The Structural Significance Of Elevation Differences

I’m sure I will get a lot of negative feedback from this page. All I ask is that you stay with me and give me a fair hearing. 

Let me state this first

I am not opposed to making elevation measurements. I am strongly opposed to the misuse of elevation measurements. Misuse is far too common, even among licensed engineers. I am not opposed to using elevation measurements to estimate the degree of bending across profiles such as from front to rear and from side to side. 

I am opposed to the idea that the average slope between two arbitrary points has any structural significance what-so-ever.                 

What the codes say

This might surprise you: the codes say nothing about how level a slab-on-ground should be. If someone can specify the specific code, edition, and section, I will agree to change my mind. But I’m confident that no building code, including any edition of the International Residential Code For One and Two Family Dwellings and anything published by the American Concrete Institute, that addresses levelness of slab-on-ground foundations. 

What the codes say by ignoring this issue  

Building codes are supposed to safeguard public safety, health, and general welfare. Since the codes say nothing about how level a slab surface should be, it is fair to conclude that levelness is not a public safety, health or general welfare issue.      

Construction tolerances exist but are not that helpful

I say not that helpful for several reasons. First, the measurements must be made within 72 hours after concrete finishing.  Second, the most applicable tolerances are these: 

local worst case: 1.25 inches difference in elevation between two points 10-feet apart

overall levelness: 1.5 inches in elevation between any two points regardless of how close or far away they are    

Sloping floors are not a safety issue

I hate to beat what should be a dead horse, but this is an important issue. Concrete driveways, ordinary suburban streets, and overpasses, especially overpasses, have far higher elevation differences that any slab foundation I have ever seen. I have never heard of anyone who thinks, say overpasses, should be made more level to increase safety.

The structural significance of elevation differences is nill. 


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