The hollow cores of concrete blocks allow an easy path for insects to reach the the wooden sole plate at the bases of walls and floors and begin feasting away.
A better method of building the foundation is to have at least the top course of block to be of solid or “termite” blocks. Sometimes, even many years ago, many higher quality builders additionally capped this top course of concrete block with a metal “termite” shield – copper being one of the best and longest lasting materials. This shield is typically a little wider than the block and the overhangs are bent downward to become an inedible and troublesome mechanical obstacle to insects.
The wood frame garage in this picture was built on a foundation made entirely of hollow core concrete blocks. An expensive repair was required which involved cutting out and replacing large wall areas, a section at a time. At the same time the foundation was "upgraded" -- some of the concrete blocks were replaced with solid block and some hollow blocks were filled with a mortar grout. The photo was taken at an early stage of repair and shows the insect tunneling in the wooden sole plate.
The homeowner had purchased the house about 15 years earlier without the advantage of a thorough pre-purchase buyer's home inspection.
The Inspector's Corner
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