A foundation slab is a concrete plate supported on the ground which distributes the loads of the building over the entire support surface.

Slabs are a type of surface foundation that performs very well on inhomogeneous terrain that, with another type of foundation, could suffer differential settlements. Also on land with very little bearing capacity. The simplest slabs are slabs of constant thickness, although there are also ribbed slabs that are thicker depending on the direction of the walls or rows of pillars.

Its calculation is similar to that of a flat roof slab, reversing the directions of the forces and applying both axial and uniform loads coming from the entire building. The beams of these slabs are inverted to remain buried in the ground and avoid obstacles to the use of the surface, which is ready to be used as a pavement although its surface is still rough.

In essence, a foundation slab behaves like a plate that on its underside suffers vertical forces proportional to the ballast module and the settlement or vertical arrow "Arrow (engineering)") of the slab. For a slab of uniform thickness the expression is:.

Where:.

Once the function that gives the vertical settlements has been determined, the bending moments in each direction are estimated:

From the bending moments, the correct thickness of the slab and the necessary reinforcement for the slab are determined.