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Deflection vs Tilt: What’s The Difference?

This house is 2” lower in one area vs another. Does that mean it needs underpinning? Not necessarily. I have seen houses up to 9” out of level with very little damage and houses out of level 1”to 2” out of level with much more damage. What gives?

The house with 9” in this case had a very rigid slab and all of the elevation difference was basically just a very steady, even tilt from one side to the other. The house with a ton of damage and only 1-2 inches had a huge bulge in the profile of the footing and floor. That “bulge is called deflection. If the bulge side is like a bow and the tilt is the string, the deflection is the difference between the bow and the string. In other words the bend in the footing/slab is what creates stress on the structure.

Take a look at the 2 profiles below. The first is tilt the second shows deflection.

deflection-vs-tilt 1

Notice how the profile shows a nice even slope from one side to the other. Also see how the actual line does not deviate from the red slope line. As you might suspect not much damage in this house.


Now take a look at this one. See how there is a big difference between the red line and actual profile? This house had lots of distress and was in bad need of leveling.

How can you determine deflection without having all of the points and subsequent contour lines to plot? Obviously you cannot.

How much tilt or deflection is too much? Structures can withstand a lot of tilt as compared with deflection as discussed above. The Foundation Performance Association has developed a consensus of tilt of 1% and deflection of L/360. Accordingly I feel those guidelines should be followed unless a new and better consensus is ever developed.

So Now what happens if we discover with our careful analysis that there is a floor slab heave and no footing movement? In my next blog, I will lay out the options including a revolutionary process developed by our team. 

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