1.The structural load are so high or soil conditions so poor that spread footings would be exceptionally large. As a general rule of thumb, if spread footing would cover more than 50 % of the building footprint area, a mat or some type of deep foundation will usually be more economical.
2.If the soil is very erratic and prone to excessive differential settlements (heaves). The structural continuity and flexural strength of a raft will bridge over these irregularities.
3.The uplift loads are larger than spread footings can accommodate. The greater weight and continuity of a mat may provide sufficient resistance.
4.The bottom of structure is located below the GWT, so water proofing is an important concern. Because mats are monolithic, they are much easier to waterproof. The weight of mat also helps resist hydrostatic uplift forces from the groundwater.