We like checking whether things are on the level—or should we say “plumb”? Is a wall plumb or is it level? Is the doorframe plumb or square? And what about walls? Are they displaced or deflected? Let’s review these terms.
Think vertical or horizontal
A level tool has a bubble floating in a vial (Illustration M077). It can check both vertical and horizontal surfaces. “Level” refers to horizontal surfaces; when a surface is perfectly level, the bubble floats within the markings.
“Plumb” refers to vertical surfaces. A table leg, foundation wall or door frame that is “plumb” is perfectly vertical. Again, the bubble in the tool floats within the markings when a surface is vertical. Plumb also can be checked with a plumb bob, which is a weight hung from a length of string. A plumb bob always hangs perfectly vertical.
“Square” refers to the relative position of two surfaces. If they meet at a 90-degree angle, the two surfaces are square. You can check this with a carpenter’s square tool. For a window, you can measure diagonally from each top corner to the opposite corner on the bottom; if the two measurements are different, the window is not square.
Deflection: The wall could move back again
When there’s evidence that a wall has moved, this indicates a need for further inspection. But what’s the correct term—is the wall deflected or displaced?
I prefer to use “deflection” when a structural element moves under load and will return to its original position when the load is removed. Floor framing is a good example. It’s designed for a small amount of deflection (Illustration S133). Often, the specification uses a deflection design criteria of L/360. This means that a floor joist appropriately selected to span 15 feet with an L/360 limit will deflect no more than 1/2 inch (180 in./360) under maximum design loads.
Displacement: The wall won’t return to normal
Unlike deflection, “displacement” assumes that the wall cannot return to its original position. For example, external pressure forces a basement block wall to move inward (Illustration B026). It’s unlikely that this wall could return to its original position, so that makes it displaced, not deflected. The exact measure of displacement is the distance an element has moved from its original position.
When measuring displacement, you should also note the length over which the displacement took place. In this illustration: If the wall is 8 feet high, note that displacement occurs over 4 feet, not 8 feet. You can measure displacement with a long level, a plumb bob or a vertical laser.
Determining the proper terms to use isn’t just splitting hairs. Home inspectors must understand the correct terms to make sure they’re providing accurate information.
Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com, he provides high-quality marketing materials that help professional home inspectors boost their business. Copyright © 2018 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.