The word foundation is a timeless metaphor of strength and security, and people quite naturally have genuine concerns about the foundations on which their homes rest. For this reason, people need to be educated about foundations in general and specific types in particular, and I include such information in every report. This is what I include about slab on-grade foundations, and you are welcome to use all or any part of it that you might find useful.
This residence has a bolted, slab foundation. Such foundations are the most modern, but they can vary considerably from older ones that have no moisture barrier beneath them and no reinforcing steel within them to newer ones that have moisture barriers beneath them and adjustable reinforcing steel within them. This type is called a post-tension slab, but is often impossible to distinguish one slab type from another in which even the size and spacing of the bolts can vary, although most are concealed. Our inspection of slabs conforms to industry standards. We examine the visible portion of the stem walls on the exterior of the structure for any evidence of significant cracks or structural deformation. However, we do not move furniture or lift carpeting and padding to look for cracks, and we do not use any specialized tools or measuring devices to establish relative elevations or determine any degree of differential settling. Significantly, many slabs are built or move out of level, but the average person would not realize this until there is a difference of more than one inch in twenty feet, which most authorities describe as being tolerable. Interestingly, many slabs are found to contain cracks when the carpet and padding are removed, but there is no absolute standard for evaluating them.
However, those that are less than 1/4" and which exhibit no significant vertical or horizontal displacement are not regarded as being structurally threatening. They typically result from common shrinkage, but can also be caused by a deficient mixture of concrete, deterioration through time, seismic activity, adverse soil conditions, and poor drainage, and if they are not sealed they can allow moisture to enter a residence, and particularly if the residence is surcharged by a hill or a slope, or if downspouts discharge adjacent to the slab. However, in the absence of any major defects, we may not recommend that you consult with a structural engineer or a foundation contractor, but this should not deter you from seeking the opinion of any such expert, and we would be happy to refer one.
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