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What Is a Plumb Bob?
The plumb bob or plumb line employs the law of gravity to establish what is “plumb” (that is, what is exactly vertical, or true). You don’t have to be a physics whiz to understand that a string suspended with a weight at the bottom will be both vertical and perpendicular to any level plane through which it passes. In a sense, the plumb bob is the vertical equivalent of the line level.
I doubt that you’ll be surprised to learn that this tool, too, has ancient origins. Evidence suggests that Egyptian architects used them to establish verticals in constructing the pyramids. (It’s also thought that, in tandem, they used flooded ditches to determine levels.)
The plumb consists of a specially designed weight and coarse string made of twisted cotton or nylon threads. (Masons prefer nylon because it stands up better over time to the dampness that comes with the trade.) At one end of the string the weight is affixed. Precisely machined and balanced bobs have pointed tips, and can be made of brass, steel, or other materials, including plastic.
Photo: shutterstock.com
How to Use a Plumb Bob
To use the tool, the string is fixed at the point to be plumbed. The weight, or bob, is then allowed to swing freely; when it stops, the point of the bob is precisely below the point at which the string is fixed above. (Note that the line must be hanging free to get an accurate reading.)
The plumb bob is useful in establishing vertical for a wall in construction or a doorjamb when hanging a door. A spirit level will also accomplish those tasks, but some jobs are much more easily done using the tool. For example, you can locate fixtures or decorations in relation to an object or surface below (or above) using the plumb line. Once it is hung and still, the top and bottom points can be marked and used as guides. The plumb line in one place can be used to sight an object in another — a pipe, for example — for plumb.
Surveyors sometimes use plumb bobs for lining up points or transferring them. Excavation and foundation contractors rely upon the plumb line, and constructing a chimney the tool can indicate whether a flue is running true vertical or veering off plumb.
Some bobs have pointed tips that can be bent by repeated use or abuse; if the point on yours is bent or out of alignment, replace it.
Plumb bobs may weigh as little as an ounce or as much as several pounds, depending upon the application. For most home toolboxes, I recommend having a single bob that weighs a few ounces. As is so often the case, it isn’t how fancy the tool is, but how well you use it.
Also called a plummet, a plumb bob is a very simple device. It consists of a small weight with a pointed bottom that is attached to the bottom of a piece of string. The string is usually made from either cotton or nylon, and the weight is typically made from brass or steel.
When the weight is allowed to dangle, it pulls the string into a vertical line that is referred to as a plumb line. The plumb bob gives architects and builders a vertical reference point during construction. It can be used for all kinds of construction projects, from building a bridge to hanging art on the walls.
The term “plumb bob” is sometimes used in golf to refer to a putting technique. The golfer squats behind the golf ball when it is on the green and extends the putter toward the hole. Then, the golfer compares the green on either side of the putter to see which way the ball will break. While the accuracy of this technique is debated among professionals, it is based on the same ideas that make the plumb bob device useful.
History of the Plumb Bob
Most historians agree that the first plumb bob was invented in Egyptian times. Used for architecture, sailing, and surveying the skies, the device likely played a major role in constructing the pyramids. The weight was usually made from a rock or some other kind of heavy object. It was installed inside a frame that allowed it to turn horizontally and vertically: acting as both a level and a plumb bob.
The word itself is said to have come from plumbum, a Latin word that means “lead”. This is likely because the metal, lead, was used for the earliest weights.
In the 1800s, the level was invented. It largely eliminated the need to use a plumb bob as a horizontal measurement device. The level was more compact, efficient, and usually gave a more accurate reading. However, the plumb bob was still very useful for buildings, bridges, and more.
The modern day plumb bob hasn’t changed much from the original version. The first weights were usually made from rocks or stones, then replaced by lead, then finally steel and brass. The weights are still pointed and come with a string to dangle the weight.
There is also a modern adaptation of the plumb bob: laser levels. These devices will project vertical and horizontal laser lines on the wall.
What Is a Plumb Bob Used for?
A plumb bob is a weight that is used to find the center of gravity in order to provide builders with a true vertical line: the plumb line.
Plumb bobs are frequently used in construction when you need to find a vertical line. For example, builders might use it to construct uniform stair steps or to check walls to make sure they are straight. Homeowners can use the plumb bob for a variety of DIY projects, such as hanging picture frames, installing a stud, or hanging a door.
Just like the design, the uses for the plumb bob have not changed much from the earliest version. The first plumb bobs were used to construct huge architectural structures for one of the most advanced civilizations of the time.
For years, plumb bobs were used to survey buildings such as cathedrals and churches. They are still used today in many historical buildings where modern tools would be disruptive — though in some cases, surveyors are turning to high-tech lasers to get their measurements.
Historically, these simple tools were used for both horizontal and vertical measurements, but today, the level is a much more accurate device for finding a true horizontal angle.
How to Use a Plumb Bob
Using a plumb bob to find the true vertical line is not complicated. Here are the steps:
1. Attach the string to a secure surface
Use a nail or a screw to secure the string of the plumb bob to the uppermost point of the wall, door, or surface that you need to measure. If you’re measuring a wall, leave about 2-3 inches between the start of the ceiling and the nail.
2. Release the string
Bring the string all the way down and let go of the weight.
3. Let the weight swing back and forth
Make sure the weight can swing freely without touching the floor. Make sure to wait for it to stop naturally. If you touch the string to make the weight stop, you’ll get an inaccurate reading.
4. Measure the distance from the wall
Once it stops swinging, use a ruler to measure the distance between the string and the wall at the top and bottom of the string. If the measurements are the same, that means your wall is plumb. If they are different, the wall is not truly straight.
Plumb Bob Golf
A Plumb Bob for the Modern Homeowner
Plumb bobs are simple to operate and very affordable. Like a level, this tool comes in handy more often than you would imagine. You can use it to hang pictures, install floating shelves, or hang a new door or window. You can even use it to find out your location with a basic understanding of celestial navigation.
Keep in mind that the weight needs to be able to swing freely in order to produce an accurate reading. This means that any sort of wind or breeze will mess up the device and give you an unreliable measurement.
The simplicity of this humble tool is one of its greatest advantages. In a pinch, you can use a piece of string and any weighty object to get a rough vertical measurement. Some builders use nails attached to a small piece of clay for extra weight — you can also use a rock or a screw.
Plumb Bob Putting
Just be aware that these alternative weights will provide a less accurate measurement than a pointed brass or steel plumb bob weight.
Plumb-bob Method
Brandon Hubbard, AIA, NCARB, LEED AP BD+C