Poured and block foundations both sit on concrete footings, or poured pads that serve as a base for the walls. 

Footings are constructed in trenches dug beneath the level of the basement floor. These trenches are wider and longer than the walls they support and function like feet to distribute the weight of the wall and the structure above it. Footings provide a firm surface to resist sinking or shifting into the ground or substrate. 

A footing trench ranges from six inches to three feet deep, depending on the building size and soil characteristics.