Suppose you have a 300 mm thick slab with uniform load distribution across the entire slab and some areas representing foundations, etc and you want to assign uniform loads on these particular areas. Do not add these areas as 0.01 mm thick slabs and the loads applied on them. Just assign these areas to the same thickness of the slab and it will not be added to the slab thickness. In Safe 14, slab assignments will overwrite the previous slab assignment.