The right fastener can be the difference between a successful and a failed project. Even more than that, the wrong one can turn a project into a hazard.
There are a huge amount of different fastener types, each of them designed for a unique purpose. So it’s not uncommon for people to get the wrong one for the job. Do you know the difference between a carriage and a lag bolt?
This article will explain exactly when to use each.
Carriage and Lag Bolts: The Basics
Before we get started with the differences, let’s have a closer look at each of the types of bolt.
What Is a Carriage Bolt?
A carriage bolt is used to fix two components together. Often it will join two pieces of wood, but it can also join wood and metal, two pieces of metal, and many others.
The distinguishing feature of a carriage bolt is the design of its head. A carriage bolt has a smooth, rounded head that sits flat on the jointed surface. Just beneath the head, before the thread starts, the neck is square. This stops the bolt from spinning whilst it is tightened.
A carriage bolt is designed for a nut to tighten along its thread. This can only be done from the end the nut is on, instead of both ends like with a hex screw.
What Is a Lag Bolt?
A lag bolt, or lag screw, as it is also known, is a heavy-duty fastener. Commonly it is used to join timber together, but when combined with a masonry anchor, a lag bolt can be used in stone, masonry, and brick.
There is a bit of confusion around the name of this fixing. In the past, it was called a bolt because its head is large and either hexagonal or square, similar to many other types of bolts. This style head also means that the lag bolt must be tightened with a socket wrench or specialized bit.
However, more recent terminology has moved towards calling the lag bolt a screw, because of the pointed end that allows the fixing to pull itself into the material when tightened.
Similarities of Carriage Bolts and Lag Bolts
There is a lot of crossover between the uses and applications of carriage and lag bolts. Below we will go into more detail about the similarities of them both.
1. Materials
Both the carriage bolt and the lag bolt can be used to join a lot of the same materials.
For example, the carriage bolt and lag bolt can join two pieces of wood together or join metal to wood. However, there are some materials that they both can’t join, which will be covered further down in this article.
2. Jointing Methods
Both the bolts work on the same jointing principle. Essentially a middle component is clamped between the bolt head and the bottom component by a tightening / twisting action.
3. Applications
Some jobs could be completed successfully by using either type of bolt. Although the process might have to be adjusted slightly depending on the bolt chosen.
For example, both bolts are suitable for timber framing, decking, and structural carpentry.
4. The Metals They Are Made From
Both carriage bolts and lag bolts are made from a range of metals.
The most common are steel and stainless steel. Stainless steel is the strongest option, it is also the most resistant to rust. Stainless steel is also the best option for circumstances where the bolts will be in contact with water regularly.
Differences Between Carriage Bolts and Lag Bolts
Even though there are a lot of similarities between carriage and lag bolts, there is a reason for both designs. Below we will explain the differences between the two types of bolts.
1. Head Shape
The head shape is the most obvious difference between the carriage and lag bolt.
The carriage bolt has a smooth, rounded head that is designed to self-lock and not be tightened. In comparison, the head of the lag bolt is hexagonal and designed for a wrench, or spanner to fit around to tighten.
2. Tightening Technique
The two bolts differ fundamentally in the way they are tightened. The carriage bolt works on a nut and bolt system, whereas the lag bolt works as a screw.
To tighten a carriage bolt, there must be space to get the nut onto the end of the thread and a tool around the nut to tighten. It can only be done from the nut end of the thread because the head is smooth and can’t be securely grasped.
In comparison, the lag bolt is twisted via the head and the pointed end of the thread pulls the head tighter against the material.
3. Materials They Can Join
As mentioned in the previous section, the carriage bolt and lag bolt are capable of jointing a lot of the same materials. However, there are some key differences.
The carriage bolt can have metal in either position from the head. On the other hand, the lag bolt’s pointed end must finish in wood. So for instance, a lag bolt could join metal to wood, but not wood to metal.
Carriage Bolts vs. Lag Bolts: Which Should You Use?
Depending on the situation, one of these is better suited than the other:
- If you need to join onto metal, use a carriage bolt
- If the head could be an obstruction or a hazard, use a carriage bolt
- If you can’t have the bolt protruding out of one side of the joint, use a lag bolt
- If you only have access to one side of the joint, use a lag bolt
Alternatives to Carriage and Lag Bolts
You don’t have to only use a carriage bolt or a lag bolt for your project, there are a lot of other options out there. Depending on the type of work you have to complete, they might be better suited.
1. Hex Bolts
Hex bolts are almost the middle ground between a carriage bolt and a lag bolt. They need a nut to tighten properly, like a carriage bolt. However, the head is hexagonal, like a lag bolt. A hex bolt can be used to join multiple materials, including wood, metal, and stone.
I wrote in detail about how carriage and hex bolts compare here.
2. Screws
Screws come in all shapes and sizes, however, they usually aren’t as big or strong as a carriage or lag bolt. The benefit of a screw is it is cheaper than a bolt, easier to install, and smaller in size.
3. Wood Glue
Sometimes, you don’t need to use a fastener to join two components. In fact, a lot of joinery and cabinetry is done without any.
Instead, wood glue can create a strong enough joint in a lot of applications. Make sure that there is sufficient surface contact between the two components and the glue, and consider cutting a traditional joint to strengthen the bond.
Summary
Carriage and lag bolts are extremely useful fasteners that can be used in a wide range of applications.
For non-visual structural carpentry, people favor the lag bolt. However, if the bolt will be visible, or if people could brush past it, the carriage bolt tends to be favored.