavy weights and sinking into the ground. How thick and deep it is depends on how heavy the building is and what the ground is like.

Raft foundation types

There are many types of raft foundations. However, the main ones are:

Solid slab raft foundation
Slab-beam type raft foundation
Cellular raft foundation
Piled raft foundation

Solid slabs are the most popular type of raft foundation. This is where all the foundation is one solid, reinforced concrete slab.

A slab-beam raft foundation has beams and thick edges. The beams and edges make the foundation stronger.

Cellular raft foundations have spaces (cells) in the slab, making them lighter than other types.
A piled raft foundation is often used in places with really bad soil – piles are put deep in the ground to give it more support.

What raft foundation type you choose depends on many things like how heavy the building is, the soil and the building's needs. A structural engineer can help pick the best type of raft foundation for a project.

Advantages of raft foundations

Many buildings use raft foundations – they have lots of benefits and are popular with architects and builders. Raft foundations are good for places with bad soil and work well on soft clay or loose sand. Other shallow foundation types normally don’t work here. They spread the weight of a building evenly over a big area and prevent the building from sinking or moving. Raft foundations are sometimes cheaper than deep foundations (like pile foundations) because they don’t need as much digging into the ground, and the slab also works as a floor slab.

Disadvantages of raft foundations

A raft foundation is not best for all soil types, like clay which changes size a lot. This soil type can make the raft foundation move and crack. Raft foundations are hard to change or make bigger when built. This could stop people from making changes to the building. A raft foundation may cost more than some foundation types because it uses more steel and concrete.

To sum up, raft foundations are good for spreading the weight of a building evenly over a bigger area. They are better for places where the ground is not very strong and the building is heavy. They can also help stop damage to the building. However, they can also be more expensive to build than other foundations and are only good for some types of buildings or soil.

A building expert can help decide if raft foundations are best for the job.

The information on this website is intended to be for general guidance – however every project is different which could affect the suitability for your particular project or circumstances. References to a product, service or material should not be considered a recommendation or any indication of fitness for any specific purpose. Where applicable work must be carried out by a qualified professional. Seek appropriate specialist advice if needed and always follow manufacturer’s instructions advice and ensure compliance with any applicable laws or regulations.

What is a raft foundation? Pros, cons & types