Raft Foundations, also called Mat Foundations, are most often used when basements are to be constructed. In a raft, the entire basement floor slab acts as the foundation; the weight of the building is spread evenly over the entire footprint of the building. It is called a raft because the building is like a vessel that ‘floats’ in a sea of soil.
Mat Foundations are used where the soil is week, and therefore building loads have to be spread over a large area, or where columns are closely spaced, which means that if individual footings were used, they would touch each other.
When Raft Foundations Are Used
A raft foundation is usually preferred under a number of circumstances:
It is used for large loads, which is why they are so common in commercial building which tend to be much larger, and therefore heavier, than domestic homes.
The soil has a low bearing capacity so the weight of the building needs to be spread out over a large area to create a stable foundation.
The ratio of individual footings to total floor space is high. Typically if the footings would cover over half of the construction area then raft foundation would be used.
If the walls of the building are so close that it would cause the individual footings to overlap, then raft foundations should be used.
Advantages of Raft Foundations
- Raft foundations are cheaper and quicker to use than traditional footings.
- The foundation and floor slab is combined, which saves time and materials.
- Less excavation is required.