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By: Fred Willcox, MTI
TREC Professional Inspector #160
Copyright© 2010
All Rights Reserved

Most of the hi-tech equipment used in house inspections in Houston is of little real value and often gives erroneous information. The use of hi-tech equipment may be a disservice to house buyers.

Let’s start with the equipment commonly used to measure foundation elevations. The device most commonly used has an accuracy rating that is in the range of the difference in the elevations of the surface of the foundation that most inspectors use to make a determination of the need for foundation leveling. This means that the differences in elevation indicated by the device may be caused solely by the range of error in the equipment itself and that the foundation may be perfectly fine.

Most of these elevation measuring instruments require constant servicing to maintain their accuracy. According the information on most manufacturer websites, an inspector would need to own three of these instruments in order to keep one instrument properly calibrated and adjusted at any one time. The other two would be at the manufacturers for adjustment and calibration. The inspectors I know have only one elevation measuring instrument. The instrument has often never been serviced, calibrated or adjusted.

Re-creation of the results of the elevation measurements is virtually impossible to recreate. This means that the degree of accuracy and the ability of the inspector to get the same result on two or more elevation reading is very questionable. This inability to get consistent results often gives the impression that the foundation is moving differentially when it may not be and probably isn’t.

Then there is the interpretation of the data collected from the use of the equipment.

Most inspectors and engineers use an improper reference for the basis of their opinions on foundation performance. The allowable deflection (one part of the foundation moving differently from another part of the foundation) used for a concrete foundation are the parameters quoted in Table R502.3.1(1) of the International Residential Code shown below.


hi-tech equipment for real estate inspections

This section of the code relates to the allowable deflections in wooden floor framing systems. It has nothing to do with concrete foundations. This amount of deflection, one (1) part vertical movement or displacement in a three hundred sixty (360) part span (the horizontal distance over which the movement is measured) is the same as a 1 inch vertical difference over a distance of 360 inches or 30 feet. This equation of 1/360 is a ratio. That means that the deflection limitation follows regardless of the distance over which the movement occurs. For example, a 1 inch deflection in 30 feet is the same as a ½ inch deflection in 15 feet or a ¼ inch deflection in 7.5 feet, etc. You would have a hard time being aware of this amount of deflection unless you were extremely sensitive to slopes.

There is a foundation performance committee here in Houston that is attempting to write and enforce rules for the rendering of opinions on the need for leveling a concrete foundation. The standard set by this committee as I understand it is that the foundation must be “level” within a slope of two (2) percent. The math on this is simple. The tangent (the side opposite which would be the “deflection” divided by the side adjacent which is the “span”) of two degrees is 0.0349207695. To get the deflection, multiply 0.0349207695 by 360. The answer is 12.57147702. That means that the allowable vertical difference in elevations or displacement under this rule would be 12.57 inches in vertical differences in the surface of the slab in a 30 foot span or 6.3 inch deflection in a 15 foot span or a 3.15 inch deflection in a 7.5 foot span.

You wouldn’t have to be a very good inspector or engineer to figure out that a foundation with this degree of vertical change would be uncomfortable to live in.

If your inspector or engineer takes elevations, the data will then be judged against a standard that says that differential movements in a foundation that you would never be aware of would require the foundation to be “leveled” or the data would be judged by the standard that says that although you will be leaning walking down the hallway the foundation may not need to be “leveled”.

Add to that the fact that the cracks in the drywall and brick veneer, the out of square door frames and out of square window frames that you hear so much about as being signs of differential foundation movements can be, and most commonly are, caused by defects and errors in the framing (the wooden wall, floor, ceiling and roof members) of the house and you can understand why many inspectors and engineers are hesitant to rely on their own judgment.

If you want to know what the elevations of the surfaces of the slab on the house are hire a land surveyor. Land surveyors are the experts in taking such elevation measurements.

You must understand that the elevations will show that the foundation is not “flat” or “level”. It is practically impossible for a concrete foundation to be constructed “flat” or “level” and even if it was the mass will not “cure” uniformly which will cause vertical differences or slopes in surface elevations.

“Movement” is a function of time. Elevation readings that are taken at one point in time cannot determine “movement” by the very definition of the word.

If you want to know if the foundation of the house that you want to buy is in need of leveling, hire a real estate inspector who is not afraid to make judgments on foundation performance and who does not need to rely on artificial standards.