Slab Foundation Second Opinion Triggers

What is a second opinion trigger?

A second opinion trigger is something that should make you lean toward recommending that your client retain an engineer to make a second (or sometimes a first) assessment of the performance of a foundation. What follows is our opinions concerning things that we believe should make home inspectors, buyers, sellers and agents consider recommending that a buyer (or seller) retain an engineer to make a performance evaluation of a foundation.

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. Designing for bearing capacity alone was a recipe for an under-performing foundation when the supporting soils were expansive. 

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 is what engineers call a rational design method. A rational design method is based on a scientific understanding of how a slab foundation interacts with the supporting soil as opposed to an empirical design method which is based on the idea that if a design method is known to give acceptable results, that method can be used even though it is not based on a scientific understanding of how slab foundations interact with the supporting soil.

It was not until 1976 that serious efforts were made to develop rational design techniques for designing slab-on-ground foundations. The research for this work was made at Texas A&M University. The research yielded new techniques for soil investigations by geotechnical engineers and new rational design techniques for structural engineers. The research work was done by Kent Wray, who was then a doctoral student under Robert Lytton, a professor at Texas A&M. The soil testing methods and design procedures engineers use today for expansive soil foundations are based on this research.

The main point that you should understand is that homes built before 1980 are more susceptible to foundation problems than slab foundations built prior.

The house is a large, expensive home

All of the slab foundation design methodologies used for expansive soils were developed for small houses with simple foundation footprints such as a rectangle. They were not intended to be used for large, expensive homes with complicated footprints.

Most engineers modify the design techniques to compensate for this. The more complex the foundation shape and the larger the foundation, the more problematic 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.

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.

For this reason, we 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

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 there are situations where they are larger than the original foundation.

It is inevitable that the original foundation and any add-on foundation 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. If the house has an add-on foundation, in our opinion, an engineering report should be ordered.

The house is on a ravine, bayou, creek or a sloped site

Houses that are on sloped sites are far more likely to experience future foundation problems than comparable houses on relatively flat sites. The standard design procedures are intended for relatively flat sites, not for sloped sites. We think home inspectors are foolish to take the risk these homes pose. In most cases, an engineering report should be done, but it should be done by a geotechnical engineer. We simply turn them down.

Homes with areas of exposed concrete floors

I am not talking about houses with detached garages. What I have in mind are houses with attached garages and houses with exposed concrete floors in the living areas.

The exposed concrete will inevitably have visible cracks. The cracks are rarely a structural concern to an engineer, but they are a concern to home buyers.

Home buyers associate visible cracks with failure. Structural engineers consider them as normal. The reason we put steel in concrete foundations is to control cracking that will inevitably take place.

The reality is that many buyers will focus on the visible cracks to the extent that they may not close on the house. We recommend sellers of these houses retain an engineer to write a report that will keep things in perspective.


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