Lag Bolts: All You Need to Know

Lag Bolts: All You Need to Know

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Safety, efficiency, and structural integrity all depend on using the right fastener for a job. One of the most commonly used types is a lag bolt. It sure is a great fastener but do you know exactly when to choose it?

This article will talk all about lag bolts – once you finish reading, you should know everything ranging from when to use them all the way to what alternatives you have at your disposal.

What Is a Lag Bolt?

Lag Bolts A lag bolt is also commonly known as a lag screw. The fixing has a lot of uses because it is one of the most robust fasteners regularly available. The strength of lag bolts means they are used for heavy-duty projects, for example, framing joists, beams, and rooves. We’ll talk more about their applications later in this article.

In terms of appearance, a lag bolt is very similar to a screw. However, a lag bolt is much thicker, with a larger, hexagonal or square head. Also, the end of the thread is pointed. This shape means the lag bolt can be driven into softer material like wood, without piloting a hole first.

The fixing gets its name ‘lag’ from its original use, which was to fix barrel lags together. As time passed and the fixings were used for broader applications, it retained the name.

Lag Bolts vs. Lag Screws: Are They The Same?

Traditionally, while very similar, bolts and screws are different types of fixings. Bolts usually require a pilot hole before insertion and a nut to tighten. In comparison, when discussing wood, screws can drive themselves in without needing a pilot hole. Furthermore, a screw doesn’t need a nut to tighten.

So are lag bolts and screws the same thing? In short, yes. The lag bolt is actually a screw, but the traditional name ‘bolt’ has remained. People are increasingly referring to the fastener as a screw, particularly people from newer generations.

Advantages and Disadvantages of Lag Bolts

Lag bolts have a lot of strengths, so they can be used in many applications. However, lag bolts are also not the ideal choice for many situations. In this section, we’ll discuss the advantages and disadvantages of lag bolts.

1. Strength

The size of lag bolts, both in length and diameter, means that they can handle heavy loads. The sheer strength of lag bolts makes it the go-to choice for many structural carpentry jobs. The larger the diameter, the stronger the bolt and the more weight it can support. However, it is essential to always check with a certified professional before undertaking any structural work.

2. Ease of Use

Lag bolts are relatively straightforward to install. They can be driven straight into the material, particularly if using them with softwood. However, if you want to create a cleaner finish, the application is more complicated, as you will need to drill another hole to the depth of the head and wider than the head. This additional hole allows the bolt to sit beneath the face of the material and not stick out.

3. Availability

Lag bolts are available from most hardware retailers. Sometimes, when working with a specialist fixing, sourcing more is difficult and can cause delays. Lag bolts are found in most places, so they can quickly be restocked if a supply runs low.

4. Size

In terms of structural carpentry, the size of the lag bolt is an advantage. However, this also means that a lag bolt is not a good option for more delicate work. For example, a lag bolt shouldn’t be used to construct a cabinet. First of all, it is completely overkill as that level of strength is not needed to hold a cabinet together. Secondly, the bolt is too big to fit into the positions and hardware for cabinets.

3 Common Lag Bolt Uses

This section will discuss some of the most common uses for lag bolts. As a fixing, lag bolts are incredibly versatile, so don’t feel constrained by what is on this list.

1. Decking

Lag bolts are very useful to fix decking boards down and join the structural components that the boards fix onto.

2. Roof Beams

A roof beam is one of the main load-bearing components of a roof. The roof beam takes the roof’s weight, including the joists and purlins. A building’s structure relies heavily on the integrity of a roof beam. Often lag bolts are used to provide a roof’s required strength.

3. General Framing

Alongside the two common lag bolt uses explained above, lag bolts are perfect for general framing. Framing could be hanging joists, securing a pergola together, or even creating a stud wall.

4 Lag Bolt Types

Lag bolts come in many different shapes and sizes. Alongside different lengths and diameters, they can also have other shaped heads and be made from different materials.

1. Square Head

Square head lag bolts have a more traditional look, and people often use them to give a more rustic style to their build. As the name suggests, the head of this type of lag bolt is square.

2. Hex Head

Hex head lag bolts have six sides to their head. The additional surface area gives more grip for spanners and socket wrenches. The hex head is the more modern style of head for a lag bolt, compared to the square head bolts.

3. Stainless Steel

Stainless steel lag bolts are incredibly strong, making stainless steel one of the most popular forms of lag bolt.

4. Silicone Bronze

Silicone bronze lag bolts are premium bolts used in the wooden boat building industry. Bronze is exceptionally resistant to rust, which is ideal for boats. Although bronze is softer than stainless steel, the bolts are still highly effective at joining components.

4 Lag Bolt Alternatives

Sometimes there are better wood joining fasteners than lag bolts, below are four alternatives to consider.

1. Wood Screws

Wood screws are much smaller and lighter than lag bolts. What wood screws lack in strength compared to lag bolts, they make up for in cost, availability, and ease of use.

2. Hex Bolts

When people think of a bolt, hex bolts are the classic ones that most envisage. With a nut, the hex bolt can join wood, metal, and many other materials with a high degree of strength.

3. Carriage Bolts

Carriage bolts have a rounded head that is less of a hazard to catch. The smooth head is ideal for children’s play equipment or joinery in public spaces.

4. Traditional Timber Framing

Traditional timber framing involves cutting and shaping wood to join together without using fasteners. The types of joints used depends on the application, but mortise and tenon, scarf, and housing joints are some of the most effective and widely used.


Lag bolts have been used for hundreds of years to join wooden components together. The design has slightly adapted to the modern age, but there has been little need as the effectiveness of the bolt is almost timeless.

Use the bolt for a range of activities, but mainly for doing heavy-duty work when smaller screws aren’t strong enough.