This article will examine the question of whether relying solely on the levelness of a slab as a basis for recommending foundation repair is an accurate way of determining the need for underpinning. The simple answer to this dilemma is no. While slab measurements are a widely used method of determining the levelness of a slab, they cannot and should not be the sole basis for determining whether a home needs underpinning based on a variety of reasons, 3 of which we will be examining.
Reasons Why Basing Foundation Repair Recommendations
Solely on Measurements is an inexact Science
- No newly constructed slab is perfectly level
- When a new home is constructed, numerous factors affect the slope of the slab, including it’s shape, support materials, extent of soil preparation, and the weight of the structure which is built on it among other things. For these reasons, and the fact that human handywork can never be absolutely perfect, a perfectly level slab at the time of construction is absolutely impossible.It is absolutely feasible, that at the time of construction of a home, a slab can have a slope of 2 inches or more, and the home will be properly designed to support this deflection. Thus, any leveling done on this house in the future to correct the slope can adversely affect the structure of the home.
- The nature of Expansive soils means that slab foundations are constantly moving
- Because of the characteristics of expansive soils in and around Austin and San Antonio the moisture content of the soil is always changing. In short moist soils expand and dry soils contract, for this reason measurements taken over extended periods of time will show different levels of deviation. As previously stated, while measuring the deviation of a slab is a widely used technique, it is not the only factor which should be considered when evaluating a foundation.
“The foundation surface changes as the soil shrinks and swells. While the foundation may become more level with time, at least some damage to the supported structure does not disappear, such as cracks in brittle wall coverings. Thus, it is possible that a house that shows cracking in brittle wall coverings may fail to be properly diagnosed because the foundation surface profile was judged to be “level” at the time of the inspection.” excerpt taken from Using Slab-on-Ground Elevation Measurements in Residential Foundation Engineering Performance Evaluations
- A level reading can be obtained on a structure that actually needs repairs
- Because of the two factors previously discussed, a slab which was not completely level after construction, and, which is experiencing soil expansion or contraction underneath the slab, at any given time, based on the conditions, can exhibit a level reading.
“Either foundation deflection (bending or angular distortion) or tilt (planar rotation) may affect structural integrity and performance. Determining the deflection and tilt of a slab-on-ground foundation is an approximation without an as built or previous floor elevation survey, because the original surface configuration is unknown. Therefore, a floor elevation survey can provide valuable information, but should not be the only basis for evaluating foundation deflection and tilt.” excerpt taken from the Guidelines for the Evaluation and Repair of Residential Foundations Section 5.4 – Deflection and Tilt
Relying on Measurements Alone Would be Irrational
As described, measurements of slopes in foundations are not the only way to tell whether a foundation is in need of repair. Relying solely on measurements to determine the condition of a foundation can lead to inaccurate evaluations, and more damage to a home. Knowing the soil make up of the area in which the home is located, the shape of the slab, observing signs of foundation problems, and familiarity with slab construction all contribute to making an informed decision on whether to recommend foundation repair or not.
“There is no rational, logical reason to refuse to make a positive diagnosis merely because the surface profile does not conform with one of two distortion profiles that are used for design purposes.” excerpt taken from Using Slab-on-Ground Elevation Measurements in Residential Foundation Engineering Performance Evaluations
A combination of Measurements, Observation,
Knowledge of Soil, and Experience
Superior Foundation Repair’s steel pier method of foundation repair has been tested over decades and been shown to effectively counteract the effects of expansive soil on slab foundations by relying on bedrock for support. When evaluating a foundation and recommending repair we combine our experience, extensive knowledge of Central Texas soils, observation of signs of structural damage, along with measurements to determine our recommendations for repair. We have recommended repair on homes which other foundation companies did not recommend repair, and conversely, we have recommended no repair on homes where other companies have recommended repair. We always stand by our evaluations of slab foundations.