The phrase “rule of thumb” has ancient antecedents and no one really knows where it came from. Most likely, it resulted from early artisans, builders and masons using their thumb as a constant for measurement – a bit rough but effective. There is a specious story about its origin in old English domestic law but that has been proven false many times over.
Today, we use “rule of thumb” to mean a consistent, reliable benchmark that can be applied to a situation to achieve at least an approximately correct measurement. For example, if you place a penny head-down into the tread of a car tire, the rule of thumb is that if the top of Lincoln’s head is showing, the tire is too worn and should be replaced.
Like many other types of construction and home repair, structural foundation repair has its own rules of thumb and some of them may be useful to homeowners considering Chicago foundation repairs.
A lot of the rules for Chicago foundation repairs involve the principles of level and plumb (“plumb” is the vertical equivalent of “level.”)
30% Rule – One typical type of foundation damage is when a poured concrete foundation wall, very common in Chicago, cracks and tilts or rotates inward, out of plumb. This can be caused by settling of the foundation or by lateral pressure from over-saturated soil outside the wall. This rule states that when the wall has moved out of plumb the equivalent of 30% of its own thickness, it must be straightened to stabilize the foundation. In a typical 8” thick wall, that means a deflection, or movement, of 2.4”
Middle-Third Rule – Similar in effect to the 30% Rule, this rule takes into account the downward force exerted on the foundation wall by the structure it supports. This force is greatest on the wall’s center of gravity, which runs down the middle third of the wall. As a wall tips or rotates, that center of gravity shifts toward the outside edge of the middle third, growing increasingly unstable. When the center of gravity passes the middle third, the wall must be straightened.
Other rules of thumb for Chicago foundation repairs are to be applied to a foundation settling or dropping and they measure how much the structure has gone out of level. These rules are typically expressed in a movement of inches (or fractions of inches) over so many feet. For example, a rule might be that a foundation that is more 1” out of level in 15 feet must be raised and stabilized. Here are some common rules of thumb for level:
National Association of Home Builders – ¼” in 8 feet.
U.S. Department of Housing & Urban Development (HUD) – ½” in 12 feet.
U.S. Navy, Facilities Engineering Command – 1” in 20 feet.
Averaged out, these three rules indicate that movement of about 1” in 25 feet, less than any one side of a typical Chicago home, mandates that the foundation be stabilized.
So, what does this mean to the Chicago homeowner? It’s a benchmark for the curious homeowner to do some measuring with a level or a plumb bob when he or she suspects that the foundation may be in trouble. It’s also a good way to assess the work of a Chicago foundation repair contractor, just to make sure he knows what he’s talking about.
The foundation repair experts at U.S. Waterproofing understand the rules of thumb and how they apply and how to use engineering data to confirm them and to plan and execute a permanent, cost-effective foundation repair plan, whether it’s to stabilize one wall or an entire home. Why not ask for a free consultation?
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