Somewhere, in the annals of home inspector training, a well-meaning home inspector instructor deemed that any foundation crack greater than 3/8” wide indicates sufficient concern that a structural engineer should be consulted. And from then on, it was home inspector gospel.
Although the 3/8” rule is a home inspection guideline, home inspectors rarely follow it, and more often recommend an engineer based on their own personal experience. Some home inspectors may recommend an engineer for any crack, say hairline or 1/32” wide (1/32” is about the thickness of a credit card). Other inspectors may not recommend an engineer for a crack under any circumstances, which is shocking to say the least, and is sometimes wrongly justified with language like, “I’ve seen cracks like this many times.” Well, firemen have seen many fires, and it’s nonsensical for them to suggest that because they’ve seen a house fire it’s irrelevant if your house is burning down.
The engineering basis for a 3/8” crack being the starting point for a structural issue is dubious at best, because much smaller cracks can indicate a big structural issue, and bigger cracks may not be an issue at all. Recently I was in a mid-90s townhouse without so much as a single crack, but the floors dropped more than 4” at the back house, failure of the foundation. While this is an extreme example, we regularly inspect houses with minor cracks that exhibit significant house movement. To be clear: house movement is what is most important, not the size of cracks.
A sound structure should not shift, move, settle, or lean. An inspection of the house structure should focus on movement, not the size of cracks. Therefore, I want to put a nail in the coffin of the 3/8” rule. I mean this, let the 3/8” rule die. In fact, no meaningful conclusions should be inferred from the length, width, or orientation of any foundation crack, interior wall crack, ceiling crack, or any other crack without also measuring how much movement has occurred. Because the only way to know the condition of the house structure is to know how much it has moved and house measurements are simple to take. If your home inspector isn’t taking measurements, then they aren’t helping you to make sure the house structure is sound.
