Individuals who ask “How much foundation movement is too much” will receive a variety of answers depending on who they ask. The answer will depend on the respondent’s vested interests and opinions about the appropriate performance standards, building codes, legal precedents, and market expectations. It is not at all uncommon for two engineers looking at the same data, to reach different conclusions. As one would expect, an engineer testifying for a plaintiff will see a situation differently than an engineer testifying for a defendant. The seller of a home will tend to minimize the importance of movement while a buyer will zero in on the issue. In part, varying opinions are the result of the fact that the standards published by the Post Tensioning Institute and the American Society of Civil Engineers are not strictly numerical and do allow an engineer to exercise some subjective judgment.
Slab-on-ground foundations are built of steel-reinforced concrete. Over time, design criteria have changed to incorporate the experiences of the engineering and building communities. In general, older slabs (those from the 1950s, 1960s, and 1970s) tend to be thinner and have less reinforcing steel than newer slabs. As a result, expectations for an existing slab should take into account the age, and hence the standards that applied at the time of construction.
Foundation slabs are not perfectly rigid or perfectly stationary. Per the Post Tensioning Institute, “Slab-on-ground foundations are not designed to control soil movement. Rather, they respond to soil movement. They are not infinitely stiff or immovable, therefore they will experience out-of-plane curvature (also known as deflection or bending) and planar tilt.”
Soil movements are caused by:
- The shrinking of expansive soils as they dry.
- The swelling of expansive soils as they absorb water.
- The compaction and settlement of fill dirt.
- Soil slips on hillsides.
As slabs move, the structures that they support also move. How and of what materials a home was built affect to a significant degree how much foundation movement is acceptable. The more rigid and brittle the materials used to build a house are, the more rapidly damage will appear. Hard plaster walls are more brittle than sheetrock, brick veneer is more brittle than wood siding, ceramic tile floors are more brittle than wood floors. The age of a home is also important. Owners are typically less tolerant of movement in homes that are less than ten years old. Purchasers will often pay a premium for older historic homes fully expecting that floors will be out of level and knowing that the doors and window will not be square.
Over the last thirty years, various bodies have had a hand in determining standards for acceptable foundation movement. Some of the bodies are:
- The American Society of Civil Engineers
- The Post Tensioning Institute
- The American Concrete Institute
- The committees that prepare the various building codes
- The marketplace
- And the courts.
As of 2021, the two numerical standards that are most widely used are 1% for tilt and L/360 for deflection. A tilt of 1% is one where foundation slopes exceed a rise or fall of one inch in a span of 100 inches. 100 inches is eight feet four inches (8’4”). Deflection is harder to describe and calculate. Deflection measures how far the surface of a slab lies from a hypothetical surface. For those that are interested in reading the published standards, the document published by the PTI is the Guide for Performance Evaluation of Slab-on-Ground Foundations PTI DC10.8-18, and the document published by the ASCE is Guidelines for the Evaluation and Repair of Residential Foundations.
Individuals can perceive slopes but often have a hard time feeling or seeing curvature. Based on 30 years of experience by the staff at Advanced Foundation Repair, we believe that most people will not notice a slope that is less than a rise or fall of one inch over twenty feet. Most people will definitely notice a slope that exceeds a rise or fall of one inch over a distance of ten feet. For the average person, what is significant is based on what they can perceive.
Unfortunately, aside from situations where specific warranty standards apply, there are no written standards. If foundation movement is causing damage that a homeowner finds unacceptable, and foundation repairs can eliminate or reduce the problem, then repairs are appropriate when an informed homeowner decides to purchase repair services.