Waffle raft is a concrete slab that made up of close grid stiffening ribs. These ribs are made by using waffle polystyrene void forms which are laid out in a grid pattern and locked together so they do not move during the concrete pour. The waffle grid must be laid down on a level surface and locked together. This type of slab is ideal for very reactive clay sites as the whole slab sits on top of the ground and is not embedded into it.