The stiffness – deflection – tilt compromise
The stiffer the foundation, the less the foundation bends. The less the foundation bends, the less distress / damage to the house.
There is a side-effect. The stiffer the foundation, the more the foundation tilts. The more the foundation tilts, the less level the foundation as measured by elevation differences.
Consider this example: if a slab foundation is 40-feet by 60-feet, is perfectly rigid and it tilts as little as 1% in each direction, the highest elevation difference would be 12-inches.
Why is this important?
Some engineers will treat an elevation difference as a repair indicator if it exceeds some arbitrary value such as 1.5-inches, 2-inches or 3-inches.
This is grossly unfair to the homeowner and even the potential buyer.
Any house foundation that has tilted more than, say, 3-inches is almost certainly a very stiff s;ab