I was once asked this question at a seminar for TREC home inspectors:
Why would a licensed engineer seal plans for a foundation that was a minimal design?
Reality check one: There is a large demand for starter homes
Owning and living in a single-family house is a well-established part of the American dream. Young couples typically start out in a rented apartment, but their goal is to buy their own house. Once they have set aside enough money for a down payment, they are going to start looking more seriously for that dream home.
The monthly payment for a starter home can be less than the monthly rent for an apartment. Starter home builders know this and target young couples in their advertising.
Reality check two: Every builder has a budget that targets a specific selling price
Starter homes today are much nicer than they were just a decade or two ago. In one sense, true starter homes are a thing of the past. One thing has not changed. Every builder who targets young couples looking for their first house knows that they must keep the price at or below what young couples can afford.
Reality check three: Builders also have budgets
Home building is a very competitive business. This is especially true of builders who specialize in starter homes. Builders have to control their costs plus overhead and profit or they will not survive.
Reality check four: Builders & owners do not want to spend any more than necessary on the foundation
It is almost axiomatic that the builder will not spend any more than necessary to build a house. They especially do not want to spend any more than necessary to build the foundation. It is also true that the large majority of buyers and homeowners are reluctant to spend any more than necessary for the foundation.
Reality check five: builders are not going to spend money on the house when they cannot recapture it when selling the house
Once the house is finished, only about 5% of the foundation can be seen. How do you sell something that you cannot see, smell or taste? I have found that the large majority of home buyers and more interested in the kitchen countertop and other aesthetic amenities than in the foundation. The result is that if the builder spends, say, $10,000 to get a stiffer foundation he or she will not be able to recoup that cost much less any money for to cover overhead and profit. The buyer will just go look at a house by another builder who puts more money into the kitchen counter-tops and uses a foundation design that is truly minimal.
Reality check six: an engineer’s seal is visible evidence that the engineer takes responsibility for all engineering decisions that went into the design
A geotechnical engineer makes engineering decisions as to the suitability of various types of foundations. Typically, the choices are slab-on-ground, pier-supported slab-on-ground and pier-supported structural slab foundations. If a slab-on-ground foundation is adequate from an engineering perspective, the normal business decision is to go with a slab-on-ground foundation.
Notice that the decision to go with a slab-on-ground foundation is normally a business decision, not an engineering decision. But it is a business decision that takes into account what the market wants as revealed by what buyers are willing to pay for.
Reality check seven: Engineers have an obligation to not waste the builder’s money
There are published guidelines for designing slab-on-ground foundations. Specifically, there is a guide published by the Texas Section of the American Society of Civil Engineers and another published by the Wire Reinforcement Institute. The TBPE has indicated that if the foundation design meets the design criteria, they are likely to consider the design to be competent.