Raft foundations are formed by a reinforced concrete slab (of a uniform thickness) and cover a wide area, often the entire footprint of a building. They spread the load of wall columns built across the entire area of the foundation and sit on top of the ground. 

There are two types commonly used: 

Flat slab raft foundations 

Cast on a bed of blinding concrete (cast over a rough surface to provide a smooth working finish) and a moisture proof membrane to prevent damp rising through the slab. Flat slab is used when the ground has a reasonable bearing capacity. The raft may not need to be reinforced and can be used for small buildings, such as two storey houses, where there is no need to thicken the raft under the load bearing walls. 

Wide toe raft foundations 

Used when the compressibility of the ground is poor and would need an uneconomically thick slab of concrete. To take the load off the external walls of the build, a reinforced concrete edge beam is built under the external walls and thickened under internal load bearing walls.