13. Slab foundation second opinion triggers

Slab foundation second opinion triggers

The house was constructed before 1980

Prior to 1980, it was common to design slab foundations with an eye toward making sure that the loads imposed on the soil by the foundation, the house, and the contents would not result in an excessive amount of settlement. It was not commonly understood in the engineering community that a slab foundation in an expansive soil area needed to be designed for stiffness, not just bearing capacity.

By the 1960s, a growing number of engineers were working on empirical methods of slab foundation design that focused on making the foundation stiffer. Some of these methods worked better than others, but there was a serious problem with all of them. They were empirical and what worked in one location might not work in another. What worked for one engineer might not work for another. In other words, design methods that resulted in successful designs in San Antonio would not necessarily work in Houston or even in Austin.

What was needed was what is called in the engineering world a rational design method. A rational design method is based on a scientific understanding of how a slab foundation interacts with the supporting soil.  tdning a i ng   

istinsyangs wrerahtaht successful design metjhocng ng

atiosnyet n wyh jciodif   

The house is a large, expensive home

The house has been underpinned in the past

The best marker or metric for predicting if a house foundation will be underpinned in the future is a history of foundation repair. This flies in the face of what too many people think they know. They have listened to far too many ads that say something like, Fix It Right, Fix For Good.

I strongly recommend that you encourage anyone who is buying a previously underpinned house to order an engineering report on the foundation.    

The house has one or more add-on foundations

I know many readers are asking what is an add-on foundation? An add-on foundation is simply a foundation that was an addition to an original foundation. They are usually small compared to the original foundation, but I have seen situations where they are larger than the original foundation.

It is inevitable that the original foundation and any add-on foundations will move differentially over time. Typically, the add-on foundation will tilt away from the original foundation. They may also drift away from each other laterally.

This does not normally cause significant structural issues, but it can cause very noticeable distress, physical distress in the house and emotional distress in the owner.        

The house is on a ravine, bayou, creek or a lot that has a lot of slope

Houses that are on sloped sites are far more likely to experience future foundation problem than comparable houses on relatively flat sites. I think home inspectors are fooling to take the risk these homes pose. I simply turn them down. 

Large expensive homes

Like homes on sloped sites, these homes are not worth the risk. The design methods we have for slab foundations on expansive soils were developed for small, simple rectangular foundations. The more complex the foundation shape and the larger the foundation, the more problematical it becomes to apply the design procedures in a straightforward way.

Many, perhaps most, of these foundations are designed conservatively so they probably do not pose more of a risk of future foundation issues than smaller, simpler houses, but that is certainly sometimes the case.

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